Acids, Bases and Salts

"Sour fruits contain various amounts of citric acid."

• We have studied matter on the basis of composition, state, solubility, types of mixture, types of compound.
• Now in this chapter we will categorise some compounds on the basis of their nature as acids, bases and salts.

1.0Introduction

The chemicals which we come across are usually categorised as acids, bases and salts.

Earlier this classification was based on different taste of chemicals.

Acids were first recognized as substances that taste sour. Vinegar tastes sour because it is a dilute solution of acetic acid. Citric acid is responsible for the sour taste of a lemon. Bases, sometimes called alkalis, are characterised by their bitter taste and they are soapy to feel. Most hand soaps and commercial preparation for unclogging drains are highly basic. Substances having taste similar to that of common salt are called salts. However, some substances have very unpleasant taste and even may be poisonous. Usually salts are prepared by chemical reaction between acids and bases.

Now, the question arises - how to test that the given substance is an acid or a base without tasting it? In this chapter, we shall discuss the common characteristics of acids and bases and their chemical nature. We shall also discuss how salts are produced and important applications of some of these salts.

2.0Acids

The term "acid" is derived from the latin word "acidus" meaning sour in taste. According to Arrhenius theory, "An acid is a substance which when dissolved in water, ionizes and releases hydrogen ions [ ${\mathrm{H}}^{+}(\mathrm{aq})]$ in solution". Example, (i) $\mathrm{HCl}(\mathrm{aq})$ Hydrochloric acid ${\mathrm{H}}^{+}(\mathrm{aq})+{\mathrm{Cl}}^{-}(\mathrm{aq})$ Hydrogen ion Chloride ion

OR

$\mathrm{HCl}(\mathrm{g})+{\mathrm{H}}_2\mathrm{O}(\ell )⟶{\mathrm{H}}_3{\mathrm{O}}^{+}(\mathrm{aq})+{\mathrm{Cl}}^{-}(\mathrm{aq})$ (ii) ${\mathrm{H}}_2{\mathrm{SO}}_4(\mathrm{aq})$ $⟶$ Sulphuric acid $2{\mathrm{H}}^{+}(\mathrm{aq})+{\mathrm{SO}}_4^{2-}(\mathrm{aq})$ Hydrogen ion Sulphate ion

3.0Bases

According to Arrhenius theory "Those substances which give hydroxide or hydroxyl ion $\left({\mathrm{OH}}^{-}\right)$in their aqueous solution are called bases". For example, (i) $\mathrm{NaOH}(\mathrm{aq})\to {\mathrm{Na}}^{+}(\mathrm{aq})+{\mathrm{OH}}^{-}(\mathrm{aq})$ Sodium hydroxide Sodium ion Hydroxyl ion

(ii) $\mathrm{KOH}(\mathrm{aq})\to$ ${\mathrm{K}}^{+}(\mathrm{aq})$ $+{\mathrm{OH}}^{-}(\mathrm{aq})$ Potassium hydroxide , Potassium ion , Hydroxyl ion

• In earlier classes, we have studied about some common indicators like - litmus paper.
• As we know, acid turn blue litmus paper red and base turns red litmus paper blue.

4.0Chemical properties of acids and bases

Action of indicators

Indicators: It is a chemical which indicates the nature of solution by changing its colour/odour. Natural indicators: Some plants are natural indicators. For example, onion extract. Litmus is obtained from a plant called lichens. Litmus paper is made by soaking paper in litmus solution. It turns red in acid and blue in alkali.

Turmeric (haldi) is commonly used as a spice in kitchen. It is yellow in colour and stains a white cloth yellow. If the yellow stain is rubbed with soap solution, the colour of the stain changes to brownish red. Why does this happen? The soap solution always contains a small amount of chemical called sodium hydroxide. It is the sodium hydroxide which changes the colour of turmeric stain from yellow to brownish red. Now, if we rub stain with lemon, it again changes to yellow colour. The lemon contains another substance called citric acid, which neutralises the effect of sodium hydroxide. Olfactory indicators: Indicators giving different odour in acidic and basic medium are called olfactory indicators. Examples: Onion extract, vanilla and clove oil.

What causes indicators to show different colours in acids and bases?

• Explanation Indicators undergo some structural changes in the presence of acids or bases which causes them to show different colours. Note: Litmus is the most commonly used indicator in the lab.

5.0Active Chemistry 1

Aim To test acids and bases in the laboratory using acid-base indicators that changes colour.

Materials required Watch glass, test tube, hydrochloric acid, sulphuric acid, nitric acid, acetic acid, sodium hydroxide, calcium hydroxide, potassium hydroxide, magnesium hydroxide, ammonium hydroxide, red and blue litmus solution, phenolphthalein solution, methyl orange solution.

Method (i) Collect the following samples from the science laboratory-hydrochloric acid ( HCl ), sulphuric acid $\left({\mathrm{H}}_2{\mathrm{SO}}_4\right)$, nitric acid $\left({\mathrm{HNO}}_3\right)$, acetic acid $\left({\mathrm{CH}}_3\mathrm{COOH}\right)$, sodium hydroxide $(\mathrm{NaOH})$, calcium hydroxide $\left[\mathrm{Ca}(\mathrm{OH}{)}_2\right]$, potassium hydroxide $(\mathrm{KOH})$, magnesium hydroxide $\left[\mathrm{Mg}(\mathrm{OH}{)}_2\right]$, and ammonium hydroxide $\left({\mathrm{NH}}_4\mathrm{OH}\right)$. (ii) Put a drop of each of the above solutions on a watch-glass and test with a drop of the following indicators. Red litmus solution, Blue litmus solution, Phenolphthalein solution, Methyl orange solution.

Observation The changes observed are as follows:

Conclusion Acids turn blue litmus red but have no effect on red litmus. Bases turn red litmus blue but have no effect on blue litmus. Phenolphthalein is colourless in acidic medium and turns pink in basic medium. Methyl orange is yellow in basic medium and red in acidic medium.

Colour of indicator in acidic/basic medium.

6.0Active Chemistry 2

Aim To test acids and bases using olfactory indicators.

Materials required Plastic bag, chopped onion, strips of clean cloth, dilute HCl solution, dilute NaOH solution, test tube, vanilla essence, clove oil.

Method (i) Take some finely chopped onion in a plastic bag along with some strip of clean cloth. Tie up the bag tightly and leave overnight in the fridge. The cloth strips can now be used to test for acids and bases. $$\begin{gathered} \text{ Aqua regia is a } \\ \text{ mixture of conc. }\mathrm{HCl} \\ \text{ and conc. }{\mathrm{HNO}}_3\text{ in } \\ 3:1\text{ ratio. It can } \\ \text{ dissolve noble metals } \\ \text{ like }\mathrm{Ag},\mathrm{Au}\text{ and }\mathrm{Pt}\text{. } \end{gathered}$$ (ii) Take two of these cloth strips and check their odour. (iii) Keep them on a clean surface and put a few drops of dilute HCl solution on one strip and a few drops of dilute NaOH solution on the other. (iv) Rinse both cloth strips with water and again check their odour. (v) Note your observations. (vi) Now take some dilute vanilla essence and clove oil and check their odour. (vii) Take some dilute HCl solution in one test tube and dilute NaOH solution in another. Add a few drops of dilute vanilla essence to both test tubes and shake well. Check the odour once again and record change in odour, if any. (viii) Similarly, test the change in the odour of clove oil with dilute HCl and dilute NaOH solutions and record your observation.

Observation - Onion, vanilla essence and clove oil loses their smell in bases but does not change their smell in acids.

Conclusion - A substance whose smell varies when it is mixed with an acidic or basic solution, can be used as an acid base indicator (called olfactory indicator).

(2) Reaction of metals with acids and bases

(i) Dilute acids like dilute HCl and dilute ${\mathrm{H}}_2{\mathrm{SO}}_4$ react with certain active metals to evolve hydrogen gas. Metal + Dilute acid $⟶$ Metal salt + Hydrogen

Lets discuss the following activity to understand the reaction between acid and metal.

7.0Active Chemistry 3

Aim To study the reaction of acids with metals.

Materials required Test tube, dilute ${\mathrm{H}}_2{\mathrm{SO}}_4$, zinc granules, soap solution, test tube stand, cork, delivery tube, candle.

Method (i) Take about 5 mL of dil. ${\mathrm{H}}_2{\mathrm{SO}}_4$ in a test tube and add a few pieces of zinc granules to it. (ii) What do you observe on the surface of zinc granules? (iii) Pass the gas being evolved through the soap solution.

Now answer (i) Why are bubbles formed in the soap solution? (ii) Take a burning candle near a gas filled bubble. What do you observe when burning candle is brought near the gas bubbles?

Observation and discussion (i) Bubbles are formed in the soap solution indicating that some gas is evolved during the reaction between the metal and the acid. (ii) The gas burns with a popping sound showing that the gas evolved is hydrogen.

Reaction involved : $\mathrm{Zn}(\mathrm{s})+{\mathrm{dil}}_2{\mathrm{SO}}_4\to {\mathrm{ZnSO}}_4(\mathrm{aq})+{\mathrm{H}}_2(\mathrm{g})$

Conclusion The active metal like zinc displaces hydrogen from dilute acids and the remaining part of the acid combines with the metal to form the corresponding salts. More examples, (i) 2 Na (s) +2 HCl (dil) $\to 2\mathrm{NaCl}(\mathrm{aq})+{\mathrm{H}}_2(\mathrm{g})$ (ii) $\mathrm{Fe}(\mathrm{s})+{\mathrm{H}}_2{\mathrm{SO}}_4$ (dil) $\to {\mathrm{FeSO}}_4(\mathrm{aq})+{\mathrm{H}}_2(\mathrm{g})$ (ii) Metals like zinc, tin and aluminium react with strong alkalies like NaOH (caustic soda) and KOH (caustic potash) to evolve hydrogen gas. Consider the following activity.

8.0Active Chemistry 4

Aim To study the reaction of a base with metal.

Materials required Test tube, granulated zinc, NaOH solution, burner.

Method (i) Place a few pieces of granulated zinc metal in a test tube. (ii) Add 2 mL of NaOH (sodium hydroxide) solution and warm the contents of the test tube. (iii) Repeat the rest of the steps as in previous activity and record your observation.

Observation It is observed that metals like zinc react with strong base like $\mathrm{NaOH},\mathrm{KOH}$, etc. to liberate hydrogen gas and form the corresponding salt. For example, $\mathrm{Zn}(\mathrm{s})+2\mathrm{NaOH}(\mathrm{aq})⟶{\mathrm{Na}}_2{\mathrm{ZnO}}_2(\mathrm{aq})+{\mathrm{H}}_2(\mathrm{g})$ Zinc Sodium hydroxide Sodium zincate Hydrogen

Conclusion Some metals react with bases to liberate hydrogen gas. Some more examples, (i) $2\mathrm{Al}(\mathrm{s})+2\mathrm{NaOH}(\mathrm{aq})+2{\mathrm{H}}_2\mathrm{O}(\ell )⟶2{\mathrm{NaAlO}}_2(\mathrm{aq})+3{\mathrm{H}}_2(\mathrm{g})$ Sodium meta aluminate (ii) $\mathrm{Sn}(\mathrm{s})+2\mathrm{NaOH}(\mathrm{aq})$ $⟶{\mathrm{Na}}_2{\mathrm{SnO}}_2(\mathrm{aq})+{\mathrm{H}}_2(\mathrm{g})$ Sodium stannite Note: Non-metals do not react with dilute acids as they do not displace hydrogen from acids.

(3) Reaction of acid with metal carbonates and metal hydrogen carbonates

All metal carbonates and hydrogen carbonates react with acids to give a corresponding salt, carbon dioxide and water.

Metal carbonate/Metal hydrogen carbonate + Acid $⟶$ Salt + Carbon dioxide + Water Let's discuss the following activity

9.0Active Chemistry 5

Aim To study reaction of metal carbonates and metal hydrogen carbonates with acids.

Materials required Test tubes, sodium carbonate, sodium hydrogen carbonate, dilute HCl solution, lime water.

Method (i) Take two test tubes. Label them as A and B. (ii) Take about 0.5 g of sodium carbonate $\left({\mathrm{Na}}_2{\mathrm{CO}}_3\right)$ in test tube A and about 0.5 g of sodium hydrogen carbonate $\left({\mathrm{NaHCO}}_3\right)$ in test tube B. (iii) Add about 2 mL dilute HCl to both the test tubes. (iv) Pass the gas produced in each case through lime water (calcium hydroxide solution)

Observation It is observed that carbon dioxide gas is evolved in each case which turns lime water milky.

${\mathrm{Na}}_2{\mathrm{CO}}_3(\mathrm{s})+2\mathrm{HCl}(\mathrm{aq})$ $⟶$ $2\mathrm{NaCl}(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\ell )+{\mathrm{CO}}_2(\mathrm{g})$ |

${\mathrm{NaHCO}}_3(\mathrm{s})+\mathrm{HCl}(\mathrm{aq})$ $⟶$ ${\mathrm{NaCl}}_{(\mathrm{aq})}+{\mathrm{H}}_2\mathrm{O}(\ell )+{\mathrm{CO}}_2(\mathrm{g})$

$\mathrm{Ca}(\mathrm{OH}{)}_2+{\mathrm{CO}}_2(\mathrm{g})$ $⟶$ ${\mathrm{CaCO}}_3(\mathrm{s})+{\mathrm{H}}_2\mathrm{O}(\ell )$

Lime water, Carbon dioxide, Calcium carbonate (White ppt.)

On passing excess ${\mathrm{CO}}_2$, the solution becomes clear due to conversion of insoluble ${\mathrm{CaCO}}_3$ into soluble calcium hydrogen carbonate.

$$\begin{gathered} {\mathrm{CaCO}}_3(\mathrm{s})+{\mathrm{H}}_2\mathrm{O}(\ell )+{\mathrm{CO}}_2(\mathrm{g})\stackrel{⟶}{\to }\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2(\mathrm{aq}) \\ \text{ Calcium hydrogen carbonate } \\ \text{ (Soluble in water) } \end{gathered}$$

Conclusion All metal carbonates and hydrogen carbonates react with acids to give a corresponding salt, carbon dioxide and water. One more example, ${\mathrm{CaCO}}_3(\mathrm{s})+2\mathrm{HCl}($ dil $)⟶{\mathrm{CaCl}}_2(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\ell )+{\mathrm{CO}}_2(\mathrm{g})$

(4) Reaction of acid and base with each other Whenever a solution of acid is treated with a solution of base, salt and water are formed as the products. This reaction is known as neutralisation reaction. In general, a neutralisation reaction can be written as :

$\text{ Acid + Base }⟶\text{ Salt + Water }$

The reaction is called neutralisation because the salt which is formed is generally neutral towards litmus. Thus, neutralisation may be defined as: "The reaction between acid and base present in aqueous solution to form salt and water."

10.0Active Chemistry 6

Aim To study the reaction between acids and bases, i.e. neutralisation reaction.

Materials required Test tube, dilute NaOH solution, phenolphthalein solution, dilute HCl solution.

Method (i) Take about 2 mL of dilute NaOH solution in a test tube and add two drops of phenolphthalein solution. (ii) Note the colour of the solution. (iii) Add dilute HCl solution to the above solution drop by drop. (iv) Note the change in colour of the reaction mixture. (v) Now add a few drops of NaOH solution to the above mixture.

Now answer (i) What is the colour of the NaOH solution on adding phenolphthalein? (ii) What change in colour takes place on adding dilute HCl ? (iii) Does the pink colour of phenolphthalein reappear on adding NaOH ? (iv) Why do you think this has happened?

Observation and discussion On adding phenolphthalein to NaOH solution, the colour becomes pink. On adding dilute HCl solution dropwise, finally the pink colour disappears and the solution again become colourless. On adding NaOH , pink colour reappears because the medium becomes basic again.

Conclusion Acid and base react together to form salt and water. Acid + Base $\to$ Salt + Water The reaction is called neutralization reaction.

Some more examples, (i) ${\mathrm{HNO}}_3(\mathrm{aq})+\mathrm{KOH}(\mathrm{aq})\to {\mathrm{KNO}}_3(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\ell )$ (ii) $\mathrm{HCl}(\mathrm{aq})+{\mathrm{NH}}_4\mathrm{OH}(\mathrm{aq})\to {\mathrm{NH}}_4\mathrm{Cl}(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\ell )$

(5) Reaction of metallic oxides with acids A metal oxide reacts with dilute acids to form salt and water. Metal oxide + Acid $\to$ Salt + Water This can be explained with the help of following activity.

11.0Active Chemistry 7

Aim To study the reaction of metal oxides with acids.

Materials required Beaker, copper oxide, dilute hydrochloric acid.

Method (i) Take a small amount of copper oxide in a beaker and add dilute hydrochloric acid slowly while stirring. (ii) Note the colour of the solution.

Now answer What has happened to the copper oxide?

Observation and discussion It is observed that the colour of the solution become blue-green and the copper oxide dissolves.

The blue green colour of the solution is due to formation of copper (II) chloride in the solution.

Conclusion Metal oxides react with the acids to form salt and water.

This shows that metal oxides are basic in nature.

Some more examples: (i) CaO (s) $+2\mathrm{HCl}(\mathrm{aq})⟶{\mathrm{CaCl}}_2(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\ell )$ (ii) $\mathrm{MgO}(\mathrm{s})+{\mathrm{H}}_2{\mathrm{SO}}_4(\mathrm{aq})⟶{\mathrm{MgSO}}_4(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\ell )$

(6) Reaction of a Non-Metallic oxide with a base

Acids react with metal oxides but base react with non-metallic oxide to form their respective salts and water. Base + Non-metallic oxide $⟶$ Salt + Water We have already discussed under acids that metal carbonates and metal hydrogen carbonates react with acids to form carbon dioxide. On passing the ${\mathrm{CO}}_2$ gas evolved through lime water $\left[\mathrm{Ca}(\mathrm{OH}{)}_2\right]$, lime water turns milky.

$\mathrm{Ca}(\mathrm{OH}{)}_2(\mathrm{aq})+{\mathrm{CO}}_2(\mathrm{g})⟶\underset{(\text{ White ppt.) }}{}{\mathrm{CaCO}}_3(\mathrm{s})+{\mathrm{H}}_2\mathrm{O}(\ell )$

Since this is similar to the reaction between a base and an acid, we can conclude that non-metallic oxides are acidic in nature.

Comparison between properties of acids and bases

Nature of oxides

12.0What do all acids and all bases have in common?

In general, all acids have similar chemical properties. We have seen that all acids generate hydrogen gas on reacting with metals, so hydrogen seems to be common in all acids.

All acids produce hydrogen ions $\left[{\mathrm{H}}^{+}(\mathrm{aq})\right]$ when dissolved in water. For example, $\mathrm{HCl},{\mathrm{H}}_2{\mathrm{SO}}_4,{\mathrm{HNO}}_3,{\mathrm{CH}}_3\mathrm{COOH}$, etc. Similarly all bases produce hydroxide ions ( ${\mathrm{OH}}^{-}$) when dissolved in water. For example, $\mathrm{NaOH},\mathrm{Mg}(\mathrm{OH}{)}_2,\mathrm{Ca}(\mathrm{OH}{)}_2,{\mathrm{NH}}_4\mathrm{OH}$, etc. Since the ions are the carriers of charge, therefore tendency to conduct electric current is the common characteristic of all acids and bases when dissolved in water.

13.0Active Chemistry 8

Aim To study the conduction of electricity by aqueous solutions of acids and bases.

Materials required Beaker, solution of glucose, alcohol, hydrochloric acid, sulphuric acid, nails, cork, battery, bulb, switch.

Method (i) Take solution of glucose, alcohol, hydrochloric acid, sulphuric acid, etc. (ii) Fix two nails on a cork and place the cork in a 100 mL beaker. (iii)Connect the nails to the two terminals of a 6-volt battery through a bulb and a switch. (iv) Now pour some dilute HCl in the beaker and switch on the current. (v) Repeat with dilute sulphuric acid. (vi) Repeat the experiment separately with glucose and alcohol solutions.

Now answer (i) What do you observe on passing electric current? (ii) What do you observe in case of glucose and alcohol solution? (iii)Does the bulb glow in all cases?

Observation and discussion The bulb is found to glow in case of acids but not in case of glucose and alcohol solutions.

Conclusion As electric current is carried through the solution by ions, this shows that acids dissociate in the solution to produce ${\mathrm{H}}^{+}(\mathrm{aq})$ ions but substances like glucose and alcohol do not dissociate to give ${\mathrm{H}}^{+}$ions and hence are not acidic. Thus, ${\mathrm{H}}^{+}$ions are responsible for the acidic properties. If we repeat the same activity using alkalis such as sodium hydroxide, calcium hydroxide etc, similar results will be observed. As bases also produce ions in water i.e. ${\mathrm{OH}}^{-}(\mathrm{aq})$ ions, they are also electrically conducting in nature.

What happens to an acid or a base in a water solution? The acidic behaviour of acids is due to the presence of hydrogen ions [ ${\mathrm{H}}^{+}(\mathrm{aq})]$, in them. The acids produce hydrogen ions only in the presence of water. So, in the absence of water, a substance will not form hydrogen ions and hence will not show its acidic behaviour. This can be well illustrated with the following activity. Note:

${\mathrm{H}}^{+}(\mathrm{aq})$ $={\mathrm{H}}_3{\mathrm{O}}^{+}$ ORAq. hydrogen ion $=$Hydronium ion

14.0Active Chemistry 8

Aim To test that dry HCl gas is not acidic but its aqueous solution is acidic.

Materials required Test tube, NaCl , concentrated sulphuric acid, moist blue litmus paper.

Method (i) Take about 1 g solid NaCl in a clean and dry test tube and set up the apparatus. (ii) Add some concentrated sulphuric acid to the test tube. (iii)Test the gas evolved successively with dry and wet blue litmus paper.

Now answer (i) In which case does the litmus paper change colour? (ii) On the basis of the above activity, what do you infer about the acidic character of (a) dry HCl gas (b) HCl solution?

Observation and discussion Dry blue litmus paper does not change colour but wet blue litmus paper changes its colour to red.

Conclusion Dry HCl gas is not acidic but HCl solution is acidic. This is because in the presence of water, HCl dissociates to give ${\mathrm{H}}^{+}$ions.

$$\begin{gathered} \mathrm{HCl}(\mathrm{g})+{\mathrm{H}}_2\mathrm{O}(\ell )\to {\mathrm{H}}_3{\mathrm{O}}^{+}\text{or }{\mathrm{H}}^{+}(\mathrm{aq})+{\mathrm{Cl}}^{-}(\mathrm{aq}) \\ \text{ Hydronium ion }\text{ Chloride ion } \end{gathered}$$

Hydrogen ions cannot exist alone but they exist after combining with water molecules. Thus, hydrogen ions must always be shown as ${\mathrm{H}}^{+}(\mathrm{aq})$ or hydronium ion $\left({\mathrm{H}}_3{\mathrm{O}}^{+}\right)$. ${\mathrm{H}}^{+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}$ Similarly, when a base is dissolved in water, it forms ${\mathrm{OH}}^{-}(\mathrm{aq})$ ion. $\mathrm{NaOH}(\mathrm{s})⟶{\mathrm{Na}}^{+}(\mathrm{aq})+{\mathrm{OH}}^{-}(\mathrm{aq})$. As we have seen that all acids generate ${\mathrm{H}}^{+}(\mathrm{aq})$ ions and all bases generate ${\mathrm{OH}}^{-}(\mathrm{aq})$ ions, this explains the formation of ${\mathrm{H}}_2\mathrm{O}(\ell )$ during neutralisation reaction.

Acid + Base $\to$ Salt + water $\mathrm{HX}+\mathrm{MOH}\to \mathrm{MX}+\mathrm{HOH}$ ${\mathrm{H}}^{+}(\mathrm{aq})+{\mathrm{OH}}^{-}(\mathrm{aq})\to {\mathrm{H}}_2\mathrm{O}(\ell )$

Bases which are soluble in water are called alkalis. All bases do not dissolve in water.

Mixing of acid or base in water is called dilution and the acid or the base is said to be diluted. During dilution, concentration of ions $\left({\mathrm{H}}_3{\mathrm{O}}^{+}/{\mathrm{OH}}^{-}\right)$per unit volume decreases. This process is generally exothermic in nature, which produces heat. Heat produced may be harmful because it may cause the mixture to splash out and cause burns. Moreover, the glass container may also break due to excessive local heating. So, precautions are recommended

• The gas burns with pop sound when a burning candle is brought near the gas bubbles.
• Carbon dioxide gas.
• There are some substances whose odour changes in acidic and basic medium. These are called olfactory indicators.

How can we say all alkalis are bases but all bases are not alkalis?

• Explanation: Only those bases which are soluble in water are called alkalis. All bases do not dissolve in water. So, we can say all alkalis are bases but all bases are not alkalis. E.g. KOH and NaOH are alkalis, while $\mathrm{Mg}(\mathrm{OH}{)}_2$ is a base.

Aim To test that dilution of a strong acid like ${\mathrm{H}}_2{\mathrm{SO}}_4$ and strong base like NaOH is exothermic.

Materials required Beaker, water, concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$, NaOH pellets.

Method (i) Take 10 mL water in a beaker. (ii) Add a few drops of concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$ to it and swirl the beaker slowly. (iii) Touch the base of the beaker. (iv) Repeat the above activity with NaOH pellets.

Caution Add sulphuric acid into water for dilution and not water into sulphuric acid as otherwise the solution may splash out and cause burns.

Now answer (i) Is there a change in temperature in the two cases? (ii) Is this an exothermic or endothermic process?

Observation and discussion In both cases, the temperature is found to rise.

Conclusion Dilution of concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$ with water as well as dissolution of NaOH in water is exothermic.

Precaution needed for dilution During dilution small quantity of acid or base should be added in water with stirring. Heat evolved in this case will be quite slow.

If water will be added to the container containing acid or base, large amount of heat will be produced immediately, which may be harmful.

15.0How strong are acid or base solution?

Acids and bases may be either strong or weak. We can compare their relative strengths on the basis of the Arrhenius theory. According to this theory, more the number of ${\mathrm{H}}^{+}$ions released by acid in water, stronger is the acid. Similarly, more the number of ${\mathrm{OH}}^{-}$ions released by base in water, stronger is the base. The concentration of ${\mathrm{H}}^{+}(\mathrm{aq})$ or ${\mathrm{OH}}^{-}(\mathrm{aq})$ ions in the acidic and basic solution respectively decreases on dilution with water.

If we find quantitatively, the amount of ${\mathrm{H}}^{+}(\mathrm{aq})/{\mathrm{OH}}^{-}(\mathrm{aq})$ ions present in a solution, we can judge how strong an acid or a base is? We can do this with the help of a universal indicator, which is a mixture of several indicators. The universal indicator shows different colours at different concentration of hydrogen ions or pH values in solution.

pH Scale

S.P.L. Sorenson, a Danish chemist in 1909 introduced the concept of measuring the concentration of hydrogen ions $\left[{\mathrm{H}}^{+}(\mathrm{aq})\right]$ in a particular solution. The ' p ' in pH stands for 'potenz' in German, meaning power.

On the pH scale we can measure pH from " 0 " (very acidic) to 14 (very alkaline). (i) Solutions with $\mathrm{pH}=0-3$ are strongly acidic, with $\mathrm{pH}=3-5$ are moderately acidic while with $\mathrm{pH}=5-7$ are weakly acidic. (ii) Solution with $\mathrm{pH}=7-9$ are weakly basic, with $\mathrm{pH}=9-12$ are moderately basic while with $\mathrm{pH}=12-14$ are strongly basic. (iii)If $\mathrm{pH}=7$, then the solution is neutral.

16.0Active Chemistry 11

Aim To test the pH values of some solutions.

Materials required pH paper, given solutions in table.

Method (i) Test the pH value of solutions given in following table. (ii) Record your observations.

Now answer (i) What is the nature of each substance on the basis of your observations?

Observations

Conclusion Solutions with $\mathrm{pH}<7$ but close to 7 are weakly acidic, solutions with $\mathrm{pH}\ll 7$ are strongly acidic, solution with $\mathrm{pH}>7$ but close to 7 are weakly basic, solutions with $\mathrm{pH}\gg 7$ are strongly basic.

Some important pH of common substances.

Important note The pH of human blood varies between 7.36 to 7.42. It is maintained by the soluble hydrogen carbonates and carbonic acid present in blood.

17.0Importance of pH in everyday life

Plants and animals are pH sensitive

The pH plays an important role in the survival of animals, including human beings. Our body works well within a narrow pH range of 7.0 to 7.8. The aquatic animals, like fish can survive in river water within a narrow range of pH change.

For example, when the pH of rain water is less than 5.6, it is called acid rain. Acid rain can lower the pH of river water to such an extent and make it so acidic that the survival of

Red blood cells can exist only over a narrow range of pH aquatic animals becomes difficult or may even kill the aquatic animals.

Soil pH and plants

The growth of plants in a particular soil is related to its pH . Actually, different plants prefer different pH range for their growth. It is therefore, quite important to provide the soil with proper pH for their healthy growth. Plants require a specific pH for their growth. If the soil is too acidic or too basic the plants grow badly or do not grow at all.

18.0Active Chemistry 12

Aim To test the pH of the soil in the backyard of our home.

Materials required Test tube, water, soil, pH paper.

Method (i) Put about 2 g soil in a test tube and add 5 mL water to it. (ii) Shake the contents of the test tube. (iii) Filter the contents and collect the filtrate in the test tube. (iv) Check the pH of this filtrate with the help of universal indicator paper.

Now answer What can you conclude about the ideal soil pH for the growth of plants in your region?

Observation and conclusion The ideal soil pH should be close to 7, i.e., neither very acidic nor very alkaline.

Treatment of acidic or basic Soil The pH of acidic soil can reach as low as 4 and that of the basic soil can go up to 8.3. Chemicals can be added to soil to adjust its pH and make it suitable for growing plants. If the soil is too acidic, then it is treated with materials like quicklime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate). All these materials are bases and hence react with the excess acid present in soil and reduce its acidity. If the soil is too basic (or too alkaline) then its alkalinity can be reduced by adding decaying organic matter (manure or compost), which contains acidic materials. Soils with high peat content or iron mineral or with rotting vegetation tend to become acidic.

Importance of pH in our digestive system

As we know our stomach produces gastric juice which contains large amount of hydrochloric acid ( pH about 1.4). The acid so produced does not harm the stomach walls, but kills germs and bacteria which enter in our digestive system along with food, thus in a way it protects us from diseases and helps in digestion. Sometimes excess of acid is produced in the stomach due to overeating or eating spicy foods. This stage is called acidity. To get relief from this pain, we take tablets known as antacids. These are weak bases which neutralise the excess of acids. For example, Magnesium hydroxide (milk of magnesia) $\mathrm{Mg}(\mathrm{OH}{)}_2$

pH changes as the cause of tooth decay

Generally, the pH in the mouth is more than 7 , as the saliva produced in the mouth is basic in nature. However, when we take food, some food particles remain in the mouth after eating and bacteria present in the mouth produce acids by degradation of food particles. This acid lowers the pH in the mouth, tooth decay starts when the pH of acid formed in the mouth falls below 5.5 and the enamel get corroded. The bacteria present in the mouth break down the sugar that we eat into acids. Lactic acid is one of these. The formation of these acids causes decrease in pH . The acids react with calcium phosphate and the enamel coating slowly breaks.

Therefore, to prevent tooth decay, it is advised to clean the mouth and use toothpastes which are generally basic, for cleaning the teeth. It neutralise the excess acid and prevent tooth decay. (5) Self-defence by animals and plants through chemical warfare

The sting of the honey bee contains formic acid, this acid causes a lot of irritation and pain. The pain can be reduced by applying baking soda paste on the affected region as the acid gets neutralised. In plant kingdom, nettle (Bichu Booti) is a herbaceous plant which grows in wild. The nettle leaves have stinging hair. When a person happens to touch the leaves of a nettle plant accidently, the stinging hair of nettle leaves inject methanoic acid

Nettle plant $(\mathrm{HCOOH})$ into the skin of the person causing burning pain. The nettle sting, being acidic can be neutralised by rubbing baking soda on the skin. Nature provides remedy for the nettle sting in the form of a 'dock' plant, which often grows besides the nettle plants. The leaves of dock plant contain some basic chemicals which neutralises methanoic acid. Some flowering plants carry their own 'built in pH indicators'. For example, the flowers of a hydrangea bush are blue in colour when grown in an acidic soil. If it is alkaline in nature, the flowers become pink.

Some naturally occurring acids

19.0Salts

Salts are the ionic compounds consisting of two parts, one part carrying a positive charge called positive ion or cation and the other part carrying a negative charge called a negative ion or anion.

Families of salts

Salts are generally formed by reaction between an acid and base. On the basis of kind of acid and base used, they contain specific cation or anion. So, we can divide salts in different families on the basis of cation or anion present. The salts having the same positive radical (or cation) or negative radical (or anion) are said to belong to the same family. Or we can say, salts can be classified into different families based on the common ion present.

20.0Active Chemistry 13

Aim Write the formulae of the salts and to identify their acids and bases and their families.

Method (i) Write down the formulae of the salts given below : (1) Potassium sulphate (2) Sodium sulphate (3) Calcium sulphate (4) Magnesium sulphate (5) Copper sulphate (6) Sodium chloride (7) Sodium nitrate (8) Sodium carbonate (9) Ammonium chloride. (ii) Identify the acids and bases from which the above salts may be obtained. (iii)Salts having the same positive or negative radicals belong to the same family. For example, NaCl and ${\mathrm{Na}}_2{\mathrm{SO}}_4$ belong to the family of sodium salts. Similarly, NaCl

Now answer How many families can you identify among the salts given in this activity?

Observation and Conclusion

Families observed :- sulphate family, chloride family, sodium family.

What will happen when egg shell is placed in conc. \mathrm{HNO}_{3} ?

• Explanation: Egg shell contains calcium carbonate as its main constituent. When dipped in concentrated ${\mathrm{HNO}}_3$, calcium carbonate reacts to evolve ${\mathrm{CO}}_2$. As a result, eggshell slowly dissolves. ${\mathrm{CaCO}}_3+2{\mathrm{HNO}}_3$ (conc.) $⟶\mathrm{Ca}{\left({\mathrm{NO}}_3\right)}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2$

\mathbf{p H} of salts: Depending upon the nature of acid and base which react to form the salt.

The nature of acid and base produced when salt reacts with water, the salts can be classified into the following four types. Salts of a strong acid and a strong base are neutral with pH value of 7 . On the other hand, salts of a strong acid and weak base are acidic with pH value less than 7 and those of a strong base and weak acid are basic in nature, with pH value more than 7.

(1) Salts of strong acid and strong base. (Neutral salts) For example, NaCl $\mathrm{NaCl}\Rightarrow \mathrm{NaOH}+\mathrm{HCl}$ Hence, the acid and the base produced neutralize each other completely, so $\mathrm{pH}=7$.

(2) Salt of strong acid and weak base. (Acidic salt) For example, ${\mathrm{NH}}_4\mathrm{Cl}$ ${\mathrm{NH}}_4\mathrm{Cl}\Rightarrow {\mathrm{NH}}_4\mathrm{OH}+\dots \mathrm{HCl}$

(3) Salt of weak acid and strong base (Basic salt) For example, ${\mathrm{Na}}_2{\mathrm{CO}}_3$

(4) Salt of weak acid and weak base (Neutral salts) For example, ${\mathrm{CH}}_3{\mathrm{COONH}}_4$

Chemistry

Aim To find the pH of the given salt solutions.

Method (i) Collect the following salt samples: (a) Sodium chloride (b) Potassium nitrate (c) Aluminium chloride (d) Zinc sulphate (e) Copper sulphate (f) Sodium acetate (g) Sodium carbonate (h) Sodium hydrogen carbonate (ii) Check their solubility in water (use distilled water only). (iii)Check the action of these solutions on litmus and find the pH using a pH paper. (iv) Report your observations in a tabular form.

Now answer (i) Which of the salts are acidic, basic or neutral? (ii) Identify the acid or base used to form the salt.

21.0Chemicals from common salt

Sodium chloride (Common salt / table salt)

We know that hydrochloric acid and sodium hydroxide combine with each other to form sodium chloride ( NaCl ) which in common language is also known as common salt. This is the salt which you sprinkle on your salads and use in your kitchens. Common salt is an ionic compound of sodium and chlorine ${\left({\mathrm{Na}}^{+}{\mathrm{Cl}}^{-}\right)}_{\mathrm{n}}$. The main source of common salt (sodium chloride) is the sea

Sodium hydroxide

Commercially, sodium hydroxide is also called caustic soda because of its corrosive action on animal and vegetable tissues.

Chlor-alkali process for obtaining sodium hydroxide When we pass electricity through a solution of sodium chloride, commonly called brine, it decomposes to form sodium hydroxide according to the following equation: $2\mathrm{NaCl}(\mathrm{aq})+2{\mathrm{H}}_2\mathrm{O}(\ell )\stackrel{\text{ Electric current }}{\to }2\mathrm{NaOH}(\mathrm{aq})+{\mathrm{Cl}}_2(\mathrm{g})+{\mathrm{H}}_2(\mathrm{g})$ On electrolysis, chlorine gas is formed at anode and hydrogen at cathode, sodium hydroxide solution is formed near the cathode. All these products are commercially important.

Bleaching powder

Chemically, bleaching powder is generally represented by the formula, ${\mathrm{CaOCl}}_2$ (called, calcium oxychloride). We know that chlorine is produced during the electrolysis of aqueous sodium chloride (brine). This chlorine gas is used for the manufacture of bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime $\left[\mathrm{Ca}(\mathrm{OH}{)}_2\right]$. Bleaching powder is represented as ${\mathrm{CaOCl}}_2$, though the actual composition is quite complex. $\mathrm{Ca}(\mathrm{OH}{)}_2+{\mathrm{Cl}}_2⟶{\mathrm{CaOCl}}_2+{\mathrm{H}}_2\mathrm{O}$

Uses of bleaching powder (a) For bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry. (b) As an oxidising agent in many chemical industries. (c) For disinfecting drinking water to make it free of germs.

Note: Bleaching powder is also known as chloride of lime.

Baking soda

The chemical name of baking soda is sodium hydrogen carbonate or sodium bicarbonate. Baking soda (or sodium bicarbonate) is represented by the formula ${\mathrm{NaHCO}}_3$. The soda commonly used in the kitchen for making tasty crispy pakoras is baking soda. Sometime it is added for faster cooking. It is produced using sodium chloride as one of the raw materials.

Note: Anhydrous sodium carbonate $\left({\mathrm{Na}}_2{\mathrm{CO}}_3\right)$ is generally called soda ash.

In laboratory, baking soda can be prepared by passing excess of ${\mathrm{CO}}_2$ gas through the saturated solution of sodium carbonate. As a result, sodium hydrogen carbonate is formed. It gets precipitated due to supersaturation. The precipitate is separated, washed and dried without heating. ${\mathrm{Na}}_2{\mathrm{CO}}_3+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}⟶2{\mathrm{NaHCO}}_3\text{ (ppt.) }$

Baking soda can be used to On commercial scale, ${\mathrm{NaHCO}}_3$ is formed as an neutralise formic acid present in red ant's sting. intermediate product, when washing soda is prepared by Solvay process or ammonia-soda process. In this method, a solution of sodium chloride (called "brine") saturated with ammonia is allowed to react with ${\mathrm{CO}}_2$, sodium hydrogen carbonate & ammonium chloride are formed. $\mathrm{NaCl}(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})+{\mathrm{CO}}_2(\mathrm{g})+{\mathrm{NH}}_3(\mathrm{g})\to {\mathrm{NH}}_4\mathrm{Cl}(\mathrm{aq})+{\mathrm{NaHCO}}_3(\mathrm{s})$ Common salt Ammonium Sodium hydrogen chloride carbonate

It can be used to neutralise an acid because it is mild, non-corrosive base due to the presence of ${\mathrm{HCO}}_3^{-}$ ion.

The following reaction takes place when it is heated during cooking. $2{\mathrm{NaHCO}}_3(\mathrm{s})\stackrel{\text{ Heat }}{\to }{\mathrm{Na}}_2{\mathrm{CO}}_3(\mathrm{s})+{\mathrm{H}}_2\mathrm{O}(\ell )+{\mathrm{CO}}_2(\mathrm{g})$

Uses of sodium hydrogen carbonate $\left({\mathrm{NaHCO}}_3\right)$

(a) For making baking powder (which is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid like tartaric acid.) When baking powder is heated, the following reaction takes place. ${\mathrm{NaHCO}}_3+{\mathrm{H}}^{+}⟶{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+$ Sodium salt of the acid (From acid) Carbon dioxide produced during the reaction is responsible for making the bread and cake to rise, making them soft and spongy.

(b) As an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.

(c) It is used in soda-acid fire extinguisher.

Why do we feel a burning sensation in the stomach when we overeat? What is the medicine used called?

• Explanation: Burning sensation is due to excess HCl produced in the stomach. The medicine used to neutralise it, is called antacid. For example, milk of magnesia.

22.0Washing soda (sodium carbonate)

The chemical formula of washing soda is ${\mathrm{Na}}_2{\mathrm{CO}}_3\cdot 10{\mathrm{H}}_2\mathrm{O}$ (sodium carbonate decahydrate). Washing soda is manufactured by Solvay process. This process is also known as ammonia soda process. The raw materials needed for the process are sodium chloride, lime stone $\left({\mathrm{CaCO}}_3\right)$ and ammonia $\left({\mathrm{NH}}_3\right)$. The reactions involved are Step-I

Step-IIDry sodium hydrogen carbonate is heated strongly to produce sodium carbonate.$2{\mathrm{NaHCO}}_3(\mathrm{s})\stackrel{\text{ Heat }}{\to }\underset{\begin{array}{c}\text{ Anhydrous sodium }\\ \text{ carbonate (soda ash) }\end{array}}{{\mathrm{Na}}_2{\mathrm{CO}}_3(\mathrm{s})}+{\mathrm{H}}_2\mathrm{O}(\ell )+{\mathrm{CO}}_2(\mathrm{g})$

Step-IIIWashing soda $\left({\mathrm{Na}}_2{\mathrm{CO}}_3\cdot 10{\mathrm{H}}_2\mathrm{O}\right)$ is obtained by recrystallisation from a saturated solution of soda ash ( ${\mathrm{Na}}_2{\mathrm{CO}}_3$ ).

Uses of washing soda

(a) Washing soda (or sodium carbonate) is used for washing clothes (laundry purposes).(b) Washing soda is used for softening hard water.(c) Sodium carbonate (soda ash) is used for the manufacture of detergents.(d) Sodium carbonate is used for the manufacture of many important compounds, such as borax (${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7$), hypo $\left({\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3\cdot 5{\mathrm{H}}_2\mathrm{O}\right)$, etc.(e) Sodium carbonate is also used in paper, glass, soap and paint industries.

Note: Since one of the source of washing soda is ashes of plants, it is sometimes called soda ash.

Crystals of some salts contain certain amount of associated water.The water associated with the crystal (or molecule) of any salt is called water of crystallisation.The salts containing water of crystallisation are called hydrated salts.

Aim To test the presence of water of crystallisation in copper sulphate crystals.

Materials required Boiling tube, copper sulphate crystals, burner, water, test tube holder.

Method (i) Heat a few crystals of copper sulphate in a dry boiling tube. (ii) Note the colour of copper sulphate after heating. (iii) Observe the water droplets in the boiling tube. (iv) Add 2-3 drops of water on the sample of copper sulphate obtained after heating.

Now answer (i) What do you observe on heating blue copper sulphate crystals? (ii) Is the blue colour of copper sulphate restored on adding water?

Observation and discussion Blue coloured copper sulphate crystals on heating leave behind white anhydrous copper sulphate and water droplets are seen in the upper cooler parts of the boiling tube. On adding 2-3 drops of water to the white residue, blue colour reappears.

Conclusion Copper sulphate crystals on heating lose water to form white anhydrous copper sulphate, which can combine again with water to form blue coloured copper sulphate.

Water of crystallisation It is the fixed number of water molecules present in one formula unit of a crystalline salt. For example,(i) Blue vitriol ${\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}$(ii) Green vitriol ${\mathrm{FeSO}}_4.7{\mathrm{H}}_2\mathrm{O}$(iii) Gypsum ${\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}$

Plaster of Paris (CaSO $4.1/2{\mathbf{H}}_2\mathrm{O}$)

Plaster of Paris is hemihydrate (hemi means half and hydrate means water) of calcium sulphate. Its molecular formula is ${\mathrm{CaSO}}_4\cdot 1/2{\mathrm{H}}_2\mathrm{O}$ or ${\left({\mathrm{CaSO}}_4\right)}_2.{\mathrm{H}}_2\mathrm{O}$.

Here is the processed markdown text according to the specified rules:

Preparation of plaster of Paris Plaster of Paris is obtained by heating gypsum ( ${\mathrm{CaSO}}_4\cdot 2\mathrm{HzO}$ ) in a kiln at 373 K (or $100^{\circ }\mathrm{C}$ ).

Properties of Plaster of Paris (1) Plaster of Paris is a white, odourless powder.(2) At ordinary room temperature, Plaster of Paris absorbs water and a large amount of heat is liberated.(3) When mixed with a limited amount of water ( $50\%$ by mass), it forms a plastic mass, evolves heat and quickly sets to a hard-porous mass within minutes. This is called the setting process.

During setting, a slight expansion in volume occurs. It is due to this that it fills the mould completely and gives sharp impression. The reaction during process is${\mathrm{CaSO}}_4\cdot \frac{1}{2}{\mathrm{H}}_2\mathrm{O}(\mathrm{s})+\frac{3}{2}{\mathrm{H}}_2\mathrm{O}(\ell )\to {\mathrm{CaSO}}_4\cdot 2{\mathrm{H}}_2\mathrm{O}(\mathrm{s})$

Plaster of Paris, Water, Gypsum (Hard mass)

Uses of plaster of Paris (1) Plaster of Paris is used in making casts and patterns for moulds and statues. (2) Plaster of Paris is used as cement in ornamental casting and for making decorative materials.

List of some important chemical compounds with their Common Names & Formulas.

23.0Basic terminology

• Olfactory - Related to the sense of smell
• Odour - Smell
• Brine - Saturated salt solution
• Crystals - Definite shaped salt
• Mild - Neither strong nor weak
• Edible - Can be eaten
• Popping - To make a short, quick explosive sound.
• Unclogging - To remove accumulated matter.
• Stain - To leave a coloured mark that is difficult to remove.
• Cation - Positively charged ion.
• Anion - Negatively charged ion.
• Pellet - Small particles created by compressing an original material.
• Swirl - To cause something to make fast circular movements.
• Splash - A small amount of liquid that falls onto something.
• Casting - The process of pouring liquid metal into a mold where it cools and solidifies.
• Ornamental - Made somewhere in order to look attractive.
• Disinfectant - A substance that destroys bacteria and is used for cleaning.