Separation of Substances

In the previous chapter, we have discussed the three physical states of matter i.e. solid, liquid & gas. We have also studied their characteristics & the effect of temperature & pressure on these physical states. In the present chapter, we shall consider the chemical nature of matter.

In literature, the word substance means, ‘a particular kind of matter with uniform properties. Thus, a pure substance in science is a single pure form of a matter and not a mixture of several different kinds of matter. A pure substance cannot be separated into other kinds of matter by any physical process. It implies that, whenever we talk about a pure substance, then in actuality we are talking of an ‘element’ or a ‘compound’.

We know that all the matter around us is not pure. If we observe some soil and some sugar placed on two different sheets of paper with a magnifying glass, we find that soil contains clay particles, some grass particles and even some dead insects etc. That is, soil contains particles of different kinds, thus it is called an impure substance (or mixture). Now observe that sugar which contains only one kind of particle, is called a pure substance.

In terms of science, a substance is a kind of matter which cannot be separated into any other types of matter by some physical means. Such a substance which has only one component & nothing else in it is called a pure substance. But it is quite difficult to get a pure substance as such. Substances are mostly mixed with one another & their combination is known as mixture.  

1.0Types of substances

On the basis of chemical nature, matter can be classified into two types:

(i) Pure substance.

(ii) Impure substance (mixture)

Many things around us such as stones, wood, etc. are used as such. However, a large number of substances available in nature are mixed with certain other substances. Thus, in order to use them we separate them by various methods. On the separation of a mixture, some of its components may be useful or all components are useful.

Following examples will illustrate the above statement. 

(i) Milk is churned to separate butter.

(ii) We gin cotton to separate seeds from the fibre.

(iii) We took out chillies from poha or daliya.

(iv) Pick out one kind of fruit from basket containing mangoes and guavas.

(v) Tea leaves are separated from the liquid with strainer.

2.0Pure Substance

A homogeneous material which contains particles of only one kind and has a definite set of properties, is called a pure substance e.g. O₂, iron, sulphur etc.

or

If a substance is composed of two or more different kinds of particles combined together in fixed proportion by weight, then also substance is regarded as a pure substance

e.g. Sodium chloride is a pure substance, because it has a fixed number of sodium and chloride ions, combined together in fixed proportion by weight. Similarly, magnesium oxide (MgO), carbon dioxide (CO₂), copper sulphate (CuSO₄) etc. are pure substances.

Characteristics of Pure Substances

(1) A pure substance is homogeneous in nature.

(2) A pure substance has a definite set of properties.

(3) The composition of a pure substance cannot be altered by any physical means.

(4) A pure substance has a definite melting point, density, boiling point etc.

For example, water is a pure substance because it has uniform composition throughout. It consists of only one kind of particle called water molecules, which cannot be separated into its constituents (H & O) by any simple physical process.

Elements 

Although only 92 elements occur naturally, there are obviously far more than 92 different kinds of matter on earth. We can find a few hundred around us. All the many kinds of matter can be classified as either pure substances or mixtures. Pure substances, in turn, can be either elements or compounds. Elements are those pure substances that cannot be separated into simpler substances. For example, iron, gold, oxygen, lead, nitrogen, silver etc.

These elements combine with each other to form various compounds.

For example, salt is a compound made up of the elements sodium and chlorine. Water is a compound made up of the elements hydrogen and oxygen.

We cannot divide sodium, chlorine, hydrogen or oxygen into further simpler substances.

An element

(i) Consists of only one kind of atom.

(ii) Cannot be broken down into a simpler type of matter by either physical or chemical means.

(iii) Can exist as either atoms (e.g. argon) or molecules (e.g., nitrogen).

About 118 elements are known today, out of which 92 elements exist in nature, remaining are synthetic (man-made).

Elements can exist as solid, liquid or gas. Two elements are liquid in their standard state i.e. mercury and bromine. Eleven elements are gaseous in their standard state. These are noble gases (He, Ne, Ar, Kr, Xe, Rn), the halogens (fluorine and chlorine). Hydrogen, oxygen and nitrogen are the other gaseous elements. All other elements are solid in their standard state.

On the basis of difference in properties, elements can be divided into metals, non-metals and metalloids.

Metals

Metals are often shiny, malleable, ductile, and can conduct electricity. A malleable material is one that can be rolled into flat sheets. A ductile material is one that can be pulled out into a long wire. Copper can be made into thin sheets and wire because it is malleable and ductile.

Conductivity is the ability of an object to transfer heat or electricity to another object. 

Most metals are good conductors of electricity. In addition, a few metals are magnetic. For example, iron (Fe), cobalt (Co), and Nickel (Ni) are attracted to magnets. They can also be made into magnets.

Most metals are solid at room temperature. Mercury (Hg) is a metal that is liquid at room temperature.

Non-metals

A nonmetal is an element that lacks most of the properties of a metal. Solid non-metals are dull, i.e. they are not shiny. They are brittle, i.e. they are neither malleable nor ductile.

If you hit most solid non-metals with a hammer, they break apart easily and non-metals are poor conductors of electricity.

Ten of the 16 non-metals found in nature are gases at room temperature. The air you breathe is mostly a mixture of two non-metals, oxygen (O₂) and nitrogen (N₂). The non-metals carbon (C), iodine (I), and sulphur (S) are solid at room temperature. Bromine (Br) is the only nonmetal that is liquid at room temperature. The elements helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) are known as noble gases. The noble gases do not ordinarily form compounds.

Metalloids

The metalloids have some characteristics of both metals and non-metals. All are solids at room temperature. They are brittle and hard. They can conduct electricity under certain conditions. 

The most common metalloid is silicon (Si). Silicon combines with oxygen to form silicon dioxide. Sand is largely made up of silicon dioxide. Sand is used in making glass.

Metalloids can vary in their electrical conductivity. Their conductivity depends on various factors, such as temperature or light. For this reason, metalloids such as silicon (Si) and germanium (Ge) are used to make computer chips. 

Symbol of Elements

The names of the chemical elements have come from many sources. Sometimes the names come from descriptions of the element’s properties, sometimes the name reflects the place where the element was discovered, and sometimes the name honours a famous scientist.

We often use abbreviations to simplify the written word. For example, it is much easier to put M.P. on an envelope than to write out Madhya Pradesh, and we often write USA instead of United States of America. 

Likewise, chemists have invented a set of abbreviations or element symbols for the chemical elements. 

These symbols usually consist of the first letter or the first two letters of the element names. The first letter is always capitalized, and the second is not. For example,

Sometimes, however, the two letters used are not the first two letters in the name. For example,

The symbols for some other elements are based on the original Latin or Greek name.

The names and symbols of the most common elements*

Where appropriate, the original name is shown in parentheses so that you can see the sources of some of the symbols

Compounds 

A compound is a substance which is formed when two or more elements are chemically joined. Water, salt, and sugar are examples of compounds. When the elements are joined, the atoms lose their individual properties and have different properties from the elements they are composed of.

A compound

(i) Consists of atoms of two or more different elements bound together,

(ii) Can be broken down into a simpler type of matter (elements) by chemical means (but not by physical means),

(iii) Has properties that are different from its component elements, and always contains the same ratio of its component atoms.

For example, when atoms of sodium (a soft, silvery metal) combine with atoms of chlorine (a toxic, yellow-green gas), the familiar white solid called sodium chloride (table salt) is formed. Similarly, when two atoms of hydrogen combine with one atom of oxygen, water is formed. Such transformations are called chemical reactions.