NCERT Solutions for Class 8 Science Chapter 8 – Nature of Matter: Elements, Compounds, and Mixtures

The 8th chapter of NCERT Solutions Class 8 Science book describes that there are three categories of matter that correspond to different categories of composition; elements, compounds, and mixtures based on either classification or composition. Chapter 8 describes the formation of all substances, how they are different, and how they may be separated through different methods. 

This chapter also teaches students how these differences will be the basis for a solid foundation in chemistry and offer a deeper understanding of how all matter works together.

The solutions to the questions provide for an organized, easy-to-follow presentation style that provides conceptual clarity and prepares students for examinations to be ready.

1.0NCERT Solutions for Class 8 Science Chapter 8 PDF Download

The following section presents access for students to download the entire Class 8 PDF of NCERT's nature of matter, elements, compounds and mixtures chapter, for ease of review and reference.

The solutions for class 8 to the science question set for chapter 8 have been created by a group of highly skilled science professionals at ALLEN, all of whom are extensively knowledgable on the nature and composition of matter, as well as the requirements set forth by the NCERT educational curriculum.

2.0NCERT Questions with Solutions Class 8 Science Chapter 8 Detailed Solutions

1. Consider the following reaction where two substances, A and B, combine to form a product C:

A + B → C

Assume that A and B cannot be broken down into simpler substances by chemical reactions. Based on this information, which of the following statements is correct?

(i) A, B, and C are all compounds and only C has a fixed composition.
(ii) C is a compound, and A and B have a fixed composition.
(iii) A and B are compounds, and C has a fixed composition.
(iv) A and B are elements, C is a compound, and has a fixed composition.

Ans. (iv) A and B are elements, C is a compound, and has a fixed composition.

It says A and B cannot be broken down into simpler substances by chemical reactions → this means A and B are elements. When elements combine, they form a compound → so C is a compound. 

Compounds always have a fixed composition (the ratio of A and B atoms is always the same).

2. Assertion: Air is a mixture.

Reason: A mixture is formed when two or more substances are mixed, without undergoing any chemical change.

(i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
(ii) Both Assertion and Reason are true, but Reason is not the correct explanation for Assertion.
(iii) Assertion is true, but Reason is false.
(iv) Assertion is false, but Reason is true.

Ans. (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.

Assertion: Air is a mixture. → This is true, because air is made of nitrogen, oxygen, carbon dioxide, etc., and they do not chemically react with each other.

Reason: A mixture is formed when two or more substances are mixed, without undergoing any chemical change. → This is also true, because in air, the gases keep their own properties (no chemical change) and this reason perfectly explains why air is a mixture.

3. Water, a compound, has different properties compared to those of the elements oxygen and hydrogen from which it is formed. Justify this statement.

Ans. Water is a compound made by combining hydrogen and oxygen.

• Hydrogen is a gas that can catch fire.
• Oxygen is a gas that helps things burn.

But when they combine, they form water, which is a liquid that puts out fire instead of burning. This shows that a compound has completely different properties from the elements it is made of.

4. In which of the following cases are all the examples correctly matched? Give reasons in support of your answers.

(i) Elements — water, nitrogen, iron, air.
(ii) Uniform mixtures — minerals, seawater, bronze, air.
(iii) Pure substances — carbon dioxide, iron, oxygen, sugar.
(iv) Non-uniform mixtures — air, sand, brass, muddy water.

Ans.
(iii) Pure substances — carbon dioxide, iron, oxygen, sugar.

This is one of the options among all of the above option in which all the examples are correctly matched.

Carbon dioxide and sugar are compounds, iron and oxygen are elements. Elements and compounds are both pure substances.

For the part (i), only iron and nitrogen are elements whereas water is a compound and air is a mixture.

For the part (ii), bronze and air are uniform mixtures whereas minerals and seawater are not.

For the part (iv), sand and muddy water are non-uniform mixtures whereas air and brass are not.

5. Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to form magnesium oxide. Classify all the substances involved in the above reactions as elements, compounds or mixtures, with justification.

Ans.

• Element: A substance that cannot be broken into simpler substances. Example: Oxygen, Iron.
• Compound: A substance made when two or more elements combine chemically. Example: Water (H₂O), Carbon dioxide (CO₂).
• Mixture: A combination of two or more substances that are not chemically joined. Example: Air, Salt + Water.

As per the reactions:

Reaction 1: Iron + moist air → Iron oxide

Iron → element (cannot be broken down)
Moist air → mixture (oxygen + water vapour + other gases)
Iron oxide → compound (new substance formed with fixed composition)

Reaction 2: Magnesium + oxygen → Magnesium oxide
Magnesium → element
Oxygen → element
Magnesium oxide → compound

6. Classify the following as elements, compounds, or mixtures in table.

Carbon dioxide, sand, seawater, magnesium oxide, muddy water, aluminium, gold, oxygen, rust, iron sulfide, glucose, air, water, fruit juice, nitrogen, sodium chloride, sulfur, hydrogen, baking soda.

Identify pure substances amongst these and list them below.

Ans. Pure Substances: Elements and compounds are pure substances. All the examples included in the list of elements and compounds are the examples of pure substances.

7. What new substance is formed when a mixture of iron filings and sulfur powder is heated, and how is it different from the original mixture? Also, write the word equation for the reaction.

Ans. When iron filings and sulfur powder are heated, they react to form iron sulfide, a new compound.

Difference: The original mixture can be separated easily, but iron sulfide cannot be separated into iron and sulfur because it’s a compound.

Word Equation:
Iron + Sulfur → Iron sulfide

8. Is it possible for a substance to be classified as both an element and a compound? Explain why or why not.

Ans. No, it is not possible for a substance to be both an element and a compound. This is because an element is made of only one kind of atom and cannot be broken down into simpler substances, while a compound is formed when two or more elements chemically combine in a fixed ratio.

9. How would our daily lives be changed if water were not a compound but a mixture of hydrogen and oxygen?

Ans. If water were not a compound but just a mixture of hydrogen and oxygen, our daily lives would be completely different and dangerous. Hydrogen and oxygen as separate gases are highly flammable, so we couldn’t safely drink or use them.

There would be no liquid water for drinking, cooking, cleaning, or bathing. Plants and animals could not survive without it. Rivers, lakes, and oceans would not exist as we know them, and weather like rain would be impossible.

In short, life on Earth as we know it would not be possible without water being a stable compound.

10. Analyse figure. Identify Gas A. Also, write the word equation of the chemical reaction.

Ans.  When metals react with an acid, it results in the formation of hydrogen gas and salt. When iron reacts with dilute sulfuric acid, it results in the formation of iron sulfate and hydrogen gas.

Word Equation for the chemical reaction:

Iron + dil. sulfuric acid → Iron sulfate + Hydrogen gas (Gas A)

Gas A (Hydrogen gas) can be confirmed by the burning candle test. When a burning candle is kept near the mouth of test tube, hydrogen gas burns with the pop-up sound that confirms Gas A is the hydrogen gas.

11. Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.

Ans.

1. Water (H₂O) – made of hydrogen and oxygen (both non-metals)
   Uses: Drinking and cooking, cleaning and bathing
2. Carbon dioxide (CO₂) – made of carbon and oxygen (both non-metals)
   Uses: Used in carbonated drinks, helps plants make food by photosynthesis.
   
   

12. How can gold be classified as both a mineral and a metal?

Ans. Gold can be classified as both a mineral and a metal because it fits the definitions of both. As a mineral, gold occurs naturally in the Earth’s crust in a pure, solid form. As a metal, it has metallic properties like being shiny, malleable, ductile, and a good conductor of heat and electricity.

3.0Key Topics Covered in Class 8 Science Chapter 8 – Nature of Matter

• Defining Elements: Substances that cannot be broken down into simpler substances by chemical reactions. They consist of only one kind of atom.
• Understanding Compounds: Pure substances are formed when two or more elements combine chemically in a fixed ratio.
• Key Property: Compounds have completely different properties from the elements they are made of (e.g., water puts out fire, while its components hydrogen and oxygen are highly flammable).
• Explaining Mixtures: Combinations of two or more substances that are not chemically joined and keep their own properties. Air is a prime example of a mixture.
• Pure Substances: This category includes both elements and compounds because of their fixed compositions.
• Chemical Reactions: Observing how metals like iron react with acids to produce hydrogen gas, or how heating a mixture of iron and sulfur creates a new compound (iron sulfide).

4.0Quick Insights About the Chapter – Nature of Matter

• Air vs. Water: Air is a mixture because its gases do not chemically react with each other. Water is a compound because hydrogen and oxygen are chemically bonded in a fixed 2:10 ratio.
• Testing for Hydrogen: Gas released during an iron-acid reaction (Hydrogen) can be confirmed by the "burning candle test," where it burns with a distinct "pop" sound.
• The Importance of Compounds: If water were just a mixture of hydrogen and oxygen gases rather than a stable liquid compound, life on Earth would be dangerous and likely impossible due to the flammability of the separate gases.
• Dual Classification: Gold can be classified as both a mineral (naturally occurring in the Earth's crust) and a metal (based on its properties like being shiny and a good conductor).