CBSE Important Questions Class 8 Science Chapter 8 – Nature of Matter: Elements, Compounds and Mixtures

Class 8 Science Chapter 8 Nature of Matter: Elements, Compounds and Mixtures is part of the new NCERT Curiosity textbook. It explains how all the matter around us can be sorted into pure substances and mixtures. Students learn what an element is, what a compound is and how a compound differs from a mixture, along with symbols of common elements, and the difference between homogeneous and heterogeneous mixtures.

These CBSE important questions for Class 8 Science Chapter 8 cover the classification of matter, the difference between elements and compounds, why a mixture is not a compound, symbols and formulae, metals and non-metals, homogeneous and heterogeneous mixtures, and separating the parts of a mixture. Every question has a clear step-by-step answer in simple English, so students can practise on their own, parents can check the answers, and teachers can use them for class tests.

Q1. What is a pure substance? How is it different from a mixture?

Answer: A pure substance is made of only one kind of particle and has the same properties throughout. Elements and compounds are pure substances. For example, pure water, oxygen and common salt.
A mixture contains two or more substances mixed together in any proportion, and each keeps its own properties. For example, air, sea water and milk.
The main differences are that a pure substance has a fixed composition and fixed melting and boiling points, while a mixture has no fixed composition and its parts can usually be separated by simple physical methods.

Q2. Define an element. Give two examples each of metals and non-metals.

Answer: An element is a pure substance made of only one kind of atom. It cannot be broken down into any simpler substance by a chemical reaction.
Metals: iron (Fe) and copper (Cu). Metals are shiny, hard, good conductors of heat and electricity, and can be beaten into sheets and drawn into wires.
Non-metals: oxygen (O) and sulphur (S). Non-metals are usually dull, brittle and poor conductors of heat and electricity.

Q3. What is a compound? Write any three differences between a compound and a mixture.

Answer: A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio by mass. Water (H₂O) and common salt (NaCl) are compounds.

Q4. Iron filings and sulphur powder are mixed together. In one case the mixture is simply stirred, and in another case it is heated strongly. Compare what you get in the two cases.

Answer: On stirring (a mixture): the iron filings and sulphur keep their own properties. The iron can still be picked out with a magnet, and the mixture looks like a dull yellow powder with grey specks.
On heating (a compound): the two react chemically and form iron sulphide (FeS), a black solid. A magnet can no longer pull out the iron, because the iron is now chemically joined to the sulphur. The new substance has properties completely different from both iron and sulphur.
This experiment shows clearly how a compound is different from a mixture.

Q5. What is a symbol of an element? Write the symbols for the following: hydrogen, oxygen, sodium, potassium, iron and copper.

Answer: A symbol is a short form used to represent an element. It is usually the first letter, written as a capital, or the first letter with one more letter written in small case.

Q6. Which of the following is a compound?
(a) Air
(b) Brass
(c) Water
(d) Sea water

Answer: (c) Water
Water is made by the chemical combination of hydrogen and oxygen in a fixed ratio of 1 : 8 by mass, so it is a compound. Air is a mixture of gases, brass is a mixture (alloy) of copper and zinc, and sea water is a mixture of water with salts dissolved in it.

Q7. What is the difference between a homogeneous and a heterogeneous mixture? Give two examples of each.

Answer: A homogeneous mixture is the same throughout; its parts cannot be seen separately. The different substances are mixed evenly. Examples: sugar dissolved in water, and air.
A heterogeneous mixture is not the same throughout; its different parts can be seen with the eye or a lens. Examples: a mixture of sand and iron filings, and oil floating on water.

Q8. Why is air considered a mixture and not a compound? Give three reasons.

Answer: Air is a mixture because:
1. Its composition is not fixed. The amount of water vapour, dust and carbon dioxide changes from place to place and from season to season.
2. The gases in air keep their own properties. Oxygen still supports burning and nitrogen still does not, just as they do on their own.
3. The gases can be separated by physical means, such as the fractional distillation of liquid air, and no chemical reaction and no heat or light is involved when air is formed.

Q9. Give one word for each of the following:
(a) A substance made of only one kind of atom
(b) The smallest particle of an element that takes part in a chemical reaction
(c) A pure substance formed by the chemical combination of two or more elements
(d) A substance that cannot be broken down further by chemical means

Answer: (a) Element
(b) Atom
(c) Compound
(d) Element

Q10. Classify the following as elements, compounds or mixtures: oxygen, brass, carbon dioxide, soil, gold, common salt.

Q11. How would you separate a mixture of sand, common salt and iron filings? Explain each step.

Answer: The mixture can be separated in three steps:
1. Magnetic separation: Move a magnet over the mixture. The iron filings stick to it and are removed, because iron is magnetic while sand and salt are not.
2. Dissolving and filtration: Add water to the remaining mixture and stir. The salt dissolves but the sand does not. Pour the mixture through filter paper; the sand stays behind as residue and the salt solution passes through as filtrate.
3. Evaporation: Heat the salt solution. The water evaporates and the common salt is left behind in the dish.

Q12. Read the Assertion (A) and Reason (R) and choose the correct option.
Assertion (A): Water is a compound and not a mixture of hydrogen and oxygen.
Reason (R): In water, hydrogen and oxygen are present in a fixed ratio by mass and have lost their own properties.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

Answer: (a) Both A and R are true, and R is the correct explanation of A
In water, hydrogen and oxygen are chemically joined in the fixed ratio 1 : 8 by mass. They no longer behave as they did before: hydrogen burns and oxygen supports burning, but water actually puts out fire. A fixed ratio and the loss of the original properties are exactly what make a substance a compound, so R correctly explains A.

Related Study Material on Extramarks

• CBSE Important Questions Class 8 Science Chapter 1
• CBSE Important Questions Class 8 Science Chapter 2
• CBSE Important Questions Class 8 Science Chapter 3
• CBSE Important Questions Class 8 Science Chapter 4
• CBSE Important Questions Class 8 Science Chapter 5
• CBSE Important Questions Class 8 Science Chapter 6
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