Chapter 8: Nature of Matter - Solutions
Question 1
Consider the following reaction where two substances, A and B, combine to form a product 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.
Answer: (iv) A and B are elements, C is a compound, and has a fixed composition.
Question 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.
Answer: (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
Question 3
Water, a compound, has different properties compared to those of the elements oxygen and hydrogen from which it is formed. Justify this statement.
Answer: Water is a compound formed by the chemical combination of two elements — hydrogen and oxygen — in a fixed ratio of 2:1.
- Hydrogen is a colourless, odourless, and highly flammable gas.
- Oxygen is also a colourless and odourless gas that supports combustion.
However, when these two gases combine chemically, they form water, which is a liquid that extinguishes fire instead of supporting it. Thus, the properties of water are completely different from those of its constituent elements, proving that a compound has properties distinct from the elements from which it is formed.
Question 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.
Answer: (iii) Pure substances — carbon dioxide, iron, oxygen, sugar.
Reason: All are pure substances — they consist of one type of particle. Carbon dioxide and sugar are compounds; iron and oxygen are elements.
Question 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.
Answer:
| Substance | Classification | Justification |
|---|---|---|
| Iron | Element | Pure substance of iron atoms |
| Moist air | Mixture | Physical combination of gases and vapour |
| Oxygen | Element | Made of only oxygen molecules |
| Iron oxide | Compound | Chemical combination of iron and oxygen |
| Magnesium | Element | Pure metal element |
| Magnesium oxide | Compound | Formed chemically from magnesium and oxygen |
Question 6
Classify the following as elements, compounds, or mixtures: 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.
Answer:
| Elements | Compounds | Mixtures |
|---|---|---|
| Hydrogen Oxygen Nitrogen Aluminium Gold Sulfur |
Water Carbon dioxide Sodium chloride Glucose Rust Magnesium oxide Iron sulfide Baking soda |
Sand Air Seawater Muddy water Fruit juice |
Question 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.
Answer: When a mixture of iron filings and sulfur powder is heated, they combine chemically to form a new substance called iron sulfide (FeS), also known as ferrous sulfide. This reaction is a chemical change, as the new compound formed has properties entirely different from those of the original elements — iron and sulfur.
Question 8
Is it possible for a substance to be classified as both an element and a compound? Explain why or why not.
Answer: No, it is not possible for a substance to be classified as both an element and a compound.
An element is a pure substance made up of only one kind of atom. It cannot be broken down into simpler substances by any chemical means. Examples: Hydrogen (H), Oxygen (O), Iron (Fe).
A compound is a pure substance formed when two or more different elements combine chemically in a fixed ratio. Examples: Water (H&₂O), Carbon dioxide (CO&₂), Sodium chloride (NaCl).
Since the basic composition of an element and a compound is fundamentally different, a single substance cannot belong to both categories at the same time.
Question 9
How would our daily lives be changed if water were not a compound but a mixture of hydrogen and oxygen?
Answer: If water were a mixture of hydrogen and oxygen instead of a compound, our daily lives would be completely different — and life, as we know it, could not exist.
- In a mixture, the components retain their individual properties. So, if hydrogen and oxygen were simply mixed, hydrogen would remain a flammable gas and oxygen would support combustion.
- We would not have a stable, tasteless, colourless liquid like water to drink or use.
- It would be impossible for plants, animals, or humans to survive, as living organisms depend on the chemical properties of water for vital processes like digestion, transport of nutrients, and temperature regulation.
Question 10
Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.
Answer: The chemical reaction inside the test tube is between Hydrochloric Acid (HCl) and Iron filings (Fe). The reaction forms Iron Chloride (FeCl&₂) and the gas evolved is Hydrogen (H&₂).
Gas A = Hydrogen
Word Equation: Hydrochloric Acid + Iron filing → Iron Chloride + Hydrogen
Question 11
Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.
Answer:
1. Carbon Dioxide (CO&₂): Made of Carbon and Oxygen (both non-metals).
- Used in fire extinguishers because it does not support combustion.
- Used in carbonated drinks like soda and soft drinks to make them fizzy.
2. Water (H&₂O): Made of Hydrogen and Oxygen (both non-metals).
- Essential for drinking, cooking, and cleaning in daily life.
- Used in industries for cooling machines and generating steam for power.
Question 12
How can gold be classified as both a mineral and a metal?
Answer: Gold can be classified as both a mineral and a metal because it fits the definitions of both categories.
As a mineral: Gold occurs naturally in the Earth’s crust in a pure form, usually as nuggets or grains in rocks and riverbeds. Any naturally occurring, solid, inorganic substance found in the Earth is called a mineral. Hence, gold is a native mineral.
As a metal: Gold shows typical metallic properties — it is shiny (lustrous), malleable, ductile, and a good conductor of heat and electricity.
