Question 1

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

$$
\mathrm {A} + \mathrm {B} \longrightarrow \mathrm {C}
$$

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.

Accepted Answer

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

Explanation: Since A and B cannot be broken down into simpler substances by chemical reactions, they are elements. When they combine chemically, they form a compound C which has a fixed composition. Compounds have properties different from their constituent elements and a fixed ratio of elements. — Step-by-step:
1. A and B cannot be broken down further, so they are elements.
2. When elements combine chemically, they form compounds.
3. Compounds have fixed composition and properties different from elements.
4. Therefore, C is a compound with fixed composition formed from elements A and B.

Question 2

Assertion: Air is a mixture.

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

Choose the correct option:
(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.

Accepted Answer

Option (i) is correct.

Explanation: Air is a mixture because it consists of several gases mixed physically without any chemical change. The reason correctly explains the assertion that a mixture is formed when substances are mixed without chemical change. — Step-by-step:
1. Air contains gases like nitrogen, oxygen, carbon dioxide physically mixed.
2. No chemical reaction occurs between these gases in air.
3. Hence, air is a mixture.
4. The reason correctly states the definition of a mixture.
5. Therefore, both assertion and reason are true and reason explains 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.

Accepted Answer

Water is a compound formed by chemical combination of hydrogen and oxygen in a fixed ratio (2:1). The properties of water are very different from those of hydrogen (a flammable gas) and oxygen (supports combustion). Water is a liquid at room temperature, does not support combustion, and has unique properties like high boiling point and surface tension. This difference arises because compounds have properties different from their constituent elements due to chemical bonding and new molecular structure. — Step-by-step:
1. Hydrogen and oxygen are elements with distinct properties.
2. When chemically combined, they form water (H2O), a compound.
3. Chemical bonding changes the properties, resulting in water's unique characteristics.
4. Hence, water's properties differ from hydrogen and oxygen.

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.

Accepted Answer

Correct matches:
(ii) Uniform mixtures—seawater, bronze, air are correctly matched because these are homogeneous mixtures.
(iii) Pure substances—carbon dioxide, iron, oxygen, sugar are correctly matched as these are pure substances (elements or compounds).

Incorrect matches:
(i) Elements — water and air are not elements; water is a compound and air is a mixture.
(iv) Non-uniform mixtures — air and brass are not non-uniform mixtures; air and brass are uniform mixtures.

Explanation:
- Elements are substances that cannot be broken down further (e.g., nitrogen, iron).
- Uniform mixtures have uniform composition throughout (e.g., seawater, bronze).
- Pure substances have fixed composition (elements or compounds).
- Non-uniform mixtures have visibly different components (e.g., sand, muddy water). — Step-by-step:
1. Identify each example's nature.
2. Water is a compound, not an element.
3. Air is a mixture, not an element or non-uniform mixture.
4. Minerals are usually compounds, not uniform mixtures.
5. Brass is a uniform mixture (alloy), not non-uniform.
6. Sand and muddy water are non-uniform mixtures.
7. Hence, only (ii) and (iii) are correctly matched.

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.

Accepted Answer

Substances involved:
- Iron (Fe): Element
- Moist air: Mixture (contains gases and water vapor)
- Iron oxide (Fe2O3 or Fe3O4): Compound
- Magnesium (Mg): Element
- Oxygen (O2): Element
- Magnesium oxide (MgO): Compound

Justification:
Elements are pure substances that cannot be broken down further. Iron, magnesium, and oxygen are elements. Moist air is a mixture of gases and water vapor. Iron oxide and magnesium oxide are compounds formed by chemical combination of elements in fixed ratios. — Step-by-step:
1. Identify each substance's nature.
2. Iron and magnesium are elements.
3. Moist air is a mixture of gases and water vapor.
4. Oxygen is an element.
5. Iron oxide and magnesium oxide are compounds formed by chemical reactions.
6. Hence, classification is as above.

Question 6

Classify the following as elements, compounds, or mixtures in Table 8.3.

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.

|  Elements | Compounds | Mixtures  |
| --- | --- | --- |
|  |   |   |

Identify pure substances amongst these and list them below.

## Pure substances

Accepted Answer

Classification:

Elements: Aluminium, gold, oxygen, nitrogen, sulfur, hydrogen
Compounds: Carbon dioxide, magnesium oxide, rust (iron oxide), iron sulfide, glucose, water, sodium chloride, baking soda
Mixtures: Sand, seawater, muddy water, air, fruit juice

Pure substances are elements and compounds. Hence, pure substances are aluminium, gold, oxygen, nitrogen, sulfur, hydrogen, carbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride, baking soda. — Step-by-step:
1. Elements are pure substances that cannot be broken down.
2. Compounds are pure substances formed by chemical combination.
3. Mixtures contain two or more substances physically combined.
4. Classify each substance accordingly.
5. List pure substances as all elements and compounds identified.

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.

Accepted Answer

When a mixture of iron filings and sulfur powder is heated, a new substance called iron sulfide is formed. This is a compound formed by chemical combination of iron and sulfur. It has different properties from the original mixture, which was a physical combination of iron and sulfur particles.

Word equation:
Iron + Sulfur → Iron sulfide — Step-by-step:
1. Initially, iron filings and sulfur powder form a mixture.
2. On heating, they chemically react to form iron sulfide.
3. Iron sulfide is a compound with properties different from iron or sulfur.
4. The reaction is represented by the word equation above.

Question 8

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

Accepted Answer

No, a substance cannot be classified as both an element and a compound. An element is a pure substance that cannot be broken down into simpler substances by chemical means. A compound is a pure substance formed by chemical combination of two or more elements in a fixed ratio. Since their definitions are mutually exclusive, a substance cannot be both. — Step-by-step:
1. Elements are simplest substances, cannot be broken down.
2. Compounds are made of elements chemically combined.
3. A substance cannot simultaneously be simplest and chemically combined.
4. Therefore, no substance can be both element and compound.

Question 9

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

Accepted Answer

If water were a mixture of hydrogen and oxygen instead of a compound, it would not have the properties of water. Hydrogen and oxygen gases are highly flammable and do not support life as water does. Water's unique properties like being a liquid at room temperature, its solvent abilities, and role in biological processes would be lost. Daily life would be severely affected as water is essential for drinking, cooking, cleaning, and sustaining life. — Step-by-step:
1. Water as a compound has unique properties.
2. As a mixture, hydrogen and oxygen would remain gases.
3. No liquid water would be present.
4. Life and daily activities dependent on water would be disrupted.
5. Hence, water being a compound is vital for life.

Question 10

Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.

Accepted Answer

Gas A is hydrogen gas (H2).

The chemical reaction is between zinc and dilute hydrochloric acid producing hydrogen gas and zinc chloride.

Word equation:
Zinc + Hydrochloric acid → Zinc chloride + Hydrogen — Step-by-step:
1. Zinc reacts with dilute hydrochloric acid.
2. Hydrogen gas is released (Gas A).
3. Zinc chloride is formed as a compound.
4. The reaction is represented by the word equation above.

Question 11

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

Accepted Answer

Two compounds made only from non-metals:

1. Water (H2O)
Uses:
- Essential for drinking and life processes.
- Used in cooking, cleaning, and agriculture.

2. Carbon dioxide (CO2)
Uses:
- Used in fire extinguishers.
- Used in carbonated drinks. — Step-by-step:
1. Identify compounds made only of non-metals.
2. List their common uses.
3. Water and carbon dioxide are common examples with multiple uses.

Question 12

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

Accepted Answer

Gold is classified as a mineral because it is a naturally occurring solid substance found in the Earth with a fixed chemical composition. It is also classified as a metal because it has metallic properties such as malleability, ductility, conductivity, and luster. Thus, gold fits both categories. — Step-by-step:
1. Minerals are natural solid substances with fixed composition.
2. Gold occurs naturally in the Earth as a solid.
3. Metals have characteristic physical properties.
4. Gold exhibits metallic properties.
5. Therefore, gold is both a mineral and a metal.

Question 13

Which of the following best defines a mixture in scientific terms?

Accepted Answer

A physical combination of two or more substances where each retains its own properties — A mixture is formed when two or more substances are physically combined without chemical reaction, and each substance retains its own properties. This distinguishes mixtures from compounds, which are chemically combined substances.

Question 14

Identify the type of mixture formed when sugar is dissolved in water.

Accepted Answer

Uniform mixture — When sugar dissolves in water, the sugar particles are evenly distributed and cannot be seen separately, forming a uniform (homogeneous) mixture.

Question 15

Which of the following is an example of a non-uniform mixture?

Accepted Answer

Sprout salad — A sprout salad is a non-uniform (heterogeneous) mixture because its components like green gram, chickpeas, onion, and tomato can be seen separately.

Elements, Compounds, and Mixtures

Elements, Compounds, and Mixtures — Study Notes

Nature of Matter: Elements, Compounds, and Mixtures

8.1 What Are Mixtures?

8.1.1 Is air a mixture?

Practice Questions — Elements, Compounds, and Mixtures

All 13 Chapters in Curiosity