Nature of Matter: Elements, Compounds, and Mixtures

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

Justification:
- Since A and B cannot be broken down into simpler substances by chemical reactions, they are elements (elements are pure substances that cannot be split into simpler substances by any chemical method).
- When two or more elements combine chemically in a fixed ratio, they form a compound. Here, A and B combine to form C, so C is a compound.
- Compounds always have a fixed composition (fixed ratio of constituent elements).
- Options (i), (ii), and (iii) are incorrect because A and B are elements, not compounds, and both elements and the compound C have fixed compositions. However, the most complete and accurate statement is option (iv).

Correct Option: (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.

Justification:
- Assertion is true: Air is indeed a mixture. It is a uniform (homogeneous) mixture of gases such as nitrogen (~78%), oxygen (~21%), argon, carbon dioxide, water vapour, etc. These components can be separated by physical methods and are present in variable proportions.
- Reason is true: A mixture is defined as a combination of two or more substances that are mixed together without any chemical change taking place. The components retain their individual properties.
- The Reason correctly explains the Assertion because air is a mixture precisely because its constituent gases (nitrogen, oxygen, etc.) are mixed together physically without any chemical reaction occurring between them.

Given: Water is a compound formed from the elements hydrogen and oxygen.

Concept: When elements combine chemically to form a compound, the compound has entirely new properties that are different from those of its constituent elements.

Justification:

| Property | Hydrogen (H) | Oxygen (O) | Water (H₂O) |
|---|---|---|---|
| State at room temperature | Gas | Gas | Liquid |
| Role in combustion | Highly flammable (burns) | Supports combustion | Does NOT burn; used to extinguish fire |
| Taste/Smell | Odourless, tasteless | Odourless, tasteless | Odourless, tasteless but essential for life |
| Ability to sustain life | Cannot be breathed directly to sustain life | Needed for respiration | Essential for all life processes |

- Hydrogen is a highly flammable gas that burns readily.
- Oxygen is a gas that supports combustion (helps things burn).
- Water, however, is a liquid that neither burns nor supports combustion — in fact, it is used to put out fires.

This clearly shows that water has properties completely different from those of hydrogen and oxygen. This is because in water, hydrogen and oxygen are chemically combined in a fixed ratio of 2:1 (H:O), forming an entirely new substance with new properties.

Conclusion: The properties of a compound are different from those of its constituent elements, which justifies the given statement.

Correct Answer: Option (iii) — Pure substances: carbon dioxide, iron, oxygen, sugar.

Reasoning for each option:

(i) Elements — water, nitrogen, iron, air → INCORRECT
- Water is a compound (made of hydrogen and oxygen in a 2:1 ratio), not an element.
- Air is a mixture of gases, not an element.
- Nitrogen and iron are correctly classified as elements.
- Since water and air are wrongly placed, this option is incorrect.

(ii) Uniform mixtures — minerals, seawater, bronze, air → INCORRECT
- Minerals are naturally occurring pure substances (or compounds), not mixtures.
- Seawater (salt dissolved in water), bronze (copper + tin), and air are correctly uniform (homogeneous) mixtures.
- Since minerals are wrongly placed, this option is incorrect.

(iii) Pure substances — carbon dioxide, iron, oxygen, sugar → CORRECT
- Carbon dioxide is a compound (C + O in fixed ratio) — pure substance. ✓
- Iron is an element — pure substance. ✓
- Oxygen is an element — pure substance. ✓
- Sugar is a compound (carbon, hydrogen, oxygen in fixed ratio) — pure substance. ✓
- All four are pure substances (either elements or compounds) that cannot be separated by physical methods. This option is correctly matched.

(iv) Non-uniform mixtures — air, sand, brass, muddy water → INCORRECT
- Air is a uniform (homogeneous) mixture, not a non-uniform mixture.
- Brass is an alloy (uniform/homogeneous mixture of copper and zinc), not a non-uniform mixture.
- Sand and muddy water are correctly non-uniform (heterogeneous) mixtures.
- Since air and brass are wrongly placed, this option is incorrect.

Therefore, option (iii) is the only correctly matched case.

Reactions involved:
1. Iron + Moist air → Iron oxide
2. Magnesium + Oxygen → Magnesium oxide

Classification of all substances:

| Substance | Classification | Justification |
|---|---|---|
| Iron (Fe) | Element | Iron is a pure substance made of only one type of atom (iron atoms). It cannot be broken down into simpler substances by chemical means. |
| Moist air | Mixture | Air is a uniform mixture of nitrogen, oxygen, argon, carbon dioxide, and water vapour. It is a mixture because its components are mixed physically without any chemical change and can be separated by physical methods. |
| Iron oxide (Fe₂O₃ / rust) | Compound | Iron oxide is formed when iron and oxygen (from moist air) combine chemically in a fixed ratio. It has properties different from iron and oxygen. It cannot be separated into its elements by physical methods. |
| Magnesium (Mg) | Element | Magnesium is a pure substance consisting of only one type of atom (magnesium atoms). It cannot be broken down further by chemical reactions. |
| Oxygen (O₂) | Element | Oxygen is a pure substance made of only one type of atom. It exists as a diatomic molecule but is still classified as an element. |
| Magnesium oxide (MgO) | Compound | Magnesium oxide is formed when magnesium and oxygen combine chemically in a fixed ratio (1:1). Its properties are different from those of magnesium and oxygen. It cannot be separated by physical methods. |

Word equations:
$$\text{Iron} + \text{Moist air (oxygen + water vapour)} \rightarrow \text{Iron oxide}$$
$$\text{Magnesium} + \text{Oxygen} \rightarrow \text{Magnesium oxide}$$

Classification Table:

| Elements | Compounds | Mixtures |
|---|---|---|
| Aluminium | Carbon dioxide | Sand |
| Gold | Magnesium oxide | Seawater |
| Oxygen | Rust (iron oxide) | Muddy water |
| Nitrogen | Iron sulfide | Air |
| Sulfur | Glucose | Fruit juice |
| Hydrogen | Water | |
| | Sodium chloride | |
| | Baking soda | |

Reasoning:
- Elements: Aluminium, gold, oxygen, nitrogen, sulfur, hydrogen — each made of only one type of atom; cannot be broken down further by chemical means.
- Compounds: Carbon dioxide (C + O in fixed ratio), magnesium oxide (Mg + O), rust/iron oxide (Fe + O), iron sulfide (Fe + S), glucose (C + H + O), water (H + O), sodium chloride (Na + Cl), baking soda (sodium hydrogen carbonate — Na, H, C, O in fixed ratio) — all formed by chemical combination of elements in fixed ratios.
- Mixtures: Sand (silicon dioxide + other minerals, non-uniform), seawater (water + dissolved salts, uniform), muddy water (water + mud, non-uniform), air (gases mixed physically, uniform), fruit juice (water + sugars + vitamins + other substances, non-uniform).

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Pure Substances (substances that cannot be separated into other kinds of matter by any physical process — includes both elements and compounds):

Aluminium, Gold, Oxygen, Nitrogen, Sulfur, Hydrogen, Carbon dioxide, Magnesium oxide, Rust (iron oxide), Iron sulfide, Glucose, Water, Sodium chloride, Baking soda.