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.

Given: A mixture of iron filings and sulfur powder is heated.

New Substance Formed: When the mixture of iron filings and sulfur powder is heated, a new compound called iron sulfide (a black mass) is formed.

Word Equation:
$$\text{Iron} + \text{Sulfur} \xrightarrow{\text{Heat}} \text{Iron sulfide}$$

Differences between the mixture (Sample A) and iron sulfide (Sample B):

| Property | Mixture of Iron and Sulfur (Sample A) | Iron Sulfide (Sample B) |
|---|---|---|
| Appearance/Colour | Yellowish-grey (yellow of sulfur + grey of iron visible) | Black/dark coloured mass |
| Texture | Grainy; individual particles of iron and sulfur can be seen | Uniform black mass; individual components not visible |
| Magnet test | Iron filings are attracted to the magnet and can be separated | Not attracted to the magnet; iron cannot be separated |
| Reaction with dilute HCl | Produces hydrogen gas (colourless, odourless) which burns with a 'pop' sound | Produces hydrogen sulfide gas (rotten egg smell); does NOT burn |
| Nature | A mixture — components retain their individual properties | A compound — has entirely new properties different from iron and sulfur |
| Composition | Variable (iron and sulfur can be mixed in any ratio) | Fixed ratio (iron and sulfur combined in a fixed ratio of 7:4 by mass) |
| Separation | Can be separated by physical methods (e.g., using a magnet) | Cannot be separated by physical methods |

Conclusion: Iron sulfide is a compound with properties entirely different from those of iron and sulfur, whereas the mixture retains the properties of both iron and sulfur.

Answer: No, it is NOT possible for a substance to be classified as both an element and a compound.

Explanation:

Element:
- An element is a pure substance made up of only one type of atom.
- It cannot be broken down into simpler substances by any chemical reaction.
- Examples: Iron (Fe), Oxygen (O₂), Gold (Au), Sulfur (S).

Compound:
- A compound is a pure substance formed when two or more different elements combine chemically in a fixed ratio.
- It can be broken down into its constituent elements by chemical methods.
- Examples: Water (H₂O), Carbon dioxide (CO₂), Iron sulfide (FeS).

Why they cannot be the same:
- By definition, an element contains only one kind of atom, while a compound contains two or more different kinds of atoms chemically combined.
- These are mutually exclusive categories — a substance either consists of one type of atom (element) or more than one type of atom chemically combined (compound). It cannot be both simultaneously.
- For example, water (H₂O) is a compound because it contains hydrogen and oxygen atoms combined chemically. It cannot be an element because it is made of two different types of atoms.

Conclusion: A substance can be classified as either an element or a compound, but never both, because the defining characteristics of elements and compounds are fundamentally different and mutually exclusive.

If water were a mixture of hydrogen and oxygen instead of a compound, our daily lives would change drastically in the following ways:

1. State of matter: A mixture of hydrogen and oxygen at room temperature would be a gaseous mixture, not a liquid. We would not have liquid water to drink, cook, or bathe with.

2. No fixed composition: As a mixture, hydrogen and oxygen could be present in any proportion. The properties of 'water' would vary from sample to sample, making it unreliable for any use.

3. Highly flammable and explosive: Hydrogen is a highly flammable gas and oxygen supports combustion. A mixture of the two would be extremely explosive (like an oxyhydrogen flame). It would be dangerous to have around.

4. No solvent properties: Liquid water is called the 'universal solvent' because of its unique chemical properties as a compound. A gaseous mixture would not dissolve salts, minerals, or other substances, making it useless as a solvent in biological and chemical processes.

5. No support for life: All living organisms depend on liquid water for their biochemical reactions. Without liquid water as a stable compound, life as we know it would not exist.

6. Easy separation: Since it would be a mixture, hydrogen and oxygen could be separated easily by physical methods. This means the 'water' could break apart without any chemical reaction, making it unstable.

7. No water cycle: The water cycle (evaporation, condensation, precipitation) depends on water being a liquid compound with specific boiling and melting points. A mixture would not have these fixed properties.

Conclusion: If water were a mixture instead of a compound, it would be a dangerous, gaseous, and unstable combination that could not support life or any of the functions water performs in our daily lives. Life on Earth would be impossible.

Assumption: Fig. 8.24 depicts the electrolysis of water (as described in Activity 8.3 of the chapter), where electricity is passed through water (with a few drops of dilute sulfuric acid) and gases are collected at the two electrodes. The gas collected in larger volume at one electrode is Gas A.

Identification of Gas A:

During electrolysis of water:
- Hydrogen gas is produced at the negative electrode (cathode) in double the volume compared to oxygen.
- Oxygen gas is produced at the positive electrode (anode).

Since the chapter context (Activity 8.3) mentions testing the gas with a burning splinter — oxygen relights a glowing splinter, and hydrogen burns with a 'pop' sound — and the larger volume of gas collected is hydrogen:

Gas A is Hydrogen (H₂).

*(If the figure shows the gas that relights a glowing splinter, then Gas A would be Oxygen. Based on the standard depiction where the larger volume gas is identified, Gas A = Hydrogen.)*

Word Equation:
$$\text{Water} \xrightarrow{\text{Electricity}} \text{Hydrogen} + \text{Oxygen}$$

$$\text{Water} \xrightarrow{\text{Electrolysis}} \text{Hydrogen gas} + \text{Oxygen gas}$$

Significance: This reaction shows that water is a compound made of hydrogen and oxygen, which can be separated only by a chemical method (electrolysis), not by any physical method. The ratio of hydrogen to oxygen produced is 2:1 by volume, confirming the fixed composition of water (H₂O).

Two compounds made only from non-metals:

1. Water (H₂O)
- Constituent non-metals: Hydrogen (H) and Oxygen (O)
- Uses:
- (i) Essential for drinking and sustaining all forms of life — all metabolic and biochemical reactions in living organisms take place in the presence of water.
- (ii) Used as a universal solvent in industries, laboratories, and for cooking; also used for irrigation in agriculture.

2. Carbon dioxide (CO₂)
- Constituent non-metals: Carbon (C) and Oxygen (O)
- Uses:
- (i) Used in fire extinguishers to put out fires, as it does not support combustion and is heavier than air, smothering the flame.
- (ii) Used in the food and beverage industry — it is dissolved under pressure in soft drinks and carbonated beverages to make them fizzy; also used as dry ice (solid CO₂) for refrigeration and preservation of food.

(Bonus example if needed): Sugar / Glucose (C₆H₁₂O₆) — made of carbon, hydrogen, and oxygen (all non-metals).
- Uses: (i) Primary source of energy for living organisms. (ii) Used in the food industry as a sweetener and in making confectionery.

Gold as a Mineral:
- A mineral is a naturally occurring, inorganic solid substance with a definite chemical composition and a crystalline structure.
- Gold occurs naturally in the Earth's crust in its pure elemental form (as nuggets or veins in rocks). It is found in nature without needing to be chemically extracted from a compound.
- Since gold is naturally occurring, has a definite composition (pure Au), and is found in solid crystalline form in nature, it satisfies all the criteria of a mineral.
- Therefore, gold is classified as a native element mineral.

Gold as a Metal:
- A metal is an element that is typically shiny (lustrous), malleable (can be beaten into sheets), ductile (can be drawn into wires), a good conductor of heat and electricity, and has a high melting point.
- Gold (Au) is an element with all the characteristic properties of metals:
- It is highly lustrous (shiny yellow appearance).
- It is extremely malleable and ductile (can be beaten into very thin sheets called gold leaf and drawn into fine wires).
- It is a good conductor of electricity (used in electronic components).
- It does not tarnish or corrode easily (highly unreactive/noble metal).
- Therefore, gold is classified as a metal.

Conclusion: Gold can be classified as both a mineral and a metal because:
- It is a mineral by virtue of being a naturally occurring, inorganic, crystalline solid with a definite composition found in the Earth's crust.
- It is a metal by virtue of being an element with characteristic metallic properties such as lustre, malleability, ductility, and electrical conductivity.
These two classifications are not mutually exclusive — gold is a naturally occurring metallic element, making it both a mineral and a metal simultaneously.