Nature of Matter: Elements, Compounds, and Mixtures — NCERT Solutions

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):

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.