NCERT Solutions: Chapter8 Nature of Matter: Elements, Compounds, and Mixtures (class 8 Science)

Entities that consist of matter: School buildings, students, trees, metal railings, footballs, school bags, books, water bottles, lunch boxes, and the clothes of students, etc. (All these possess mass and occupy physical space).

Entities that do not consist of matter: Light, sound, heat, etc. (These are forms of energy that do not have mass or occupy physical volume).

A compound is a unique substance that forms when two or more elements combine chemically. Compounds form as a result of chemical reactions. The elements in compounds are held together by chemical bonds in fixed definite proportions by mass.

A compound that effectively absorbs carbon dioxide (CO₂) from the air could significantly help mitigate climate change and other environmental challenges. Removing CO₂, a major greenhouse gas, from the atmosphere could reduce global warming, ocean acidification, and other harmful effects associated with excess atmospheric CO₂.

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

Explanation: The question states that A and B cannot be broken down into simpler substances by chemical reactions. This means A and B are elements, because elements are pure substances made of only one kind of atom and cannot be broken down chemically. When A and B combine chemically to form C, the result is a compound. A compound is formed when two or more elements combine in a fixed ratio through a chemical reaction. Therefore, A and B are elements, and C is a compound with a fixed composition.

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

Explanation: Air is indeed a mixture because it is composed of various gases like nitrogen, oxygen, and carbon dioxide, which are mixed together physically without any chemical reaction taking place between them. The individual component gases retain their distinct chemical properties within the air.

Water has properties that are completely different from those of its constituent elements, hydrogen and oxygen:

• Physical State: Water is a liquid at room temperature, while both hydrogen and oxygen are colourless gases.
• Combustion Behaviour: Hydrogen is highly inflammable (burns with a pop sound) and oxygen is a supporter of combustion. In contrast, water does not burn and is widely used as a fire extinguisher.

This dramatic difference arises because when a compound is formed, its constituent elements undergo a chemical reaction where old bonds break and new chemical bonds form, creating a completely new substance with distinct molecular properties.

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

Reason: A pure substance is composed of only one type of constituent particle and has a fixed chemical composition throughout:

• Iron and Oxygen are pure elements.
• Carbon dioxide (CO₂) and Sugar (C₁₂H₂₂O₁₁) are pure chemical compounds.

All four are authentic pure substances. (In options i, ii, and iv, mixtures like air and compounds like water are incorrectly grouped under elements or non-uniform mixtures).

Chemical Reactions:
1. Iron (Fe) + Oxygen (O₂) + Water (H₂O) → Iron Oxide (Fe₂O₃)
2. Magnesium (Mg) + Oxygen (O₂) → Magnesium Oxide (MgO)

When iron filings and sulfur powder are heated, they undergo a chemical reaction to form a new substance called Iron Sulfide (FeS).

• Original Physical Mixture: Iron filings and yellow sulfur powder are simply mixed physically without chemical bonding. Each retains its distinct identity: iron filings can be separated using a magnet, and sulfur can dissolve in carbon disulfide.
• New Chemical Compound (Iron Sulfide): Heating triggers an exothermic chemical reaction where iron and sulfur atoms form strong chemical bonds.
• Difference in Properties: Iron sulfide is a black, brittle solid compound. It is non-magnetic (cannot be attracted by a bar magnet), and its components cannot be separated by physical methods.

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

No, a substance cannot be classified as both an element and a compound simultaneously.

Explanation: By definition, an element is a pure substance composed of only one single type of atom that cannot be broken down into simpler substances by chemical reactions. In contrast, a compound is formed when two or more different elements chemically combine in a fixed ratio. Because a substance cannot simultaneously contain only one type of atom and multiple different types of chemically bonded atoms, the two classifications are mutually exclusive.

Water's vital role in nature depends entirely on it being a stable chemical compound with unique liquid properties. If it were merely a physical mixture of hydrogen and oxygen gases, life as we know it would be impossible:

• No Liquid Drinking Water: At room temperature, hydrogen and oxygen are gases; there would be no liquid water to drink, resulting in immediate dehydration of all living beings.
• Extreme Explosive Hazard: A physical mixture of hydrogen and oxygen gas is highly explosive; the smallest spark or flame would cause violent detonations.
• Collapse of Agriculture & Flora: Plants would have no liquid moisture to absorb from soil, destroying all agriculture.
• Extinction of Aquatic Life: Oceans, rivers, and lakes would vanish into atmospheric gases, killing all marine life.

By analysing the experimental setup in Fig. 8.24, iron filings react with dilute hydrochloric acid inside the test tube:

Chemical Equation:
$$\text{Fe (s)} + 2\text{HCl (aq)} \rightarrow \text{FeCl}_2\text{ (aq)} + \text{H}_2\text{ (g)}\uparrow$$

Identification: The gas liberated in the upper test tube, Gas A, is Hydrogen gas (H₂).

Word Equation:
$$\text{Hydrochloric Acid} + \text{Iron filings} \rightarrow \text{Iron Chloride} + \text{Hydrogen gas}$$

1. Carbon Dioxide (CO₂):
• Composed of Carbon and Oxygen (both non-metals).
• Use 1: Used in fire extinguishers to extinguish fires by displacing oxygen.
• Use 2: Used by green plants as an essential reactant during photosynthesis to synthesize glucose.

2. Sulfur Dioxide (SO₂):
• Composed of Sulfur and Oxygen (both non-metals).
• Use 1: Widely used as a food preservative in dried fruits, juices, and wines to inhibit microbial growth.
• Use 2: Used as an essential intermediate in the industrial manufacture of sulfuric acid (H₂SO₄).

Gold as a Mineral: A mineral is defined as a naturally occurring, inorganic solid with a definite chemical composition and crystalline structure. Gold occurs naturally in the Earth's crust in its uncombined native elemental state embedded in quartz veins or alluvial river deposits, formed by natural geological processes. Hence, it is a native mineral.

Gold as a Metal: After extraction and purification, gold exhibits all classic metallic physical and chemical properties:

• High metallic lustre (shiny yellow sheen).
• Exceptional malleability (can be beaten into ultra-thin sheets).
• High ductility (can be drawn into microscopic wires).
• Excellent electrical and thermal conductivity.
• Widely used in jewellery, electronic contacts, and international monetary reserves.

Three-Panel Comic Strip Narrative:

1. Panel 1 (Element): Character displays a pure gold wedding ring (Au). Dialogue: "I am made of only one type of gold atom! You cannot break me down by any chemical means. I am an Element used in jewellery and electronic circuits."
2. Panel 2 (Compound): Character offers a glass of pure water (H₂O). Dialogue: "Hydrogen and oxygen combined chemically in a strict 2:1 ratio to make me. I have entirely new properties and can only be separated by electrolysis. I am a Compound vital for all life!"
3. Panel 3 (Mixture): Character prepares a vibrant fruit salad. Dialogue: "Apples, bananas, and grapes tossed together physically! Each keeps its original taste and sweetness, and you can separate us easily with a fork. We are a Mixture!"

• Laundry Detergent: A complex mixture consisting of chemical compounds such as Sodium Carbonate (Na₂CO₃), Sodium Percarbonate, surfactants, linear alkylbenzene sulfonates, synthetic perfumes, and dyes.
• Commercial Toothpaste: A mixture of abrasives like Calcium Carbonate (CaCO₃), Sodium Monofluorophosphate (fluoride compound), water, Sorbitol, and flavouring agents.
• Potato Chips Packet: A mixture of dehydrated potatoes, edible vegetable oil, and seasoning salts such as Sodium Chloride (NaCl) and monosodium glutamate.
• Carbonated Soft Drink: A liquid mixture composed of Water (compound H₂O), high-fructose corn syrup/sugar (compound C₁₂H₂₂O₁₁), dissolved Carbon Dioxide gas (compound CO₂), and phosphoric acid.

Team 1 — Elements:
"We are the fundamental building blocks of the entire universe! Without elements, no matter could ever exist. Oxygen sustains all cellular respiration, iron carries life-giving blood throughout our bodies and constructs cities, and silicon drives modern computing. All compounds and mixtures are merely combinations of us. Hence, elements are paramount!"

Team 2 — Compounds:
"Elements on their own can be toxic and unusable; pure sodium explodes in water, and chlorine is a lethal gas, yet combined chemically they become harmless table salt (NaCl)! Water (H₂O) covers 70% of the Earth and sustains all life. Compounds form life-saving antibiotics, proteins, and DNA. Without compounds, life is chemically impossible. Hence, compounds are most important!"

Team 3 — Mixtures:
"Real life does not exist in isolated, purified test tubes! The atmosphere we breathe is a mixture of nitrogen and oxygen; the blood circulating in our veins is a complex mixture; the fertile soil sustaining agriculture is a mixture. Modern engineering relies on metal mixtures (alloys like steel and brass). Mixtures make daily life possible and functional. Hence, mixtures are the most important!"