Solutions-NCERT-Class-8-Science-Curiosity-Chapter-8-Nature of Matter: Elements, Compounds, and Mixtures-CBSE

Entities that consist of matter are school building, students, trees, metal railing, football, school bag, books, water bottle, lunch box, and clothes of students etc.

Entities that do not consist of matter - light, sound, heat etc.

• Elements combine to form a compound through a chemical reaction, where they join in a fixed ratio and form chemical bonds.
• The new substance formed has properties different from the elements that make it.

Examples:

(i) Hydrogen + Oxygen → Water (H₂O)

• 2 hydrogen atoms combine with 1 oxygen atom to form water, a liquid.

(ii) Sodium + Chlorine → Sodium Chloride (NaCl)

• Sodium (a reactive metal) and chlorine (a poisonous gas) combine to form common salt, a safe edible compound.

If scientists discover a compound that can absorb carbon dioxide (CO₂) from the air, it can help the environment in many ways.

• Reduces air pollution: Too much CO₂ makes the air dirty. The compound would soak up CO₂, making the air cleaner.
• Fights global warming: CO₂ traps heat and causes the Earth to get warmer. Removing CO₂ from the air can slow down climate change.
• Protects plants, animals, and humans: Cleaner air means healthier living conditions for all living things.
• Improves climate control: If used in factories or power plants, it can capture CO₂ before it reaches the atmosphere, reducing greenhouse gases.

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

• 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, C is a compound with a fixed composition.

(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 without any chemical reaction between them. The properties of these individual gases are retained within the air.

Water is a compound, and it behaves very differently from the elements that make it.

• Hydrogen is a gas that burns easily.
• Oxygen is a gas that helps things burn.

But when they combine together, they form water, which is a liquid that actually puts out fire.

This shows that a compound has completely new properties that are not the same as the elements it is made of.

(i) Elements — water, nitrogen, iron, air.

Incorrect

Reason:

• Nitrogen and iron → elements ✔️
• Water → compound ✖️
• Air → mixture ✖️

So all examples are not elements.

(ii) Uniform mixtures — minerals, seawater, bronze, air.

Incorrect

Reason:

• Seawater → uniform mixture ✔️
• Bronze → alloy (uniform mixture) ✔️
• Air → uniform mixture ✔️
• Minerals → mostly compounds, not mixtures ✖️

Because minerals are not mixtures, the list is incorrect.

(iii) Pure substances — carbon dioxide, iron, oxygen, sugar.

Correct

Reason:

All these are pure substances because each has only one type of particle:

• Carbon dioxide → a compound ✔️
• Iron → an element ✔️
• Oxygen → an element ✔️
• Sugar → a compound ✔️

So all examples are correctly matched.

(iv) Non-uniform mixtures — air, sand, brass, muddy water.

Incorrect

Reason:

• Sand → non-uniform mixture ✔️
• Muddy water → non-uniform mixture ✔️
• Air → uniform mixture ✖️
• Brass → alloy (uniform mixture) ✖️

So this list is not correct.

Final Answer: Only option (iii) is correctly matched.

Iron reacts with moist air to form iron oxide.

Iron (Fe) + Oxygen (O₂) + Water (H₂O) → Iron Oxide (Fe₂O₃)

Magnesium burns in oxygen to form magnesium oxide.

Magnesium (Mg) + Oxygen (O2) → Magnesium Oxide (MgO)

Classification of each substance as element, compound, or mixture.

(1) Iron (Fe) → Element

• Reason: It is a pure substance made of only one type of atom (iron atoms).

(2) Moist Air → Mixture (non-uniform or uniform depending on dust presence)

• Reason: Air is a mixture of gases (nitrogen, oxygen, carbon dioxide, etc.). "Moist" air also contains water vapour. No chemical bonding; the gases are just physically mixed.

(3) Iron Oxide (Fe₂O₃ / Fe₃O₄) → Compound

• Reason: Formed by chemical reaction between iron and oxygen. Elements combine in a fixed ratio.

(4) Magnesium (Mg) → Element

• Reason: Pure substance made of only magnesium atoms.

(5) Oxygen (O₂) → Element

• Reason: It contains only oxygen molecules (each molecule has 2 oxygen atoms).

(6) Magnesium Oxide (MgO) → Compound

Reason: Formed by chemical reaction between magnesium and oxygen. Contains Mg and O in a fixed proportion.

Final Classification Table :

Pure Substances: Aluminium, gold, oxygen, nitrogen, sulfur, hydrogen, carbon dioxide, magnesium oxide, iron sulfide, glucose, water, sodium chloride, baking soda.

When a mixture of iron filings and sulfur powder is heated, a new substance called iron sulfide is formed.

Difference from the original mixture :

Word Equation :

Iron + Sulfur → Iron sulfide

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

Elements and compounds are two different, mutually exclusive categories:

Element :

• Made of only one kind of atom.
• Cannot be split into simpler substances by chemical methods.
• Examples: Iron (Fe), Oxygen (O₂), Gold (Au).

Compound :

• Made of two or more different elements chemically combined in a fixed ratio.
• Has properties different from the elements that form it.
• Examples: Water (H₂O), Carbon dioxide (CO₂).

A substance cannot be made of only one type of atom (element), and at the same time two or more different types of atoms chemically combined (compound).

These definitions are opposite, so a single substance can fit only one category.

• If water were a mixture of hydrogen and oxygen instead of a compound, life would not exist.
• A hydrogen–oxygen mixture is highly explosive, so even a spark could cause constant blasts.
• There would be no liquid oceans, rivers, or rain, only gases that cannot support plants, animals, or humans.
• Daily activities like drinking, cooking, and cleaning would be impossible without liquid water.
• The water cycle and climate regulation would collapse, making Earth unstable.
• In short, our planet would be uninhabitable without water as a stable compound.
• Water being a compound (H₂O) is essential for life and stability on Earth.

By analysing the figure, it is found that there will be a chemical reaction inside the test tube between dilute HCl and Fe.

2HCl + Fe → FeCl₂ + H₂ ↑

So the reaction forms Iron Chloride (FeCl₂) and the gas above will be Hydrogen (H₂).

Hydrochloric acid + Iron filing → Iron chloride + Hydrogen (g)

Thus, Gas A = Hydorgen

(1) Carbon dioxide (CO₂) :

Elements: Carbon + Oxygen (both non-metals)

Uses:

• Used in fire extinguishers.
• Used by plants in photosynthesis to make food.

(2) Ammonia (NH₃) :

Elements: Nitrogen + Hydrogen (both non-metals)

Uses:

• Used to make fertilizers.
• Used as a refrigerant in cooling systems.

Gold can be classified as both a mineral and a metal because these two words describe different types of classifications.

(1) Gold as a metal : Gold is a chemical element (symbol: Au).

It has typical metallic properties, such as:

• Shiny appearance
• Good conductor of heat and electricity
• Can be hammered into thin sheets (malleable)
• Can be drawn into wires (ductile)

So, based on physical and chemical properties, gold is a metal.

(2) Gold as a mineral :

A mineral is a naturally occurring, solid substance found in the Earth’s crust, with a definite composition.

• Pure gold occurs naturally in rocks and riverbeds as gold deposits.
• Since it occurs naturally and has a definite chemical composition (only gold atoms), it is classified as a mineral.

So, based on its natural occurrence in Earth, gold is a mineral.

Therefore, gold fits both categories.