CBSE Class 7 Science Advanced Chemistry Classification of Matter MCQs Set 04

Answer: (b) BDAC

Explanation:
1. Take the mixture and let it stand (B).
2. Heavy sand settles at the bottom (D).
3. Pour off the clear water (A).
4. The sand remains in the beaker (C).
5. The order is BDAC: option (b).

Teacher's Note:
1. This is sedimentation followed by decantation.
2. Pouring off comes before noting what is left behind.

Answer: (b) (i) → (B); (ii) → (C); (iii) → (A); (iv) → (E); (v) → (D)

Explanation:
1. Distillation separates a dissolved solid from its liquid: (i) → (B).
2. Filtration removes an insoluble solid: (ii) → (C).
3. Sublimation needs a sublimable substance: (iii) → (A).
4. Sedimentation works when the solid is heavier than the liquid: (iv) → (E).
5. Magnetic separation needs a magnetic solid: (v) → (D).
6. This is option (b).

Teacher's Note:
1. Match each method with the condition it needs.
2. Match the easiest ones, magnetic and sublimation, first.

Answer: (a) 1

Explanation:
1. Helium is a noble gas that exists as single atoms.
2. So its atomicity is 1.

Teacher's Note:
1. Noble gases are monoatomic.
2. Compare with O₂, which is 2, and O₃, which is 3.

Answer: (b) mostly gases

Explanation:
1. Most non-metals are poor conductors and are neither sonorous nor malleable.
2. Many non-metals exist as gases at room temperature.
3. The answer is mostly gases.

Teacher's Note:
1. Contrast with metals, which are conductors, sonorous and malleable.
2. Remember carbon (graphite) as a special case.

Answer: (d) Phosphorous

Explanation:
1. Iron, magnesium and zinc are metals and conduct electricity.
2. Phosphorus is a non-metal and a bad conductor.
3. The answer is phosphorus.

Teacher's Note:
1. Metals conduct; most non-metals do not.
2. Graphite is the exception.

Answer: (b) decreases

Explanation:
1. The faster molecules escape and heat is taken from the liquid.
2. So the remaining liquid becomes cooler: the temperature decreases.

Teacher's Note:
1. Evaporation causes cooling.
2. This is how an earthen pot keeps water cool.

Answer: (c) The molecules of gases have minimum forces of attraction.

Explanation:
1. Solids have tightly packed molecules with the strongest forces, so (a) and (b) are wrong.
2. Gases are poor conductors of heat, so (d) is wrong.
3. Gas molecules are far apart and have the weakest forces: (c).

Teacher's Note:
1. Forces of attraction: solid greatest, gas least.
2. Gases conduct heat poorly because molecules are far apart.

Answer: (a) Diamond

Explanation:
1. Diamond is the hardest natural substance.
2. Graphite is soft and slippery, and phosphorus and sulphur are soft too.
3. The answer is diamond.

Teacher's Note:
1. Both diamond and graphite are carbon, but their structures differ.
2. Diamond is used for cutting glass.

Answer: (d) Common salt

Explanation:
1. Helium, sodium and iodine are elements, each made of one kind of atom.
2. Common salt (NaCl) contains sodium and chlorine in a fixed ratio, so it is a compound.

Teacher's Note:
1. A compound has two or more elements chemically combined.
2. Salt is a standard example.

Answer: (b) surface, bulk

Explanation:
1. Evaporation takes place only at the surface of a liquid.
2. Boiling takes place throughout the liquid.
3. The answer is surface, bulk.

Teacher's Note:
1. Surface and slow for evaporation, bulk and fast for boiling.
2. Bubbles forming inside show boiling.

Answer: (b) sublimation

Explanation:
1. A solid that turns straight into vapour without becoming liquid undergoes sublimation.
2. Camphor and naphthalene are examples.

Teacher's Note:
1. Sublimation skips the liquid state.
2. The reverse is also called sublimation or deposition.

Answer: (d) Formation of fog and mist in winter mornings.

Explanation:
1. Earthen pots, wet cloth on the forehead and shivering after a swim all use cooling caused by evaporation.
2. Fog and mist form by condensation of water vapour, not by evaporation.
3. So the answer is (d).

Teacher's Note:
1. Fog is tiny water droplets, formed when vapour cools.
2. Evaporation cools; condensation warms the surroundings.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A is true: the elements in a compound are joined chemically and can only be separated by chemical methods.
2. R is true: a compound has the same composition and properties throughout.
3. But being homogeneous does not explain why chemical methods are needed, so R is not the explanation. The answer is (b).

Teacher's Note:
1. Check A and R separately, then ask whether R explains A.
2. The real reason is the chemical bonding between the elements.

Answer: (c) A is true, but R is false.

Explanation:
1. A is true: water and alcohol are miscible and mix completely into one layer.
2. R is false: immiscible liquids such as oil and water form two layers and make a heterogeneous mixture.
3. So A is true and R is false: (c).

Teacher's Note:
1. Words like 'all' or 'always' in a reason should make you look for an exception.
2. Oil and water is the standard exception.

Answer: (c) A is true, but R is false.

Explanation:
1. A is true: steel is an alloy of iron and carbon with a uniform composition.
2. R is false: a solid–solid mixture can be heterogeneous, such as sand and salt.
3. So A is true and R is false: (c).

Teacher's Note:
1. Alloys are the standard homogeneous solid mixtures.
2. A mix of sand and sugar crystals shows the exception.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: water and petrol form two separate layers, so the mixture is heterogeneous.
2. R is true: they do not dissolve in each other.
3. Immiscibility is exactly why two layers form, so R explains A. The answer is (a).

Teacher's Note:
1. Immiscible liquids give heterogeneous mixtures.
2. Miscible liquids give homogeneous mixtures.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: water wets glass and clings to it.
2. R is true: the attraction between water and glass (adhesion) is greater than the attraction among water molecules (cohesion).
3. That is exactly why water sticks, so R explains A: (a).

Teacher's Note:
1. Adhesion is attraction between unlike molecules; cohesion is between like molecules.
2. Mercury does the opposite, as its cohesion is stronger.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: water is a compound of hydrogen and oxygen.
2. R is true: each molecule has 2 H atoms and 1 O atom, a fixed combination of different atoms.
3. That fixed chemical combination makes it a compound, so R explains A: (a).

Teacher's Note:
1. A compound has atoms of different elements joined in a fixed ratio.
2. H₂O has H and O in the ratio 2 : 1.

Answer: (c) A is true, but R is false.

Explanation:
1. A is true: attraction between like molecules is called cohesion.
2. R is false: cohesive force is not always stronger. For water on glass, adhesion is stronger.
3. So A is true and R is false: (c).

Teacher's Note:
1. Beware of the word 'always'.
2. Water on glass versus mercury on glass shows both cases.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: sodium and chlorine lose their own properties in common salt.
2. R is true: a compound forms by chemical combination of atoms.
3. The chemical combination is why properties change, so R explains A: (a).

Teacher's Note:
1. In a mixture the components keep their properties; in a compound they do not.
2. Chemical combination makes a new substance.