CBSE Class 7 Science Advanced Chemistry Classification of Matter MCQs Set 03

Answer: (d) LPG

Explanation:
1. Camphor, odonil and table salt are solids, so their molecules are packed closely and tightly.
2. LPG is kept as a liquid, and in a liquid or gas the molecules are loosely packed.
3. So the odd one out is LPG.

Teacher's Note:
1. Group the substances by physical state first.
2. Closely packed means solid; loosely packed means liquid or gas.

Answer: (a) Water kept in freezer.

Explanation:
1. Molecules that only vibrate to and fro about fixed positions belong to a solid.
2. Water in a freezer turns into ice, which is a solid.
3. Heated water, perfume vapour and melting ice at room temperature all have molecules that can move about, so the answer is (a).

Teacher's Note:
1. Vibration about a fixed position is the only motion allowed in a solid.
2. Ice at room temperature melts, so it is not a good example.

Answer: (c) Z > Y > X

Explanation:
1. X is incompressible, so it is a solid. Y diffuses only into some substances, so it is a liquid. Z has no fixed volume or shape, so it is a gas.
2. Thermal expansion is least in solids, more in liquids and greatest in gases.
3. So the order is Z > Y > X, which is option (c).

Teacher's Note:
1. Identify X, Y, Z as solid, liquid and gas before comparing.
2. Gases expand the most on heating because their molecules are farthest apart.

Answer: (a) X undergoes evaporation rapidly but Y undergoes evaporation slowly.

Explanation:
1. A liquid with a lower boiling point is more volatile.
2. X boils at 40°C, which is lower than 90°C for Y, so X evaporates faster at room temperature.
3. Neither liquid boils at room temperature, so (d) is wrong. The answer is (a).

Teacher's Note:
1. Low boiling point goes with high volatility.
2. Do not confuse evaporation, which happens at any temperature, with boiling.

Answer: (d) Ozone

Explanation:
1. Hydrogen (H₂), nitrogen (N₂) and chlorine (Cl₂) each have 2 atoms in a molecule, so their atomicity is 2.
2. Ozone is O₃ and has atomicity 3.
3. So ozone is the odd one.

Teacher's Note:
1. Atomicity means the number of atoms in one molecule of an element.
2. Remember ozone as the 3-atom form of oxygen.

Answer: (a) Lithium and sodium

Explanation:
1. A substance floats on water if its density is less than that of water.
2. Lithium and sodium both have densities lower than water, so both float.
3. Calcium and magnesium are denser than water and sink, so the answer is (a).

Teacher's Note:
1. Alkali metals such as Li, Na and K are light enough to float on water.
2. Calcium and magnesium sink.

Answer: (d) (i) → (C); (ii) → (A); (iii) → (D); (iv) → (B)

Explanation:
1. Graphite is a good conductor of electricity: (i) → (C).
2. Carbon fibre has high tensile strength: (ii) → (A).
3. Diamond has high density: (iii) → (D).
4. Bromine is a liquid at room temperature: (iv) → (B).
5. These pairs match option (d).

Teacher's Note:
1. Match the sure pair first: bromine is the liquid non-metal.
2. Graphite is the one non-metal that conducts electricity.

Answer: (c) DCBA

Explanation:
1. Evaporation is faster at a higher temperature and with a larger surface area.
2. C and D are at 50°C, and A and B are at 25°C. D has a wider surface than C, and B has a wider surface than A.
3. The order of rate is D > C > B > A, taking the 25°C pair as the slower pair. This is DCBA, option (c).

Teacher's Note:
1. Rank first by temperature, then by surface area within each temperature.
2. Here the 25°C pair is taken as slower than the 50°C pair.

Answer: (a) CABD

Explanation:
1. Go from the broadest group to the narrowest.
2. Pure substances (C) include elements (A), elements include metals (B), and metals include the monoatomic metals (D).
3. The order is C, A, B, D, which is option (a).

Teacher's Note:
1. Picture the groups as boxes inside boxes.
2. Pure substance, element, metal, then the particular type of metal.

Answer: (c) Saw dust and water

Explanation:
1. Water and alcohol are miscible and need distillation, and sugar in water is a solution that needs evaporation or distillation.
2. Sand settles, so it can also be separated by sedimentation and decantation.
3. Saw dust floats and is insoluble, so only filtration works. The answer is (c).

Teacher's Note:
1. Ask whether the solid sinks, floats or dissolves.
2. A floating insoluble solid cannot be decanted, so filtration is the method.

Answer: (c) sublimation

Explanation:
1. Iodine changes directly from solid to vapour on heating, which is sublimation.
2. Sand does not sublime, so it stays behind.
3. The method is sublimation.

Teacher's Note:
1. Sublimable solids include iodine, camphor, naphthalene and ammonium chloride.
2. Heat the mixture, then collect the vapour on a cold surface.

Answer: (c) A is vaporisation, C is condensation and F is melting.

Explanation:
1. X is a liquid (slightly compressible), Y is a gas (no definite volume) and Z is a solid (negligible space between molecules).
2. A takes X to Y, which is vaporisation. F takes Z to X, which is melting. C takes Y to Z, which the book calls condensation.
3. Only option (c) matches all three.

Teacher's Note:
1. Name X, Y and Z first, then read each arrow direction.
2. Arrow from liquid to gas is vaporisation, and from solid to liquid is melting.

Answer: (a) Sediment

Explanation:
1. The diagram shows a mixture being poured gently along a glass rod from a beaker after the heavy solid has settled.
2. This is sedimentation and decantation, and the settled solid at the bottom is the sediment.

Teacher's Note:
1. Sedimentation lets heavy particles settle; decantation pours off the liquid above.
2. Filtrate and residue are words for filtration, not for this set-up.

Answer: (a) Perfume

Explanation:
1. A liquid has one free surface (its top surface). A solid has many free surfaces and a gas has none.
2. Perfume is a liquid, so the answer is (a).

Teacher's Note:
1. Wax and talcum powder are solids; steam is a gas.
2. One free surface is a clue for a liquid.

Answer: (c) Heating of camphor

Explanation:
1. All three movements (vibration, rotation and translation) occur in the gas state.
2. Heating camphor converts it directly into vapour by sublimation, so its molecules finally move in all three ways.
3. The other processes end in a liquid or a solid. The answer is (c).

Teacher's Note:
1. Look for the process whose end product is a gas.
2. Fog and melting give liquid, and freezing gives solid.

Answer: (b) The order of compressibility is also the same.

Explanation:
1. Diffusion is highest in gases, then liquids, then solids, so A is a gas, B is a liquid and C is a solid.
2. Compressibility also goes gas > liquid > solid, so (b) is true.
3. Density, intermolecular force and rigidity increase in the opposite order, so (a), (c) and (d) are false. The answer is (b).

Teacher's Note:
1. Decide which property rises from gas to solid and which falls.
2. Diffusion and compressibility fall from gas to solid; density, force and rigidity rise.

Answer: (b) (C) and (D)

Explanation:
1. A changes to gas below its boiling point, so A evaporates (a slow surface process that causes cooling). B changes to gas only at its boiling point, so B boils (a quick, bulk process).
2. (A) is true, and (B) is true.
3. (C) is false because evaporation, not boiling, is associated with A and it is evaporation that causes cooling. (D) is false because boiling is a bulk phenomenon, not a surface one.
4. The false statements are (C) and (D): option (b).

Teacher's Note:
1. Decide first which liquid evaporates and which boils.
2. Evaporation: surface, slow, cooling. Boiling: bulk, fast, at one fixed temperature.

Answer: (d) Sulphur

Explanation:
1. Atomicity of silver is 1, bromine is 2 (Br₂), phosphorus is 4 (P₄) and sulphur is 8 (S₈).
2. The largest is sulphur.

Teacher's Note:
1. Learn the atomicities: S₈, P₄, Br₂ and metals are monoatomic.
2. Sulphur has the highest among common school examples.

Answer: (c) Sodium and potassium

Explanation:
1. Sodium and potassium are soft metals and can be cut with a knife.
2. Calcium and magnesium are harder.
3. So the pair is sodium and potassium.

Teacher's Note:
1. Soft metals: Li, Na and K.
2. These are stored under kerosene because they react quickly with air and moisture.

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

Explanation:
1. Zinc is brittle at room temperature, so it is the exception for malleability and ductility: (i) → (C).
2. Silver is found in the free (native) state: (ii) → (D).
3. Bismuth is a poor conductor of electricity for a metal: (iii) → (A).
4. Gallium is a metal that exists as a liquid near room temperature: (iv) → (B).
5. This is option (b).

Teacher's Note:
1. Think of what is unusual for each metal.
2. Bismuth conducts poorly, and gallium melts at about 30°C.

Answer: (a) CDBA

Explanation:
1. Evaporation is faster when the temperature is higher and the air is drier (lower humidity).
2. Summer is hotter than winter. Vizag, being on the coast, is more humid than Hyderabad.
3. Slowest is winter in Vizag (C), then winter in Hyderabad (D), then summer in Vizag (B), and fastest is summer in Hyderabad (A).
4. The increasing order is CDBA: option (a).

Teacher's Note:
1. Temperature matters more than humidity here, so compare seasons first.
2. Higher humidity slows evaporation.

Answer: (b) ACDB

Explanation:
1. Sugar solution is a homogeneous mixture (A).
2. Sugar is a compound (C).
3. Water and oil is a heterogeneous mixture (D).
4. Sodium is an element (B).
5. So the order is ACDB: option (b).

Teacher's Note:
1. Sort each item into its class, then follow the order the question lists.
2. Sugar is a compound of carbon, hydrogen and oxygen.

Answer: (a) C, B, A

Explanation:
1. Sodium chloride dissolves in water but carbon does not, so first dissolve the mixture in water (C).
2. Filter to remove the carbon (B).
3. Distil the filtrate to get sodium chloride back from the solution (A).
4. The methods are C, B, A: option (a).

Teacher's Note:
1. Dissolve what dissolves, filter what does not.
2. Salt is recovered from its solution by distillation or evaporation.

Answer: (b) Salt and odonil

Explanation:
1. The set-up has a china dish heated gently with an inverted funnel over it, which is the apparatus for sublimation.
2. Odonil sublimes and salt does not, so this pair can be separated.
3. The answer is (b).

Teacher's Note:
1. Inverted funnel over a heated dish means sublimation.
2. The other pairs have no sublimable substance.

Answer: (b) C, E, B

Explanation:
1. Cooling takes place when the substance loses heat: the changes from gas to liquid (B), from liquid to solid (E) and from gas to solid (C).
2. A, D and F take in heat (vaporisation, sublimation and melting).
3. So the cooling processes are C, E and B: option (b).

Teacher's Note:
1. Changes towards the solid side release heat.
2. Changes towards the gas side absorb heat.

Answer: (b) Filtrate

Explanation:
1. The liquid that passes through the filter paper and collects in the beaker is the filtrate.
2. The solid left on the filter paper is the residue.

Teacher's Note:
1. Remember: residue stays, filtrate falls through.
2. Distillate belongs to distillation.

Answer: (d) ADEBC

Explanation:
1. Add carbon disulphide (A) to dissolve the sulphur.
2. Filter, and add hot water to the residue (D) to dissolve the sodium chloride.
3. Filter again (E); carbon stays on the paper and is collected (B).
4. Heat the salt solution to get sodium chloride (C).
5. The order is ADEBC: option (d).

Teacher's Note:
1. Pick a solvent for each component in turn.
2. Each dissolving step must be followed by filtration.

Answer: (d) DCBA

Explanation:
1. Take the mixture (D).
2. Prepare the filter paper cone in the funnel (C).
3. Pour the mixture gently; the liquid collects as filtrate (B).
4. The solid left is the residue (A).
5. The order is DCBA: option (d).

Teacher's Note:
1. Have the mixture ready, set up the funnel, filter, then name the products.
2. The residue is named last because it is seen after filtering.

Answer: (a) (i) → (C); (ii) → (A); (iii) → (D); (iv) → (B)

Explanation:
1. Sublimation uses a sand bath for gentle heating: (i) → (C).
2. Magnetic separation uses a bar magnet: (ii) → (A).
3. Distillation uses a Liebig's condenser: (iii) → (D).
4. Filtration uses filter paper: (iv) → (B).
5. This is option (a).

Teacher's Note:
1. Link each method to the apparatus that does its main job.
2. Option (b) repeats (B), which cannot be right in a one-to-one match.

Answer: (c) 3

Explanation:
1. Ozone is represented by O₃.
2. Each molecule has 3 atoms, so the atomicity is 3.

Teacher's Note:
1. Atomicity is the number of atoms in one molecule.
2. Oxygen gas is O₂ with atomicity 2.

Answer: (d) All of the above

Explanation:
1. Metals generally have high melting and boiling points.
2. They are lustrous (shiny) and have high tensile strength.
3. All three are characteristics of metals, so the answer is all of the above.

Teacher's Note:
1. List the physical properties of metals when you revise.
2. Also remember sonorous, malleable and ductile.

Answer: (b) Graphite

Explanation:
1. Diamond, phosphorus and sulphur are non-metals that do not conduct electricity.
2. Graphite is the exception, because it has free electrons.
3. The answer is graphite.

Teacher's Note:
1. Graphite is used as electrodes and in pencil leads.
2. It is the only common non-metal conductor.

Answer: (c) humidity

Explanation:
1. Rate of evaporation increases with surface area, temperature and wind speed.
2. It decreases when humidity (water vapour in air) is higher.
3. So it is inversely related to humidity.

Teacher's Note:
1. Inversely means one goes up when the other goes down.
2. Clothes dry slowly on humid days.

Answer: (c) Solids have only one free surface.

Explanation:
1. Statements (a), (b) and (d) are correct for solids and gases.
2. A solid has many free surfaces, while a liquid has only one.
3. So the wrong statement is (c).

Teacher's Note:
1. Link number of free surfaces to the state: solid many, liquid one, gas none.
2. Read 'wrong' carefully so you pick the false statement.

Answer: (d) Potassium

Explanation:
1. Iron, aluminium and magnesium are hard.
2. Potassium is a very soft metal and can be cut with a knife.
3. The answer is potassium.

Teacher's Note:
1. Soft metals are Li, Na and K.
2. Cut surface of potassium tarnishes fast in air.

Answer: (c) Oxygen

Explanation:
1. Calcium oxide, common salt and water are compounds made of two or more elements.
2. Oxygen is made of only one kind of atom, so it is an element.

Teacher's Note:
1. An element has one kind of atom only.
2. Names with 'oxide' or 'chloride' usually point to compounds.

Answer: (c) Takes place at any temperature

Explanation:
1. Boiling is fast, happens throughout the liquid and keeps the temperature constant.
2. It takes place only at the boiling point, not at any temperature.
3. So the incorrect statement is (c).

Teacher's Note:
1. Evaporation happens at any temperature; boiling happens only at the boiling point.
2. This is a favourite comparison question.

Answer: (c) freezing point

Explanation:
1. The change of a liquid into a solid is freezing.
2. The temperature at which it happens is the freezing point.

Teacher's Note:
1. Melting point is solid to liquid, freezing point is liquid to solid.
2. For pure substances they are at the same temperature.

Answer: (a) evaporation of the perfume causes cooling due to absorption of heat from the surroundings, that is, the palm

Explanation:
1. Perfume is a volatile liquid and evaporates quickly.
2. Evaporation needs heat, which it takes from the palm, so the palm cools.
3. The answer is (a).

Teacher's Note:
1. Evaporation always causes cooling.
2. Sweating cools the body for the same reason.

Answer: (c) DBCA

Explanation:
1. Put the mixture in a china dish and invert a funnel over it (D).
2. Plug the stem with cotton and cover with a wet cloth (B).
3. Heat gently so camphor vapour forms (C).
4. The vapour condenses on the funnel, leaving salt behind (A).
5. The order is DBCA: option (c).

Teacher's Note:
1. Set up first, then heat, then collect.
2. The wet cloth helps the vapour condense.