CBSE Class 7 Science Advanced Chemistry Classification of Matter MCQs Set 02

Answer: (a) adhesive force is stronger than cohesive force.

Explanation:
1. Water molecules are attracted to glass (adhesive force) and to each other (cohesive force).
2. In a glass tube the pull of glass on water is greater than the pull between water molecules.
3. So the water climbs the wall and curves upward at the edge, giving a concave meniscus: adhesive force is stronger than cohesive force.

Teacher's Note:
1. Concave meniscus means adhesion wins; convex meniscus means cohesion wins.
2. Mercury in glass shows the opposite curve.

Answer: (b) effervescence is observed in case of X but not Y.

Explanation:
1. Iron is an element and reacts with dilute sulphuric acid to give hydrogen gas, so bubbles appear in X.
2. Rust is a compound of iron, and in it iron has lost its own properties.
3. Rust does not release hydrogen gas, so there is no effervescence in Y.

Teacher's Note:
1. The elements in a compound do not behave as they do when free.
2. Bubbles of gas show that a gas is being released.

Answer: (b) Y

Explanation:
1. Petrol is very volatile and evaporates quickly, and evaporation causes cooling.
2. The rate of evaporation increases with surface area.
3. The figure shows that container Y is the widest, so its petrol has the largest exposed surface.
4. So Y evaporates fastest and cools most: Y.

Teacher's Note:
1. Same volume in a wider container means a larger surface.
2. More evaporation means more cooling.

Answer: (a) sublimation

Explanation:
1. Odonil is a solid block placed in a washroom.
2. It slowly changes directly from the solid state into vapour and spreads its fragrance.
3. A direct solid to vapour change is sublimation.

Teacher's Note:
1. The block does not become liquid first.
2. Camphor and naphthalene balls behave in the same way.

Answer: (a) evaporation leads to cooling.

Explanation:
1. With the tongue out, the saliva has a large surface exposed to the air.
2. The saliva evaporates and takes heat from the body.
3. So the body is cooled: evaporation leads to cooling.

Teacher's Note:
1. Dogs have few sweat glands, so they cool themselves this way.
2. Evaporation always takes heat from the surroundings.

Answer: (a) filtration and distillation.

Explanation:
1. Camphor is insoluble in water, so filtration separates it from the salt solution.
2. Common salt is dissolved in the water; distillation separates water from the salt.
3. So the methods are filtration and distillation.

Teacher's Note:
1. Remove the insoluble solid first, then separate the solution.
2. Sublimation suits dry solid mixtures, not a mixture that contains water.

Answer: (b) close packing of molecules in solids.

Explanation:
1. Solid molecules are tightly packed and held by strong forces.
2. On heating they can move only a little further apart.
3. So the expansion is small because of the close packing of molecules.

Teacher's Note:
1. Solids have little space and strong forces, hence very small expansion.
2. Option (d) is wrong because solids have negligible space between molecules.

Answer: (c) wax-coated glass, plastic

Explanation:
1. A convex meniscus forms when the cohesive force among water molecules exceeds the adhesive force between water and the tube.
2. This happens with wax-coated glass and plastic, which water does not stick to.
3. So the tubes are wax-coated glass and plastic.

Teacher's Note:
1. Plain glass gives a concave meniscus for water.
2. Think of water beading on a waxed surface.

Answer: (b) rust powder, iron powder.

Explanation:
1. Rust is hydrated ferric oxide, which has iron, oxygen and hydrogen combined, so it is a compound.
2. Iron powder is an element.
3. A compound first, then an element, gives rust powder, iron powder.

Teacher's Note:
1. Check the order: compound first, element second.
2. Lime and rust are both compounds.

Answer: (b) is more in non-coastal areas than in coastal areas.

Explanation:
1. Coastal areas have more moisture in the air, so humidity is high.
2. High humidity reduces the rate of evaporation.
3. Non-coastal areas are drier, so evaporation is faster there: more in non-coastal areas.

Teacher's Note:
1. Humidity and evaporation rate go in opposite directions.
2. Think of clothes drying faster in dry weather.

Answer: (d) naphthalene balls and camphor.

Explanation:
1. Naphthalene balls and camphor turn directly from solid to vapour, which is sublimation.
2. An incense stick burns, and a perfume is a liquid that evaporates.
3. So the correct pair is naphthalene balls and camphor.

Teacher's Note:
1. Burning and evaporation are not sublimation.
2. Odonil does sublime, but its partner in options (a) and (c) does not.

Answer: (c) The constituents of a compound and a mixture can be separated by chemical and physical methods, respectively.

Explanation:
1. The elements in a compound lose their properties and are separated only by chemical methods.
2. The parts of a mixture keep their properties and are separated by physical methods.
3. In that order, the statement is chemical and physical, respectively.

Teacher's Note:
1. Compound goes with chemical; mixture goes with physical.
2. Mind the order in 'respectively'.

Answer: (b) Y

Explanation:
1. Petrol evaporates quickly at room temperature.
2. The rate of evaporation is directly proportional to the surface area.
3. Container Y is the widest, so it has the largest surface area.
4. So the rate of evaporation is highest in Y.

Teacher's Note:
1. Compare the widths of the containers.
2. Larger exposed surface means more molecules can escape.

Answer: (a) sublimation

Explanation:
1. Odonil is a solid and its fragrance spreads as vapour.
2. The solid turns directly into vapour without becoming liquid.
3. This change is sublimation.

Teacher's Note:
1. Solid to vapour directly is sublimation.
2. Deposition is the reverse change, vapour to solid.

Answer: (b) bromine

Explanation:
1. Steam, oxygen and hydrogen chloride are gases, with negligible forces.
2. Bromine is a liquid, so its molecules attract each other much more strongly.
3. So the strongest forces are in bromine.

Teacher's Note:
1. Spot the one substance that is not a gas.
2. Forces are strongest in solids, weaker in liquids, weakest in gases.

Answer: (b) close packing of molecules in solids.

Explanation:
1. Molecules in a solid are packed tightly and attract each other strongly.
2. So heating can push them only slightly apart.
3. This tight arrangement, the close packing of molecules, explains the small expansion.

Teacher's Note:
1. Close packing means very little room to spread out.
2. Solids do not have a large intermolecular space.

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

Explanation:
1. Assertion: dogs stretch out their tongues in summer. This is true.
2. Reason: evaporation causes cooling. This is also true.
3. The saliva on the tongue evaporates and cools the body, so R explains A: (a).

Teacher's Note:
1. Check each statement separately, then check whether R explains A.
2. Evaporation takes heat from the dog's body.

Answer: (d) carbon

Explanation:
1. Diamond is the hardest natural substance.
2. It is a form of carbon.

Teacher's Note:
1. Diamond is pure carbon.
2. It is the hard exception among the brittle nonmetals.

Answer: (d) molten NaCl

Explanation:
1. Dilute and concentrated sulphuric acid and aqueous \(\ce{NaCl}\) are mixtures with water.
2. Molten \(\ce{NaCl}\) is just sodium chloride, a compound, and every compound is a pure substance.
3. So the answer is molten NaCl.

Teacher's Note:
1. Melting only changes the state of a pure substance.
2. 'Aqueous' tells you water is present.

Answer: (b) d, c, b, a

Explanation:
1. A ball is simply an example of matter: d.
2. Water is a compound: c.
3. Argon is an element: b.
4. Dilute acid is a mixture of acid and water: a.
5. So the order is d, c, b, a.

Teacher's Note:
1. Identify each item first, then arrange them as asked.
2. Argon is a noble gas, hence an element.

Answer: (c) milk

Explanation:
1. Soda water and sugar water are uniform solutions, and air is a uniform mixture of gases.
2. Milk has fat particles that are spread unevenly, so it is heterogeneous.

Teacher's Note:
1. Air and solutions are homogeneous.
2. In milk, fat particles stay suspended in water.

Answer: (d) a, c

Explanation:
1. Magnetic separation removes the iron filings first (a).
2. Sublimation then separates the camphor from the sand (c).
3. Distillation and sedimentation need a liquid and are not used.
4. So the sequence is a, c.

Teacher's Note:
1. Use only the methods that suit solid mixtures.
2. Magnet first, then heat.

Answer: (b) Effervescence is observed in X but not in Y.

Explanation:
1. Iron is an element and reacts with dilute sulphuric acid to release hydrogen gas, so X bubbles.
2. Rust is a compound of iron in which iron does not keep its properties, so there is no bubbling in Y.

Teacher's Note:
1. Effervescence is the release of gas bubbles.
2. Elements and their compounds behave differently.

Answer: (d) A →b, B →a, C →c

Explanation:
1. Earthen pots keep water cool through evaporation: A →b.
2. Removing suspended matter from drinking water uses filtration: B →a.
3. Odonil spreads as vapour from a solid, which is sublimation: C →c.
4. So the answer is A →b, B →a, C →c.

Teacher's Note:
1. Link each principle with the everyday item that uses it.
2. Pots cool by evaporation, odonil works by sublimation.

Answer: (b) diffusion of gas/vapour

Explanation:
1. The fragrance reaches us as vapour that mixes with the air.
2. This spreading of a gas or vapour among other gas molecules is called diffusion.
3. Compressibility is about squeezing, and (d) pairs it with expansibility, so (d) is wrong.

Teacher's Note:
1. Fragrance spreading means mixing of vapour with air.
2. Option (d) pairs compressibility with expansibility, and compressibility has no role in the spreading of a smell.

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

Explanation:
1. A: baking soda is made of sodium, hydrogen, carbon and oxygen combined chemically, so it is a compound. True.
2. R: its properties are completely different from those of its elements. True.
3. Elements losing their own properties is the very reason it counts as a compound, so R explains A: (a).

Teacher's Note:
1. Elements that lose their properties form a compound.
2. Judge whether R gives the reason for A.

Answer: (d) Compounds are homogeneous and mixtures can be homogeneous or heterogeneous.

Explanation:
1. Compounds are always homogeneous.
2. A mixture may be homogeneous (such as alcohol and water) or heterogeneous (such as sulphur and water).
3. So the statement that is true is (d).

Teacher's Note:
1. A compound has uniform composition; a mixture may or may not.
2. 'Always' in (c) makes it wrong.

Answer: (b) is more in a non-coastal area than in a coastal area.

Explanation:
1. The rate of evaporation falls as humidity rises.
2. Coastal areas have higher humidity than non-coastal areas.
3. So the rate is higher in non-coastal areas.

Teacher's Note:
1. Evaporation is inversely related to humidity.
2. Dry air accepts more vapour.

Answer: (d) naphthalene balls and camphor.

Explanation:
1. Camphor and naphthalene balls change directly from solid to vapour.
2. Incense sticks burn and perfume evaporates, so they are not sublimation.
3. The right pair is naphthalene balls and camphor.

Teacher's Note:
1. Check that both items in a pair really sublime.
2. Odonil sublimes, but its partner in (a) and (c) does not.

Answer: (d) carbon, potassium

Explanation:
1. Intermolecular forces are strongest in solids.
2. Carbon and potassium are both solids at room temperature.
3. The other pairs include a liquid, so the answer is carbon, potassium.

Teacher's Note:
1. Each member of the pair has to be a solid.
2. Remember mercury and bromine as the familiar liquids.

Answer: (a) Gases are highly compressible and diffuse very easily.

Explanation:
1. Large gaps between gas molecules make gases compressible and let them mix easily: (a) is true.
2. Gas forces are negligible, so (b) is false.
3. Solids are almost incompressible, so (c) is false.
4. Solid molecules are tightly packed, so (d) is false.

Teacher's Note:
1. Match each property to the state: gases are compressible, solids are rigid.
2. Check every option before choosing.

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

Explanation:
1. A: in the rainy season the air holds a lot of moisture, so evaporation is slow. True.
2. R: the rate of evaporation is inversely proportional to humidity, not directly. False.
3. So A is true and R is false: (c).

Teacher's Note:
1. Humidity and evaporation rate are inversely related.
2. One wrong word can make the reason false.

Answer: (d) carbon fibre

Explanation:
1. Gas carbon, diamond and graphite are not ductile and have no tensile strength.
2. Carbon fibre is ductile and has high tensile strength.
3. So the odd one is carbon fibre.

Teacher's Note:
1. Carbon fibre is the exceptional nonmetal in the properties table.
2. Group the three similar items first.

Answer: (d) During formation of mixtures there is a change in the molecular composition.

Explanation:
1. Statements (a), (b) and (c) are all true.
2. A mixture is made by physical mixing, so the molecules do not change.
3. So (d) is false.

Teacher's Note:
1. A change in molecular composition is a chemical change.
2. Read for the word 'false' in the question.

Answer: (a) d, c, b, a

Explanation:
1. Copper is an element, so it has 1 constituent element.
2. Water has hydrogen and oxygen: 2 elements.
3. Calcium carbonate has calcium, carbon and oxygen: 3 elements.
4. Sodium bicarbonate has sodium, hydrogen, carbon and oxygen: 4 elements.
5. The ascending order is d, c, b, a.

Teacher's Note:
1. Count the different elements in each formula.
2. Ascending means smallest number first.

Answer: (d) air

Explanation:
1. Milk, muddy water and smoke are not uniform throughout.
2. Air is a uniform mixture of gases.
3. So the homogeneous mixture is air.

Teacher's Note:
1. All gas mixtures are homogeneous.
2. Look for a uniform appearance and composition.

Answer: (a) b, a, c

Explanation:
1. Use the magnet first to remove the iron filings (b).
2. Cover the rest with an inverted funnel and heat gently so that iodine sublimes and collects on the funnel (a).
3. Sawdust is what remains behind (c).
4. So the order is b, a, c.

Teacher's Note:
1. Magnetic separation does not need any other step first.
2. Sublimation of iodine leaves the sawdust behind.

Answer: (b) rust powder, iron powder.

Explanation:
1. Rust (hydrated ferric oxide) is a compound and iron is an element.
2. So the pair, compound first, is rust powder, iron powder.

Teacher's Note:
1. Pay attention to the order asked for.
2. Lime is also a compound.

Answer: (a) A →c, B →b, C →d, D →e, E →a

Explanation:
1. Iodine sublimes, so A →c.
2. Sawdust is removed from water by filtration: B →b.
3. Salt from sea water is obtained by evaporation: C →d.
4. Iron is magnetic and sulphur is not: D →e.
5. Copper sulphate dissolved in water is separated by distillation: E →a.

Teacher's Note:
1. Work out the sure pairs first and strike off options.
2. A dissolved solid in a liquid needs distillation or evaporation.

Answer: (a) diffusibility.

Explanation:
1. Gas molecules move fast and have large gaps between them.
2. They mix with each other on their own, which is diffusion.
3. That is why mixtures of gases are uniform.

Teacher's Note:
1. Spontaneous mixing is diffusion.
2. Compressibility and density do not decide mixing.

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

Explanation:
1. A: washing soda has sodium, carbon and oxygen combined chemically in a fixed ratio, so it is a compound. True.
2. R: in a compound the elements lose their properties, so sodium does not keep its property here. False.
3. So A is true and R is false: (c).

Teacher's Note:
1. Elements never keep their properties inside a compound.
2. Sodium is highly reactive, which washing soda is not.