CBSE Class 7 Science Advanced Chemistry Classification of Matter MCQs Set 01

Answer: (d) all the above

Explanation:
1. Metals are usually solids at room temperature, so (a) is true.
2. They conduct heat well, so (b) is true.
3. They also have high tensile strength, so (c) is true.
4. All three statements hold, so the answer is (d) all the above.

Teacher's Note:
1. When an option says 'all the above', test each statement one by one.
2. Mercury and zinc are well-known exceptions, but the word 'generally' allows for them.

Answer: (d) sublimation

Explanation:
1. Ammonium chloride changes directly from solid to vapour on heating, while chalk powder does not.
2. A mixture of one sublimable and one non-sublimable solid is separated by sublimation.
3. Filtration and evaporation need a liquid, and distillation is used for liquids, so they do not apply here.

Teacher's Note:
1. Look for the substance that sublimes in a solid mixture.
2. Ammonium chloride, camphor, iodine and naphthalene are the usual sublimable solids.

Answer: (d) potassium

Explanation:
1. Iron, aluminium and magnesium are hard enough to be used for tools and structures.
2. Sodium and potassium are the soft metals; they can be cut with a knife.
3. So the soft metal is potassium.

Teacher's Note:
1. Remember sodium and potassium as the soft exceptions among metals.
2. Both are so soft that they are cut with a knife and stored in kerosene.

Answer: (d) phosphorus

Explanation:
1. Zinc, copper and aluminium are metals, and metals conduct electricity well.
2. Phosphorus is a nonmetal, and nonmetals are generally bad conductors.
3. So the bad conductor is phosphorus.

Teacher's Note:
1. Metals conduct; most nonmetals do not.
2. Graphite is the nonmetal exception that does conduct.

Answer: (a) atom

Explanation:
1. The smallest particle of an element is its atom.
2. Most atoms cannot exist alone, but a few (the noble gases) can, so the phrase 'may or may not have independent existence' fits an atom.
3. A molecule always has independent existence, and a compound is not a particle.

Teacher's Note:
1. Atom is the tiniest particle of matter.
2. Do not mix it up with a molecule, which can always exist freely.

Answer: (c) Ozone

Explanation:
1. Calcium oxide has calcium and oxygen, common salt has sodium and chlorine, and water has hydrogen and oxygen, so each is a compound.
2. Ozone, \(\ce{O3}\), is made of oxygen atoms only, so it is an element.

Teacher's Note:
1. A formula showing just one kind of symbol indicates an element.
2. Ozone is a form of oxygen, so it is an element.

Answer: (a) a liquid to its gaseous state below the boiling point of the substance

Explanation:
1. Evaporation changes a liquid into vapour from its surface.
2. It can happen at any temperature below the boiling point.
3. Options (c) and (d) describe melting, not evaporation.

Teacher's Note:
1. Evaporation is a surface change, while boiling is a bulk change at a fixed temperature.
2. Remember the words 'below the boiling point'.

Answer: (c) filtration

Explanation:
1. Sawdust is insoluble and floats on water.
2. Floating particles cannot settle, so sedimentation and decantation will not work well.
3. Passing the mixture through filter paper holds back the sawdust, so the method is filtration.

Teacher's Note:
1. Match the method to the behaviour of the solid: floating or suspended solids need filtration.
2. Only heavy particles that settle suit decantation.

Answer: (b) The proportion of constituent elements in a compound is fixed.

Explanation:
1. Compounds are homogeneous, so (a) is false.
2. In a compound the elements are always combined in a fixed proportion by mass, so (b) is true.
3. Elements lose their properties in a compound, so (c) is false.
4. A compound forms by chemical combination, so (d) is false.

Teacher's Note:
1. Fixed proportion by mass is the key feature of a compound.
2. Check each statement against the properties of compounds one by one.

Answer: (c) A mixture of water and saw dust

Explanation:
1. Sugar, common salt and glucose dissolve in water and spread uniformly, so these are homogeneous.
2. Sawdust does not dissolve, so it is not evenly mixed with water.
3. The heterogeneous mixture is water and sawdust.

Teacher's Note:
1. Dissolving solids give homogeneous mixtures; insoluble solids give heterogeneous ones.
2. Check solubility first.

Answer: (c) milk

Explanation:
1. Soda water and sugar water are uniform solutions.
2. Liquid ammonia is a single compound, not a mixture.
3. Milk holds fat particles and other substances that are not uniformly dissolved, so milk is a heterogeneous mixture.

Teacher's Note:
1. First drop pure substances like liquid ammonia from the list.
2. Of the mixtures left, find the one whose parts are not uniform.

Answer: (d) molten NaCl

Explanation:
1. Dilute sulphuric acid, concentrated sulphuric acid and aqueous \(\ce{NaCl}\) are solutions of a substance in water, so each is a mixture.
2. Molten \(\ce{NaCl}\) is sodium chloride alone, in liquid form.
3. So the pure substance is molten NaCl.

Teacher's Note:
1. 'Aqueous' means dissolved in water, so it is a mixture.
2. Melting a substance does not change what it is made of.

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

Explanation:
1. Gas molecules are far apart, so gases are highly compressible and mix easily with each other: (a) is true.
2. Forces between gas molecules are negligible, so (b) is false.
3. Solids are incompressible, so (c) is false.
4. Solid molecules are tightly packed, so (d) is false.

Teacher's Note:
1. Link large intermolecular space with high compressibility and diffusion.
2. Gases have the weakest forces and solids the strongest.

Answer: (d) both (c) and (b)

Explanation:
1. The scent of a perfume spreads through the air because its vapour moves among the air molecules; this is diffusion.
2. The vapour also spreads out to fill the whole space available, which is expansibility.
3. So both (c) and (b) are used.

Teacher's Note:
1. Perfumes need the vapour to spread and fill the room.
2. Compressibility is the squeezing property and has no role here.

Answer: (c) the constituents are combined chemically.

Explanation:
1. Baking soda is made of sodium, hydrogen, carbon and oxygen.
2. These are joined chemically in a fixed ratio, which makes it a compound.
3. The elements do not keep their properties and cannot be separated physically, so only (c) is correct.

Teacher's Note:
1. The test for a compound is chemical combination.
2. Statements (a), (b) and (d) describe mixtures.

Answer: (c) sodium, potassium

Explanation:
1. Mercury, sodium and potassium are the metals with low melting points.
2. Magnesium, manganese and calcium melt at high temperatures.
3. Only sodium and potassium are both low-melting.

Teacher's Note:
1. Learn the list: mercury, sodium, potassium (and gallium) melt at low temperatures.
2. In each pair, both metals must be low-melting.

Answer: (d) carbon

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

Teacher's Note:
1. Diamond and graphite are both forms of carbon.
2. Diamond is the exception to the rule that nonmetals are soft or brittle.

Answer: (a) evaporation

Explanation:
1. Sea water holds salt dissolved in water.
2. Heating the water in shallow pans makes the water evaporate and leaves the salt behind.
3. So the method is evaporation.

Teacher's Note:
1. Dissolved solids cannot be removed by filtration.
2. Evaporation recovers only the solid, not the water.

Answer: (b) aecbd

Explanation:
1. First add carbon disulphide to dissolve the sulphur (a).
2. Filter the mixture and add hot water to the residue, so that nitre dissolves (e).
3. Filter again, keeping the residue (c).
4. The residue is the carbon powder (b).
5. Heat the filtrate to get \(\ce{KNO3}\) (d).
6. The order is a, e, c, b, d, i.e. aecbd.

Teacher's Note:
1. Remove sulphur with carbon disulphide first, then deal with nitre and carbon.
2. Hot water dissolves nitre but not carbon.

Answer: (c) badc

Explanation:
1. Place the mixture in a dish and invert a funnel over it (b).
2. Wrap the funnel with a wet cloth and plug the stem (a).
3. Heat gently so that camphor turns into vapour (d).
4. The vapour cools on the funnel to give solid camphor, while sand stays in the dish (c).
5. The order is b, a, d, c, i.e. badc.

Teacher's Note:
1. Set up the apparatus first, heat next, and collect last.
2. Option (4) has five letters and cannot be right.

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

Explanation:
1. Solid molecules only vibrate about fixed positions, so (a) is true.
2. Solids cannot be squeezed, so (b) is true.
3. A solid can have any number of free surfaces; only liquids have one, so (c) is wrong.
4. Gases diffuse quickly, so (d) is true.

Teacher's Note:
1. Remember: solids any number, liquids one, gases none.
2. Read the question carefully, since it asks for the wrong statement.

Answer: (d) sulphur

Explanation:
1. Helium is monoatomic, so its atomicity is 1.
2. Fluorine is diatomic, \(\ce{F2}\), so its atomicity is 2.
3. Ozone is \(\ce{O3}\), so its atomicity is 3.
4. Sulphur is \(\ce{S8}\), so its atomicity is 8, the maximum.

Teacher's Note:
1. Count the atoms in one molecule of each element.
2. Sulphur and phosphorus are polyatomic elements with 8 and 4 atoms.

Answer: (d) A →b B →c C →a

Explanation:
1. Iodine sublimes, so iodine + sand is separated by sublimation: A →b.
2. Sawdust is insoluble in water, so filtration separates them: B →c.
3. Salt dissolved in water is recovered by evaporation: C →a.
4. So the correct option is A →b, B →c, C →a.

Teacher's Note:
1. Match the most definite pair first (iodine and sublimation).
2. Think whether the solid is dissolved or not for filtration and evaporation.

Answer: (a) They have variable composition

Explanation:
1. In a mixture the substances can be mixed in any proportion, so the composition is variable: (a) is true.
2. A mixture can be homogeneous or heterogeneous, so (b) and (c) are false.

Teacher's Note:
1. 'Always' statements about mixtures are usually wrong.
2. Variable composition is a basic property of a mixture.

Answer: (c) millions of molecules of water

Explanation:
1. A water molecule is \(\ce{H2O}\), which has 2 hydrogen atoms and 1 oxygen atom.
2. But even a tiny drop holds a very large number of such molecules.
3. So a drop contains millions of molecules of water.

Teacher's Note:
1. Option (a) describes a single molecule, not a drop.
2. Molecules are far too small to be counted in a visible drop.

Answer: (d) metalloid

Explanation:
1. Germanium shows some properties of metals and some of nonmetals.
2. Such an element is a metalloid.

Teacher's Note:
1. Germanium, arsenic, antimony, selenium and tellurium are the metalloids in this chapter.
2. It is a solid at room temperature.

Answer: (a) decantation

Explanation:
1. Mud is heavier than water and settles at the bottom.
2. The clear water can then be poured off, which is decantation (after sedimentation).

Teacher's Note:
1. Muddy water is a mixture of an insoluble solid that settles.
2. Sublimation needs a sublimable solid and magnetic separation needs a magnetic one.

Answer: (a) mixture

Explanation:
1. Lime water is a solution of calcium hydroxide in water.
2. A solution is a mixture, so lime water is a mixture.

Teacher's Note:
1. Any solution is a mixture.
2. Calcium hydroxide alone would be a compound, but lime water also contains water.

Answer: (a) adhesive forces between water and glass.

Explanation:
1. Water and glass are two different substances.
2. The attraction between molecules of two different substances is an adhesive force.

Teacher's Note:
1. Different substances give adhesive forces; the same substance gives cohesive forces.
2. Water wets glass because the adhesive force is stronger.

Answer: (a) melting point

Explanation:
1. Solid to liquid is melting.
2. The temperature at which this occurs is the melting point.
3. Boiling point is for liquid to gas.

Teacher's Note:
1. Match the word with the change: melting for solid to liquid, boiling for liquid to gas.
2. Evaporation and condensation are processes, not temperatures.

Answer: (d) air

Explanation:
1. Milk, muddy water and smoke have unevenly distributed particles.
2. Air is a mixture of gases, and gases mix uniformly.
3. So the homogeneous mixture is air.

Teacher's Note:
1. Any mixture of gases is homogeneous.
2. Smoke contains solid particles, so it is not uniform.

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

Explanation:
1. Statements (a), (b) and (c) are true.
2. A mixture forms by physical mixing, so there is no change in the molecular composition.
3. So statement (d) is false.

Teacher's Note:
1. Physical mixing cannot change what the molecules are made of.
2. A change in molecular composition would mean a chemical reaction.

Answer: (b) bromine

Explanation:
1. Hydrogen chloride, fluorine and chlorine are gases at room temperature, so their forces are weak.
2. Bromine is a liquid, so the forces between its molecules are much stronger.
3. The answer is bromine.

Teacher's Note:
1. Liquids have stronger forces than gases.
2. Look for the substance in the liquid state at room temperature.

Answer: (a) diffusiblity

Explanation:
1. Gas molecules move quickly with large gaps between them.
2. They therefore mix with each other spontaneously and evenly, a property called diffusibility.

Teacher's Note:
1. Mixing on its own is diffusion.
2. Compressibility has nothing to do with mixing.

Answer: (a) chemically in fixed ratio by weight.

Explanation:
1. A compound forms by chemical combination.
2. The elements are always combined in a fixed ratio by weight.
3. So the answer is chemically in fixed ratio by weight.

Teacher's Note:
1. Compound means chemical and fixed; mixture means physical and any ratio.
2. Cross out options containing 'physically'.

Answer: (d) carbon, potassium

Explanation:
1. Forces are strongest in solids, then liquids, then gases.
2. Bromine and mercury are liquids; gallium is a solid and bromine a liquid; bromine is a liquid and sodium a solid.
3. Carbon and potassium are both solids, so this pair has the strongest forces: carbon, potassium.

Teacher's Note:
1. Both substances in the pair must be solids.
2. Check the state of each substance at room temperature.

Answer: (a) sodium, potassium

Explanation:
1. The soft metals are sodium and potassium.
2. Magnesium, calcium and manganese are comparatively hard.
3. So the pair is sodium, potassium.

Teacher's Note:
1. Sodium and potassium can be cut with a knife.
2. Both are stored in kerosene because they are reactive.

Answer: (d) carbon fibre

Explanation:
1. Gas carbon, diamond and graphite are nonmetallic forms of carbon with no tensile strength and no ductility.
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 exception among nonmetals in the table of properties.
2. Look for the one that differs from the other three.

Answer: (c) filtration and distillation.

Explanation:
1. Sand is insoluble, so filtration removes it.
2. Glucone-D is dissolved in water; distillation collects the water as distillate and leaves glucone-D in the flask.
3. So all three are collected by filtration and distillation.

Teacher's Note:
1. Evaporation would lose the water; distillation lets you collect it.
2. Sublimation does not apply since none of these sublimes.

Answer: (c) Rate of evaporation is inversely proportional to surface area of a liquid.

Explanation:
1. Statements (a), (b) and (d) are true.
2. The rate of evaporation is directly proportional to the surface area, not inversely, so (c) is false.

Teacher's Note:
1. More surface means more molecules can escape.
2. Rate: direct with area and temperature, inverse with humidity.

Answer: (d) bcad

Explanation:
1. Take the mixture in a beaker (b).
2. Prepare the funnel with a folded, moistened filter paper (c).
3. Pour the mixture into the cone and collect the filtrate (a).
4. Name the solid left on the paper as the residue (d).
5. So the order is b, c, a, d, i.e. bcad.

Teacher's Note:
1. Mixture first, set-up second, pouring third, observation last.
2. Filtrate is what passes through; residue is what stays.