CBSE Class 7 Science Advanced Chemistry Classification of Matter Short Answer Questions Set 01

Short Answer Questions

Teacher's Note:
1. A table of four points is the best format.
2. Always mention 'surface' and 'bulk'.

Question 2. Classify the elements based on atomicity with examples.
Answer:
The number of atoms in a molecule of an element is its atomicity. Based on it, elements are of three types.
1. Monoatomic elements: each molecule has one atom. Examples: \(\ce{Cu}\), \(\ce{Ag}\), \(\ce{He}\).
2. Diatomic elements: each molecule has two atoms. Examples: \(\ce{H2}\), \(\ce{O2}\), \(\ce{N2}\).
3. Polyatomic elements: each molecule has more than two atoms. Examples: \(\ce{O3}\), \(\ce{P4}\), \(\ce{S8}\).
In simple words: Elements are one-atom, two-atom or many-atom molecules.

Teacher's Note:
1. Mono means one, di means two, poly means many.
2. Quote at least two examples for each type.

Teacher's Note:
1. Four points are enough.
2. Fixed proportion by mass is another feature of compounds.

Question 4. Explain the suitable method of separation of ammonium chloride from common salt.
Answer:
The suitable method is sublimation, because ammonium chloride sublimes and common salt does not.
1. Put the mixture in a flat basin and cover it with an inverted funnel. Wrap the funnel with wet filter paper and plug its stem with cotton or paper.
2. Keep the basin on a sand bath and heat gently with a Bunsen burner.
3. Ammonium chloride changes into white vapour, which cools on the funnel wall and collects as a white powder (the sublimate).
4. Common salt stays in the basin. Scrape the powder off the funnel to get ammonium chloride.
In simple words: Heat the mixture: ammonium chloride turns to vapour and settles on the funnel, while the salt stays behind.

Teacher's Note:
1. Draw the funnel set-up if time permits.
2. Name the sublimate and the residue clearly.

Question 5. Explain the process of sedimentation and decantation.
Answer:
This method separates heavy insoluble solid particles from a liquid, for example sand from water.
1. Take the mixture in a beaker and leave it undisturbed. The heavy sand particles settle at the bottom. This is sedimentation.
2. Pour the clear water slowly into another beaker, guiding it down a glass rod so that the sediment is not disturbed. This is decantation.
Result: clear water is collected in the second beaker and the sand remains in the first.
In simple words: Let the heavy solid settle, then gently pour off the clear liquid.

Teacher's Note:
1. Sedimentation comes first, decantation follows.
2. The glass rod prevents splashing and mixing.

Question 6. What is a homogeneous and a heterogeneous mixture? Give one example for each.
Answer:
Homogeneous mixture: a mixture whose constituents are uniformly distributed throughout. Example: alcohol and water.
Heterogeneous mixture: a mixture whose constituents are not uniformly distributed. Example: sulphur and water (or sand and water).
In simple words: Even mixture is homogeneous; uneven mixture is heterogeneous.

Teacher's Note:
1. Dissolving solids give homogeneous mixtures.
2. Give one example for each as asked.

Question 7. Explain classification of matter based on its molecular composition with suitable examples.
Answer:
Matter is classified into pure substances and mixtures.
1. Pure substance: matter made up of identical molecules. It is either an element or a compound.
2. Element: molecules contain only one kind of atom. Examples: hydrogen, oxygen, sodium.
3. Compound: molecules contain two or more different kinds of atoms. Examples: water, carbon dioxide.
4. Mixture: matter with different kinds of molecules. It is homogeneous (alcohol and water, air) or heterogeneous (sulphur and water, milk).
5. Elements are further divided into metals, metalloids and nonmetals.
In simple words: Matter is pure or a mixture; pure matter is an element or a compound.

Teacher's Note:
1. Draw the classification chart along with the answer.
2. Give one example at each stage.

Teacher's Note:
1. Use the given entries to work out which row is which state.
2. 'Only one free surface' belongs to liquids and 'rapid diffusion' to gases.

Question 9. Explain the factors affecting the rate of evaporation.
Answer:
1. Surface area: the rate of evaporation is directly proportional to the surface area. A larger exposed surface lets more molecules escape.
2. Temperature: a rise in temperature increases the rate, since molecules gain energy and escape more easily.
3. Humidity: the rate is inversely proportional to the moisture in the air. Humid air slows evaporation, dry air speeds it up.
In simple words: Evaporation is faster with larger surface and higher temperature, and slower in humid air.

Teacher's Note:
1. Mention the type of proportion for each factor.
2. Add an everyday example, such as clothes drying.

Question 10. Discuss the characteristics of (a) an element (b) a mixture (c) a compound.
Answer:
(a) Element: made of only one kind of atom; cannot be broken down into simpler substances by physical or chemical means; it is a pure, homogeneous substance (for example hydrogen).
(b) Mixture: made by physically mixing two or more substances in any proportion; constituents keep their properties; they can be separated by physical methods; it may be homogeneous or heterogeneous (for example air, sulphur in water).
(c) Compound: two or more elements combined chemically in a fixed proportion by mass; the elements lose their own properties; they can be separated only by chemical methods; it is homogeneous (for example water).
In simple words: An element has one kind of atom, a mixture is just mixed, and a compound is chemically combined.

Teacher's Note:
1. Cover composition, properties and separation for each.
2. Use one example per part.

Teacher's Note:
1. Noble gases are the elements that exist as single atoms.
2. Make the answer neat with a table.

Question 12. Distinguish between malleability and ductility with an example.
Answer:
Malleability is the property of a metal by which it can be beaten into thin sheets. Example: aluminium and gold can be hammered into foil.
Ductility is the property by which a metal can be drawn into thin wires. Example: copper can be drawn into wires.
In simple words: Malleable means beaten into sheets; ductile means drawn into wires.

Teacher's Note:
1. Both are properties of metals.
2. Zinc is neither malleable nor ductile.

Question 13. What is intermolecular force of attraction? Mention two types of intermolecular force of attraction acting between molecules.
Answer:
Intermolecular force of attraction is the force of attraction that acts between neighbouring molecules of matter.
Types: 1. Cohesive force, between molecules of the same substance. 2. Adhesive force, between molecules of different substances (for example water and glass).
In simple words: It is the pull between molecules, of two types: cohesive and adhesive.

Teacher's Note:
1. Cohesive is like with like.
2. Mention an example of adhesive force.

Question 14. What is diffusion? Give one application.
Answer:
Diffusion is the spontaneous mixing of one substance with another because of the random movement of molecules. Gases diffuse quickly and liquids slowly; solids do not diffuse.
Application: the smell of a perfume or of cooked food spreads through a room because the vapour diffuses into the air.
In simple words: Diffusion is molecules spreading on their own among other molecules.

Teacher's Note:
1. Gases diffuse fastest because their molecules move rapidly with large gaps.
2. Spreading of perfume is the usual example.

Question 15. Give schematic representation of classification of matter based on its composition.
Answer:
Matter is classified as shown below.

Matter divides into pure substances and mixtures. Pure substances divide into elements and compounds, and elements divide into metals, metalloids and nonmetals. Mixtures divide into homogeneous and heterogeneous mixtures.
In simple words: The chart splits matter into pure substances and mixtures, and then branches further.

Teacher's Note:
1. Reproduce the chart with boxes and arrows.
2. Check that elements show all three branches.

Question 16. Describe the process of separation of mixture of iron filings, camphor and sand.
Answer:
The mixture is separated by magnetic separation followed by sublimation.
1. Spread the mixture evenly in a basin and move a strong bar magnet over it repeatedly. The iron filings cling to the magnet. Scrape them off with a piece of cardboard.
2. Cover the remaining mixture with an inverted funnel with its outer surface wrapped in wet filter paper and its stem plugged with cotton.
3. Heat gently. Camphor sublimes and its vapour condenses on the cool wall of the funnel. Scrape it off.
4. Sand remains in the basin.
In simple words: Take out iron with a magnet, then heat so that camphor goes up as vapour and sand stays.

Teacher's Note:
1. The order matters: magnet first, heat later.
2. Name the method used for each component.