CBSE Class 7 Science Advanced Chemistry Chemistry in Daily Life Short Answer Questions Set 01

Short Answer Questions

Question 1. How is sodium chloride extracted from sea water?
Answer:
1. Sea water is evaporated, which leaves crude salt containing sand and clay.
2. The crude salt is dissolved in water, and the insoluble impurities are removed.
3. The hot saturated solution is cooled, and pure sodium chloride crystallizes out (recrystallization).
4. A little sodium iodide or potassium iodide is added to make common (iodized) salt.
In simple words: Evaporate sea water, dissolve the crude salt, filter, recrystallize.

Teacher's Note:
1. Recrystallization separates a solid solute by cooling a hot saturated solution.
2. Learn the order of the steps: this is a common sequencing question.

Question 2. What is blue vitriol? Write its method of preparation and its uses.
Answer:
Blue vitriol is hydrated copper sulphate, \(\ce{CuSO4.5H2O}\).
Preparation: dissolve cupric sulphide, cupric oxide or cupric carbonate in dilute sulphuric acid.
Uses: its solution is sprayed on citrus trees and grape vines to kill insects, and it is used as a fungicide in Bordeaux mixture.
In simple words: Blue vitriol is blue copper sulphate crystals, used to protect plants.

Teacher's Note:
1. Copper oxide plus dilute sulphuric acid gives copper sulphate and water.
2. Note the colour of the crystals: blue.

Question 3. How is plaster of Paris manufactured? Mention its applications.
Answer:
Gypsum, \(\ce{CaSO4.2H2O}\), is heated for a long time. It loses part of its water of crystallization and plaster of Paris, \(\ce{CaSO4.1/2H2O}\), is obtained.
Applications: it is mixed with water to a paste, which sets hard, so it is used to set fractured bones, cast statues and toys, make decorative materials, modern blackboard chalk and ceiling panels.
In simple words: Heating gypsum drives off some water and gives plaster of Paris, which sets hard with water.

Teacher's Note:
1. Remember the formula carefully: half a molecule of water.
2. The setting property is the basis of all its uses.

Question 4. How are bricks manufactured?
Answer:
Red bricks are made from clay.
The right amount of water is added to clay and kneaded well.
The clay is pressed into the shape of bricks.
The bricks are heated at about 1000 °C, which hardens them.
Bricks are used to build walls.
In simple words: Moist clay is moulded into bricks and baked hard.

Teacher's Note:
1. Baking turns soft clay into a hard solid.
2. Bricks are a structural material for walls.

Teacher's Note:
1. Add uses: utensils and shoe soles (thermoplastic); water tanks, pipes and cabinets (thermosetting).
2. Both kinds are polymers.

Teacher's Note:
1. Scum forms because calcium and magnesium react with soap.
2. Both clean by emulsification.

Question 7. Write a short note on the cleansing action of detergent.
Answer:
Detergents dissolve in water and act as surfactants.
They form a thin film around oil and grease spots on clothes and help remove them along with the dirt.
Because an emulsion is formed, the process is called emulsification, and detergents are called emulsifiers.
In simple words: A detergent surrounds grease and lifts it away with water.

Teacher's Note:
1. Surfactant is a substance that lowers surface tension.
2. The oil drops are held in water as an emulsion.

Question 8. Write a short note on medicines.
Answer:
Long ago, people used parts or extracts of medicinal plants such as tulsi, vasaka and neem for treating diseases.
Scientists analysed the composition of these substances and made similar molecules artificially. They also developed new molecules that are more effective and give quicker relief; some can cure serious diseases. Such molecules are medicines.
Examples: penicillin, tetracycline and sulphadiazine. Quinine, extracted from the bark of the cinchona plant, is used for malaria.
In simple words: Medicines are chemicals that cure or relieve diseases, first taken from plants and now often made artificially.

Teacher's Note:
1. Name at least two examples in an answer.
2. Quinine is a plant-derived medicine.

Question 9. How are paints and plastic emulsions prepared?
Answer:
Paints are colouring materials dissolved in a suspension of basic lead carbonate in linseed oil.
Plastic emulsion is a kind of paint made by dissolving coloured plastics in amyl acetate. It lasts much longer than basic lead carbonate paints.
In simple words: Ordinary paint is colour in lead carbonate and linseed oil; plastic emulsion uses plastics in amyl acetate.

Teacher's Note:
1. Plastic emulsion is more durable.
2. Learn the key words: linseed oil and amyl acetate.

Question 10. What do you mean by grouting? Where does this technique find application?
Answer:
Grouting is a construction technique in which pure cement, or cement mixed with sand and water in a fixed proportion, is applied in the gaps between building materials such as bricks and tiles.
It makes the surface smooth and strengthens the structure.
In simple words: Grouting fills the gaps between bricks or tiles with cement.

Teacher's Note:
1. It is common when laying floor tiles.
2. Pure cement is the material used for grouting.

Question 11. Write a short note on compounds of silicon.
Answer:
Sand (silicon dioxide): natural mineral; purified sand is the raw material for glass and cement.
Sodium silicate: dissolves in water; used for preserving eggs.
Silicone: a polymer made from silicon; heat resistant, non-sticky, water repellent and a bad conductor of electricity, so it is used in non-stick pans, waterproof clothes, wax polishes and electrical insulators.
Ferrosilicon: an alloy of iron and silicon, a very hard steel.
Silicon carbide: the hardest compound, used in grinding wheels.
In simple words: Silicon forms sand, sodium silicate, silicone, ferrosilicon and silicon carbide, each with its own use.

Teacher's Note:
1. Link every compound to one use.
2. Silicone and silicon are different things.

Teacher's Note:
1. Inert means not reacting easily.
2. Radon is radioactive, so it is used in medicine.

Question 13. Mention the physical properties of aluminium.
Answer:
Aluminium has a silvery-white lustre.
It is highly malleable and ductile, and a very good conductor of electricity.
It is very light in weight and resists corrosion.
It is relatively soft compared to other metals.
It is the most abundant metal in the earth's crust.
In simple words: Aluminium is a light, shiny, soft, corrosion-resistant and conducting metal.

Teacher's Note:
1. Light weight and corrosion resistance explain its uses in aircraft and foil.
2. Aluminium is a metal, though the third most abundant element overall.

Question 14. Describe the process of galvanization. How is it useful?
Answer:
Galvanization is the process of coating iron articles with zinc.
Hot iron sheets are dipped in molten zinc and passed through hot, heavy rollers. On cooling, a coating of zinc forms over the iron.
The coating protects the iron from rusting. Galvanized iron is used for roofing, tanks and rainwater drain pipes.
In simple words: Dipping iron in molten zinc coats it and prevents rust.

Teacher's Note:
1. Zinc is more reactive than iron.
2. A scratched zinc coat can wear off, as in water pipes.

Question 15. Discuss a few applications of graphite.
Answer:
1. Pencil leads are made by mixing graphite with clay; more clay makes a harder pencil.
2. Crucibles for melting small quantities of metals are made from graphite and fine clay, moulded and sintered.
3. It is a lubricant for hot machine parts where ordinary oils would char.
4. It is used as electrodes in electrolytic cells.
5. It is used as a moderator in nuclear reactors to slow down neutrons.
In simple words: Graphite is used in pencils, crucibles, lubricants, electrodes and nuclear reactors.

Teacher's Note:
1. Each use depends on a different property.
2. Know why graphite can serve as a lubricant at high temperature.

Question 16. Mention a few properties of mercury.
Answer:
Mercury is a silvery-white metal and is liquid at normal temperature.
Its melting point is −39 °C and its boiling point is 357 °C.
It is a very good conductor of electricity and heat.
It dissolves almost all metals; the solution of a metal in mercury is called an amalgam.
In simple words: Mercury is the metal that is liquid at room temperature and forms amalgams.

Teacher's Note:
1. Gold and silver amalgams fill teeth.
2. Mercury is used in thermometers.