CBSE Class 7 Science Advanced Chemistry Chemistry in Daily Life Short Answer Questions Set 02

Short Answer Questions

Question 1. Write a short note on marble.
Answer:
Marble is chemically calcium carbonate, like limestone.
It is formed from limestone (the remains of sea shells) and is a hard rock that takes a polish.
It is widely used for flooring, statues and monuments, such as the Taj Mahal.
In simple words: Marble is a polished form of calcium carbonate used in buildings and statues.

Teacher's Note:
1. Marble and limestone have the same formula.
2. Limestone is also used for cement, glass and lime.

Question 2. Describe briefly the major techniques applied to the molten glass obtained during the manufacture of glass to get the desired variety of glass.
Answer:
(a) Blowing: used for laboratory glassware and bottles.
(b) Optical glass: molten glass is pressed into a circular shape, from which optical glasses are made.
(c) Sheet or float glass: molten glass is poured on a bath of molten tin, where it spreads to a uniform thickness, and sheet glass is drawn from it.
The shaped glass is then annealed (cooled slowly in a tunnel-type chamber).
In simple words: Molten glass is blown, pressed or floated on tin, and then annealed.

Teacher's Note:
1. Name the product for each technique.
2. Float glass gets a flat surface from the liquid tin.

Question 3. Discuss the general properties of glass.
Answer:
Glass is a supercooled liquid. When molten glass cools slowly, its viscosity increases so much that it stops flowing, and it looks like a solid.
It is transparent, highly resistant to chemicals, a bad conductor of heat and electricity, and can be moulded easily into shapes.
In simple words: Glass is a transparent, chemical-resistant, insulating supercooled liquid.

Teacher's Note:
1. The reason glass looks solid is its very high viscosity.
2. Add examples for each property.

Question 4. Write a short note on hydrochloric acid.
Answer:
It is prepared industrially by heating common salt with concentrated sulphuric acid, or by dissolving hydrogen chloride gas in water.
It is used to make silver chloride for photographic printing paper, as a laboratory reagent, to clean metal surfaces in industry and as a bathroom cleaner at home.
In simple words: Hydrochloric acid is made from salt and sulphuric acid and used for cleaning and in industry.

Teacher's Note:
1. Its formula is \(\ce{HCl}\) in water.
2. Keep two preparation methods in mind.

Question 5. “Sand and clay, which are abundantly available in the earth's crust, have significant roles in the comfort zone of modern life.” Comment on this statement.
Answer:
Sand is an important component of all mortars (lime mortar and cement mortar), and also of cement concrete and reinforced cement concrete (RCC), which build infrastructure in various fields of modern life.
Clay plays an important role in ceramics, bricks, building materials, sanitary equipment and household articles.
Because of these useful properties, they contribute greatly to the comfort of modern life.
In simple words: Sand and clay give us buildings, bricks, ceramics and sanitary ware.

Teacher's Note:
1. Back your comment with examples.
2. Both materials are cheap and easily available.

Question 6. Though fat provides the maximum amount of energy, it forms a minor part of our diet, while a large part of the diet contains carbohydrates. Why?
Answer:
Both fats and carbohydrates give energy and contain carbon, hydrogen and oxygen, but the structure of fat is more complex.
Carbohydrates break down quickly to glucose, a simple molecule, which is oxidized to carbon dioxide and water, releasing energy.
Fat takes a long time to be used. Fats are the reservoir of energy, while carbohydrates are the instant source, so carbohydrates form the major part of the diet.
In simple words: Carbohydrates give quick energy, fats are stored for later, so we eat more carbohydrates.

Teacher's Note:
1. Think of fat as a store and carbohydrate as ready cash.
2. Too much fat in the diet is also unhealthy.

Question 7. An apartment owner called a plumber to replace a leaking pipe line. The plumber found that the galvanized iron pipes had deteriorated and corroded, and advised replacing them with PVC pipes. Why did the plumber give this suggestion?
Answer:
PVC (polyvinyl chloride) pipes are thermoplastic, completely rust free and less expensive than galvanized iron pipes.
Although galvanization prevents rusting, pipes in continuous contact with water may lose the zinc coating over time and then corrode.
Hence PVC pipes are more durable for water lines.
In simple words: PVC does not rust, is cheaper and lasts longer than galvanized iron in water lines.

Teacher's Note:
1. The zinc coat wears off in running water.
2. PVC is a plastic polymer.

Question 8. A chemistry teacher said that, although bakelite and polyethylene both belong to the polymer family, only bakelite is used for making electrical insulators or handles of kitchenware. Why cannot polyethylene be used for this purpose?
Answer:
Polyethylene is a thermoplastic, so it softens and melts on heating. It cannot be used for handles or insulators that must stay hard when hot.
Bakelite is a thermosetting plastic. Once set into a hard mass, it cannot be softened or remelted, and it is a bad conductor of electricity.
In simple words: Polyethylene melts when hot; bakelite stays hard.

Teacher's Note:
1. Link the behaviour on heating to the use.
2. Electrical insulators must resist heat.

Question 9. An element X is non-malleable and non-ductile. X is extensively used to protect another element Y from corrosion. Identify X and Y and name the process. Also compare the reactivities of X and Y.
Answer:
X is zinc and Y is iron.
The process is galvanization (coating iron with zinc).
Zinc (X) is more reactive than iron (Y).
In simple words: Zinc coats iron to protect it, and zinc is the more reactive metal.

Teacher's Note:
1. Check the clue: non-malleable and non-ductile means zinc.
2. Because zinc is more reactive, it protects iron.

Question 10. Explain the principle involved in the use of chlorine in the sterilization of drinking water.
Answer:
Chlorine is highly soluble in water. On dissolving, it forms hypochlorous acid.
Hypochlorous acid decomposes to give nascent oxygen, which kills germs and bacteria.
Hence chlorine makes drinking water safe.
In simple words: Chlorine forms an acid that releases oxygen which kills germs.

Teacher's Note:
1. Nascent oxygen is freshly formed, very active oxygen.
2. Use only a small amount of chlorine in water.

Question 11. A sample of hard water is taken in two containers, A and B. A detergent is added to container A and soap to container B. The water in A is more efficient in cleaning than the water in B. Justify.
Answer:
Soap is less effective in hard water. Hardness is due to mineral salts, mostly of calcium and magnesium.
These salts react with soap to form an insoluble precipitate called scum, and lather formation is reduced.
A detergent forms soluble compounds with calcium and magnesium, so it does not form scum. Hence the water in A cleans better.
In simple words: Soap makes scum in hard water, while detergent does not.

Teacher's Note:
1. Scum is the curdy precipitate.
2. Detergent keeps working, so cleaning is better.

Question 12. Cement replaces lime mortar in the field of construction. Explain.
Answer:
Both cement mortar and lime mortar harden on exposure to air. But cement is a stronger binding material than lime, and through complex chemical reactions it forms a harder mass over time.
Combined with gravel, it forms concrete, which is much harder, and with steel it forms RCC, which is still stronger.
Hence cement is the better construction material.
In simple words: Cement binds more strongly and forms concrete and RCC.

Teacher's Note:
1. Compare strength and durability.
2. Mention concrete and RCC.

Question 13. People used to apply a coating of sodium silicate on eggs for preservation. Justify.
Answer:
When an egg is dipped in a solution of sodium silicate, a layer of sodium silicate forms on the shell.
This layer is impervious to water, air and heat, so bacterial growth is prevented and the egg can be preserved for a long time.
In simple words: The silicate coat seals the egg and keeps bacteria out.

Teacher's Note:
1. Water glass is another name for sodium silicate.
2. Air and water are needed for bacteria to grow.

Question 14. Name the metal which is extensively used in thermometers. Justify the reason for the same.
Answer:
The metal is mercury.
Mercury is a liquid at room temperature and expands evenly on heating. It has a high density, a high boiling point (357 °C), a shiny appearance that is easy to see, and it does not stick to the glass container.
In simple words: Mercury is a liquid metal that expands steadily when heated.

Teacher's Note:
1. It is the only metal that is liquid at room temperature.
2. Clinical thermometers use mercury.

Question 15. Pyrex or borosil glass is used for making laboratory apparatus, whereas crown glass is used in the manufacture of optical instruments. Give a reason for the same.
Answer:
Pyrex (borosil) glass contains borax, which reduces its expansion on heating and gives heat resistance. So it is used for laboratory apparatus.
Crown glass contains red lead, which greatly increases its density and transparency. This makes it useful for optical instruments.
In simple words: Borax gives heat resistance; red lead gives optical quality.

Teacher's Note:
1. Match the ingredient to the property.
2. Pyrex can withstand thermal shock.