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

Long Answer Questions

Teacher's Note:
1. Memorize one example per type.
2. N, P and K are the three main nutrients.

Teacher's Note:
1. Only proteins contain nitrogen.
2. Carbohydrates are quick energy, fats are stored energy.

Question 3. How is cement manufactured? Mention the areas of application of cement.
Answer:
The raw materials are limestone and clay.
Dry process: limestone and clay are taken in a definite proportion, pulverized to a fine powder and heated to 1500 °C to 1600 °C to form clinkers. The clinkers are pulverized again and a small amount of gypsum is added.
Wet process: water is added to the pulverized mixture to form a slurry, which is heated strongly to form clinkers; then as above.
Applications: grouting; cement mortar (cement : sand = 1 : 4) for joining bricks and plastering; concrete (cement : gravel : sand = 1 : 2 : 3) for roofs, pillars and beams; RCC for bridges, railway sleepers, prefabricated structures, columns, beams and roofs.
In simple words: Limestone and clay are ground, heated to clinkers, ground again with gypsum; cement then goes into mortar, concrete and RCC.

Teacher's Note:
1. Remember the temperature 1500 to 1600 °C.
2. Know the ratios of mortar and concrete.

Teacher's Note:
1. Make a table to remember types.
2. Annealing is always the last step.

Teacher's Note:
1. Link soap to its extra ingredient.
2. Soft soap uses potassium hydroxide.

Question 6. (i) What is the composition of pencil leads?
(ii) How is the hardness of a pencil related to the composition of pencil lead?
(iii) How are crucibles made with the help of graphite?
(iv) Why is graphite a more preferred lubricant than ordinary lubricating oils?
(v) What is the role of graphite in a nuclear reactor?
Answer:
(i) Pencil lead is a mixture of graphite and clay.
(ii) The more clay it contains, the harder the pencil.
(iii) Graphite is mixed with fine clay, moulded to the desired size and shape, and sintered.
(iv) Graphite resists heat and does not char, so it can lubricate machine parts that become very hot by friction, whereas ordinary oils char.
(v) It works as a moderator and slows down the neutrons produced when atoms of uranium or thorium split.
In simple words: Graphite is mixed with clay for pencils and also serves in crucibles, as a lubricant and as a moderator.

Teacher's Note:
1. More clay means a harder and lighter-marking pencil.
2. A moderator slows neutrons.

Question 7. (i) Name the metal which is used in the manufacture of high voltage electric transmission wire.
(ii) Why is aluminium used in the manufacture of household articles?
(iii) Write the name and the formula of the monomer of polyethylene.
(iv) Among which alloys is the percentage of zinc less (brass/bronze)?
(v) Why is silicone used in the manufacture of non-stick pans?
Answer:
(i) Aluminium.
(ii) Aluminium is light, corrosion resistant and malleable, and its alloys are light and impact resistant, so it is used for utensils and household fixtures.
(iii) Ethylene, \(\ce{C2H4}\).
(iv) Bronze (10% zinc, compared with 20 to 40% in brass).
(v) Silicone is heat resistant, non-sticky, water repellent and unreactive to chemicals.
In simple words: Aluminium for wires and utensils; ethylene is the monomer; bronze has less zinc; silicone is non-stick.

Teacher's Note:
1. Count the zinc percentages in the alloys table.
2. A monomer is the single repeating molecule.

Question 8. (i) What are hydrocarbons?
(ii) Give the formula of the simplest hydrocarbon.
(iii) What is biogas?
(iv) Give two advantages of biogas.
(v) Name the products obtained on burning methane gas.
Answer:
(i) Organic compounds containing only carbon and hydrogen.
(ii) Methane, \(\ce{CH4}\).
(iii) A mixture of gases formed when farm waste and cow dung decompose in the absence of air; methane is its major component.
(iv) It is an excellent fuel with a very high energy output for its mass, and it burns without harmful gas or solid residue (and it makes use of farm waste).
(v) Carbon dioxide and water vapour.
In simple words: Hydrocarbons have only C and H; methane burns cleanly to carbon dioxide and water.

Teacher's Note:
1. Biogas is made from waste.
2. Methane is also the main part of natural gas.

Question 9. (i) Name the inert gas used for filling weather observation balloons.
(ii) Why is radon used in cancer treatment?
(iii) Why is an alloy of tin and lead used in the manufacture of fuse wire?
(iv) Name the gas used in advertising sign boards.
(v) Why is argon used in filling electric bulbs?
Answer:
(i) Helium.
(ii) Radon is radioactive, and its radiation can be used to destroy cancer cells.
(iii) Both tin and lead have low melting points, so the alloy melts easily and breaks the circuit when too much current flows.
(iv) Neon.
(v) Argon is inert and prevents oxidation of the hot tungsten filament, making it last longer.
In simple words: Helium for balloons, neon for signs, argon in bulbs, radon in medicine, lead-tin for fuses.

Teacher's Note:
1. Learn the noble gas uses as a list.
2. Fuse wire must melt easily.

Question 10. Mention the appropriate term corresponding to the following sentences.
(i) A solution obtained by passing carbon dioxide through sugar solution flavoured with desired substances.
(ii) A substance formed by a highly concentrated sugar solution flavoured with desired substances.
(iii) A hard mass resembling the Portland rock used by the people for the construction of buildings.
(iv) The hardest compound of silicon, used in all kinds of grinding wheels.
(v) A complex organic compound used as food, which helps in the development of cells, growth of new cells and repair of damaged cells.
Answer:
(i) Soft drink.
(ii) Syrup.
(iii) Portland cement.
(iv) Silicon carbide.
(v) Protein.
In simple words: Soft drink, syrup, Portland cement, silicon carbide and protein.

Teacher's Note:
1. Each term appears in the chapter notes.
2. Carbon dioxide under pressure makes soda water.

Question 11. In an ‘aqua regia’ quiz competition, the quiz master asked one of the teams to name the two hardest substances, one of which is naturally occurring and the other is man-made. None of the teams could answer, so he gave a clue that both substances are used for cutting glass and drilling rocks and contain one common element. One of the teams then gave the correct answer. What could be the right answer?
Answer:
The two substances are diamond (natural) and silicon carbide (man-made, made from carbon and silicon).
The common element is carbon.
In simple words: Diamond and silicon carbide, which share carbon.

Teacher's Note:
1. Silicon carbide is made by heating carbon with silicon compounds.
2. Both are very hard, so they cut glass and drill rock.

Question 12. In a chemistry laboratory, a research scholar observed at a certain step that a yellow-coloured gas (X) was released, which was found to be poisonous. It was later made to react with a highly reactive metal (Y). A new compound (Z) was formed which is used for preservation of fish, meat and pickles. Identify X, Y and Z.
Answer:
The greenish-yellow poisonous gas X is chlorine.
The highly reactive metal Y is sodium.
Z is sodium chloride, used to preserve fish, meat and pickles. \(\ce{2Na + Cl2 -> 2NaCl}\)
In simple words: Chlorine plus sodium gives common salt.

Teacher's Note:
1. Salt as a preservative is the clue.
2. Sodium is highly reactive.

Question 13. Three naturally occurring elements X, Y and Z are good conductors of electricity and very precious. Z is more reactive than X and Y, and on exposure to air for a long time turns black. Z forms an alloy with Y and is used in jewellery, and X is used as a catalyst. Identify X, Y and Z.
Answer:
X is platinum, used as a catalyst.
Y is gold.
Z is silver, which turns black on long exposure to air due to the formation of silver sulphide. The silver-gold alloy is used in jewellery.
In simple words: Platinum is the catalyst, gold and silver form the jewellery alloy, and silver tarnishes.

Teacher's Note:
1. Silver is more reactive than gold and platinum.
2. Tarnish is silver sulphide.

Question 14. A reddish brown metal P and a silvery white metal Q are highly ductile and good conductors of heat and electricity. P plays a major role in the preparation of brass and bronze, whereas Q is the third most abundant element in the earth's crust, after oxygen and silicon, but Q is used as a substitute for P. Give reasons.
Answer:
P is copper (reddish brown; main metal of brass and bronze).
Q is aluminium (silvery white; third most abundant element in the earth's crust).
Both conduct heat and electricity well. Copper is costly, and aluminium is cheaper and lighter, so aluminium is used as a substitute for copper (for example in transmission lines and cookware).
In simple words: Copper is the costly metal and aluminium is a cheaper, lighter substitute.

Teacher's Note:
1. Aluminium is the most abundant metal.
2. Cost and weight decide substitution.

Question 15. A gaseous element X is used in the manufacture of bleaching powder. On reaction with hydrogen, X forms a substance Y, which after dissolution in water is used to clean metal surfaces. Identify X and Y. Justify the whole process involved.
Answer:
X is chlorine and Y is hydrogen chloride, whose aqueous solution is hydrochloric acid. \(\ce{H2 + Cl2 -> 2HCl}\)
Hydrogen chloride dissolves in water to give hydrochloric acid, which is used to clean metal surfaces. Metal surfaces have a coating of metal oxides, and hydrochloric acid reacts with these oxides and removes the coating.
In simple words: Chlorine plus hydrogen gives HCl, and its solution cleans metal oxide coatings.

Teacher's Note:
1. Acids react with metal oxides.
2. HCl is also a bathroom cleaner.

Question 16. A salt X of a metal which is used in the galvanizing process, on heating, produces an oxide of that metal Y and a gas Z. Z is used in the preparation of soda water, and Y is used as a base of white paint. Identify X, Y and Z.
Answer:
The metal used in galvanizing is zinc.
X is zinc carbonate, Y is zinc oxide and Z is carbon dioxide.
On heating: \(\ce{ZnCO3 -> ZnO + CO2}\). Carbon dioxide is used in soda water, and zinc oxide is the base of white paint.
In simple words: Heating zinc carbonate gives zinc oxide and carbon dioxide.

Teacher's Note:
1. Soda water uses carbon dioxide.
2. Zincite is another name for zinc oxide.

Question 17. A non-metallic element X is a good conductor of electricity in one form and a bad conductor in another form. Comment on the thermal conductivity of both forms. Identify the element and mention the two forms of X.
Answer:
X is carbon.
The two forms are graphite (good conductor of electricity) and diamond (bad conductor of electricity).
Diamond is a good thermal conductor, whereas graphite is a poor conductor of heat.
In simple words: Carbon exists as graphite (conducts electricity) and diamond (does not).

Teacher's Note:
1. Graphite is the exception among non-metals.
2. Forms of one element are called allotropes.

Question 18. A salt A on heating forms the oxide of a metal B and a gas C. B is used in white washing. When C is passed through water it forms D. Identify A, B, C and D. Justify the usage of C for the above purpose.
Answer:
A is calcium carbonate, B is calcium oxide (lime), C is carbon dioxide and D is carbonic acid (a weak acid).
\(\ce{CaCO3 -> CaO + CO2}\) on heating. CO2 dissolves in water to form carbonic acid.
Quick lime is added to water to give calcium hydroxide, which is used for white washing. On the wall, calcium hydroxide reacts with carbon dioxide of the air and forms a white powdery coating of calcium carbonate. Carbon dioxide also turns lime water milky.
In simple words: Calcium carbonate gives lime and carbon dioxide, and lime gives a white coating when used on walls.

Teacher's Note:
1. Lime water turning milky is the test for carbon dioxide.
2. The white wash hardens by taking in carbon dioxide.

Question 19. A non-metal X is lustrous, insoluble in water but soluble in alcohol. When it reacts with another metallic element Y, it forms a substance Z which is used in cloud seeding or artificial rain. Identify X, Y and Z. What are the advantages of artificial rain? Are there any substances other than Z that can be used for artificial rain? If so, what are they?
Answer:
X is iodine, Y is silver and Z is silver iodide.
Advantage: artificial rain is very useful in agriculture at the time of drought or water scarcity.
Other substances: silver nitrate and dry ice (solid carbon dioxide) can also be used.
In simple words: Iodine and silver give silver iodide, which seeds clouds.

Teacher's Note:
1. Cloud seeding helps in drought.
2. Dry ice is solid carbon dioxide.