Everyday Chemistry Questions CBSE Class 9 Chemistry Worksheets Set 03

Questions with Answers

Question 1. What is dry cleaning?
Answer:
Dry cleaning means removing dirt and oily stains from clothes with an organic liquid, such as petrol or a similar solvent, without using water. The solvent dissolves grease and oils from the fabric.
In simple words: Cleaning cloth with a solvent instead of water.

Teacher's Note:
1. Grease dissolves in organic solvents, not in water.
2. Dry-cleaning solvents are volatile, so the cloth dries quickly.

Question 2. What is the difference between shaving soap and toilet soap?
Answer:
Toilet soaps are hard soaps made of sodium salts of fatty acids. Shaving soaps contain mainly potassium salts of fatty acids, which are softer and give a thick, creamy lather; glycerine is added to keep the lather from drying quickly.
In simple words: Toilet soap is sodium soap (hard), shaving soap is mainly potassium soap (soft).

Teacher's Note:
1. Sodium soaps are hard; potassium soaps are soft.
2. Soaps are made by saponification of fats and oils with an alkali.

Question 3. Why should we add copper while making gold ornaments?
Answer:
Pure gold (24 carat) is very soft and would wear out or bend easily. Adding a little copper makes the metal harder and stronger, so it can be used for ornaments. For example, 22 carat gold contains 22 parts of gold and 2 parts of other metal.
In simple words: Copper makes gold hard enough for jewellery.

Teacher's Note:
1. An alloy is a mixture of metals.
2. Carat tells the purity of gold: 24 carat is pure.

Question 4. What is demineralised water?
Answer:
It is water from which all dissolved ionic impurities (mineral salts) have been removed, for example by passing it through ion-exchange resins. It is used in batteries, irons and laboratories.
In simple words: Water that has no dissolved mineral ions.

Teacher's Note:
1. Distilled water is also free of ions.
2. Hard water contains dissolved calcium and magnesium salts.

Question 5. Slippery bathrooms may be cleaned with bleaching powder. What is the action?
Answer:
The slipperiness is caused by a growth of algae and fungus and by oily matter. Bleaching powder is an oxidising agent and it kills the growth and removes the oily film, so the floor is no longer slippery.
In simple words: Bleaching powder kills the fungus and oxidises the film.

Teacher's Note:
1. Bleaching powder is calcium oxychloride.
2. It also disinfects water.

Question 6. What is meant by hardening of oils?
Answer:
Vegetable oils are unsaturated liquids. When hydrogen is passed through them in the presence of a nickel catalyst, they become saturated solid fats such as vanaspati ghee. This process is called hydrogenation or hardening of oils.
In simple words: Adding hydrogen to a liquid oil, using nickel as a catalyst, turns it into a solid fat.

Teacher's Note:
1. Hydrogenation converts double bonds into single bonds.
2. Hydrogenated fats keep longer without becoming rancid.

Question 7. What is the composition of stainless steel?
Answer:
Stainless steel is an alloy of iron and carbon (steel) with about 10 to 18 per cent chromium and some nickel (about 8 per cent). Chromium makes it resistant to rusting.
In simple words: Steel with chromium and nickel that does not rust.

Teacher's Note:
1. An alloy is harder and more corrosion resistant than its pure metals.
2. Cutlery and kitchen sinks are made of stainless steel.

Question 8. How can a mixture of water and alcohol, which mix completely, be separated?
Answer:
By fractional distillation. Alcohol boils at about 78 °C and water at 100 °C. When the mixture is heated, the vapours rich in alcohol rise first, pass through the fractionating column and are condensed and collected separately.
In simple words: Heat the mixture and collect the alcohol first, since it boils at a lower temperature.

Teacher's Note:
1. Miscible liquids are separated using their difference in boiling points.
2. Simple distillation is enough if the boiling points differ greatly.

Question 9. Why do copper vessels get a greenish coating on long exposure to air?
Answer:
Copper slowly reacts with oxygen, carbon dioxide and moisture of the air to form basic copper carbonate, which is green. This is called corrosion of copper.
In simple words: Copper reacts with moist air to form green basic copper carbonate.

Teacher's Note:
1. Corrosion is slow chemical attack of a metal by air and moisture.
2. The green coating can be removed with lemon juice or tamarind.

Question 10. What is soda water?
Answer:
Soda water is water in which carbon dioxide gas has been dissolved under pressure. When the bottle is opened, the pressure falls and the gas escapes with fizzing. It is also called aerated water.
In simple words: Water with carbon dioxide dissolved in it under pressure.

Teacher's Note:
1. Solubility of a gas increases with pressure.
2. Warm soda loses its gas faster.

Question 11. A gold ring is discoloured when it comes into contact with a drop of mercury. Why?
Answer:
Mercury forms an alloy with gold called an amalgam, which is silvery and dull. When the ring is heated, mercury evaporates and the ring becomes bright again.
In simple words: Mercury and gold form an amalgam.

Teacher's Note:
1. An amalgam is an alloy of mercury with another metal.
2. Mercury vapour is poisonous, so heating should be done only in a well-ventilated place.

Question 12. What are N.P.K. fertilizers?
Answer:
These are fertilisers which supply the three major plant nutrients: nitrogen (N), phosphorus (P) and potassium (K). Nitrogen is supplied as ammonium or nitrate salts or urea, phosphorus as phosphates and potassium as potassium salts.
In simple words: Fertilisers giving nitrogen, phosphorus and potassium.

Teacher's Note:
1. Nitrogen helps leaf growth, phosphorus helps roots and potassium helps overall health.
2. Overuse of fertilisers pollutes soil and water.

Question 13. Name an element which can be used as a lubricant.
Answer:
Graphite, an allotrope of carbon. Its carbon atoms are arranged in layers that can slide over one another easily because the layers are held weakly, so it is slippery.
In simple words: Graphite is slippery because its layers slide over each other.

Teacher's Note:
1. Diamond is the other common allotrope of carbon.
2. Graphite also conducts electricity.

Question 14. What is 18 carat gold?
Answer:
It is an alloy of gold which has 18 parts of pure gold by weight and 6 parts of another metal, usually copper, in every 24 parts, that is 75 per cent gold.
In simple words: An alloy of 75 per cent gold and 25 per cent copper (or other metals).

Teacher's Note:
1. 24 carat gold is 100 per cent pure.
2. Percentage of gold = (carat / 24) x 100.

Question 15. What is LPG?
Answer:
LPG stands for liquefied petroleum gas. It is a mixture of hydrocarbons, mostly propane and butane, kept as a liquid under pressure in cylinders. A strong-smelling substance (ethyl mercaptan) is added so that leaks can be noticed.
In simple words: A mixture of propane and butane stored as a liquid under pressure and used as cooking fuel.

Teacher's Note:
1. On opening the valve the pressure falls and the liquid changes to gas.
2. Ventilate the room and do not light a flame if there is a gas smell.

Question 16. Printed matter cannot be washed off or bleached. Why?
Answer:
Printing ink contains finely divided carbon (carbon black) as its colouring matter. Carbon is not attacked by bleaching agents and does not dissolve in water, so the print cannot be washed off or bleached.
In simple words: The carbon in printing ink is insoluble and chemically unreactive.

Teacher's Note:
1. Carbon is very unreactive at ordinary temperature.
2. Writing ink made of dyes can fade in sunlight, but carbon does not.