Page 61
1. What would be the electron dot structure of carbon dioxide which has the formula CO2?
Answer:
2. What would be the electron dot structure of sulphur which is made up of eight atoms of sulphur?
Answer:
Page 68 – 69
1. How many structural isomers can you draw for pentane?
Answer:
Three. They are n-pentane, iso-pentane, and neo-pentane.
2. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
Answer:
(i) Tetravalency (ii) Catenation.
3. What will be the formula and electron dot structure of cyclopentane?
Answer:
Molecular formula: C5H10.
Electron dot structure:
4. Draw the structures for the following compounds: (i) Ethanoic acid (ii) Bromopentane (iii) Butanone (iv) Hexanal
Answer:
(i) Ethanoic acid (CH3COOH)
(ii) Bromopentane (C5H11Br)
(iii) Butanone (CH3—CH2—COCH3)
(iv) Hexanal (C5H11CHO)
5. How would you name the following compounds?
Answer:
(i) Bromoethane
(ii) Methanal
(iii) 1–Hexyne
Page 71
1. Why is the conversion of ethanol to ethanoic acid an oxidation reaction?
Answer:
Addition of oxygen to a substance is called oxidation. Here, oxygen is added to ethanol by an oxidizing agent (alkaline potassium permanganate or acidified potassium dichromate) and it is converted into acid.
2. A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?
Answer:
Burning of ethyne in air produces a sooty flame due to incomplete combustion. It is not enough to melt metals for welding.
Page 74
1. How would you distinguish experimentally between an alcohol and a carboxylic acid?
Answer:
| Test | Alcohol | Carboxylic acid |
|---|---|---|
| Litmus test | No change in colour. | Blue litmus solution turns red. |
| Sodium hydrogen carbonate test | C2H5OH + NaHCO3 → No reaction | CH3COOH + NaHCO3 → CH3COONa + H2O + CO2. Brisk effervescence due to evolution of CO2. |
2. What are oxidising agents?
Answer:
Oxidising agents are substances which give oxygen to another substance or remove hydrogen from a substance. E.g., acidic K2Cr2O7. It oxidises ethanol into ethanoic acid.
Page 76
1. Would you be able to check if water is hard by using a detergent?
Answer:
No, because detergents can lather well even in hard water. They do not form insoluble calcium or magnesium salts (scum).
2. People use a variety of methods to wash clothes. Usually after adding the soap, they ‘beat’ the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?
Answer:
When the cloth wetted in soap solution is agitated or beaten, the micelles containing oily or greasy dirt get removed from the surface of the cloth and go into water.