Q1.
(Step 9) Group the compounds showing similar properties listed in Table 9.2.
Answer
These are the observations you should record in Table 9.2.
| Compound | Solubility in | Electrical conductivity in | |||
|---|---|---|---|---|---|
| water | kerosene | petrol | solid state | water | |
| Camphor | Insoluble | Soluble | Soluble | No | No |
| Sodium chloride | Soluble | Insoluble | Insoluble | No | Yes |
| Copper sulfate | Soluble | Insoluble | Insoluble | No | Yes |
| Sugar | Soluble | Insoluble | Insoluble | No | No |
| Naphthalene | Insoluble | Soluble | Soluble | No | No |
The compounds fall into three groups.
- Group 1 — ionic compounds: sodium chloride and copper sulfate. Soluble in water, insoluble in kerosene and petrol; do not conduct as solids but do conduct in water. Both also have high melting points.
- Group 2 — covalent compounds that are insoluble in water: camphor and naphthalene. They dissolve instead in kerosene and petrol, and never conduct. Both have low melting points and a strong smell — they sublime easily.
- Group 3 — a covalent compound that is soluble in water: sugar. It behaves like Group 1 in the solubility column but like Group 2 in the conductivity column — it dissolves without producing ions.
Why it happens: the rule behind every entry is “like dissolves like”. Water molecules are polar — they have slightly charged ends — so they can surround and pull apart the charged ions of an ionic lattice. Kerosene and petrol are non-polar, so they have no such grip on ions, but they mix happily with non-polar molecules like camphor and naphthalene. Sugar is a special case: it is covalent, but it carries several —OH groups that water can hold on to, so it dissolves — as whole neutral molecules, which is why the bulb stays dark.
Safety first: petrol and kerosene are highly flammable, so keep them away from flames and work in a ventilated place. Do not touch the electrodes while the battery is connected, and use a low-voltage battery only.