Which row shows how the boiling point and the melting point of water change when a soluble impurity is added to the water?
| boiling point | melting point | |
|---|---|---|
| A | increases | increases |
| B | decreases | decreases |
| C | increases | decreases |
| D | decreases | increases |
▶️ Answer/Explanation
Ans: C
When a soluble impurity is added to water:
1. Boiling point increases – This is called boiling point elevation. The impurity particles interfere with the water molecules’ ability to escape into the gas phase, requiring more energy (higher temperature) for boiling to occur.
2. Melting point decreases – This is called freezing point depression. The impurity disrupts the formation of the regular ice crystal lattice, making it harder for water to freeze, thus requiring a lower temperature for freezing to occur.
This phenomenon is why salt is spread on icy roads (lowers melting point) and why adding salt to cooking water makes it boil at a slightly higher temperature.
Pure solid copper(II) nitrate can be obtained from a mixture of copper(II) nitrate and copper powder.
Three stages in the method are listed.
X – add water and stir
Y – crystallise
Z – filter
After the three stages, the copper(II) nitrate is washed and dried.
What is the correct order of stages X, Y and Z to obtain pure solid copper(II) nitrate from the mixture?
A) X → Y → Z
B) X → Z → Y
C) Y → X → Z
D) Z → X → Y
▶️ Answer/Explanation
Ans: B
The correct sequence is:
- X – Add water and stir: Copper(II) nitrate is soluble in water while copper powder is not. Stirring helps dissolve the copper(II) nitrate.
- Z – Filter: This removes the insoluble copper powder, leaving a solution of pure copper(II) nitrate.
- Y – Crystallise: By evaporating some water or cooling the solution, pure copper(II) nitrate crystals form.
Key points:
- You must dissolve first to separate the soluble and insoluble components
- Filtration must come before crystallization to remove impurities
- Crystallization is the final step to obtain the pure solid
This is a standard purification technique for separating a soluble compound from an insoluble one.
Some information about solid silver chloride and solid sodium chloride is shown.
- Silver chloride and sodium chloride do not dissolve in kerosene.
- Silver chloride is insoluble in water, but sodium chloride is soluble in water.
- The boiling point of silver chloride is 1547°C and the boiling point of sodium chloride is 1413°C.
Which processes are used to separate a mixture of solid silver chloride and solid sodium chloride?
A add kerosene, stir and then filter
B add water, stir and then filter
C add water, stir and then leave to crystallise
D add water, stir and then perform fractional distillation
▶️ Answer/Explanation
Ans: B
The key to separation is the difference in solubility:
1. Both compounds don’t dissolve in kerosene, so option A won’t work
2. Sodium chloride dissolves in water while silver chloride doesn’t
3. Therefore, adding water will dissolve NaCl but leave AgCl as solid
4. Filtration can then separate the insoluble AgCl (on filter paper) from the NaCl solution
Crystallization (option C) isn’t needed as we’re not trying to recover the NaCl yet, and fractional distillation (option D) is impractical due to the extremely high boiling points of both solids.
Question
. An alkane molecule of molecular formula \(C_8H_{18}\) undergoes cracking. The equation for the
reaction is shown.
\(C_8H_{18} → Q + 2R\)
Substance R has two carbon atoms per molecule and decolourises aqueous bromine.
What is substance Q?
A butane
B butene
C ethane
D ethene
Answer/Explanation
Ans: A