The exercise
Five of the twenty exercise questions, worked through. They are the ones that test whether the chapter went in rather than whether it was remembered.
Question 1 — the vocabulary
Explain the meaning of the following words
a. mixture b. homogeneous mixture c. solvent d. solute e. solution f. solubility
The answer
- Mixture — matter in which two or more components are mixed without being chemically changed, separable again by physical methods.
- Homogeneous mixture — a mixture with one and the same composition throughout.
- Solvent — of the components mixed, the one present in the larger amount.
- Solute — the remaining components, present in the smaller amounts.
- Solution — another name for a homogeneous mixture.
- Solubility — the maximum mass of a solute that dissolves in 100 g of a solvent at a given temperature.
Question 4 — explaining three observations
Explain the following observations scientifically
a. Jackfruit latex cannot be washed off with water. b. Styrofoam can be dissolved in petrol. c. Gas bubbles leave the solution the instant the cap of a soda bottle is opened.
The answer
a. Latex is a non-polar substance and water is a polar solvent. Like dissolves like, and these two are not alike, so the latex does not dissolve and cannot be washed away.
b. Styrofoam and petrol are both non-polar, so by the same rule the styrofoam dissolves in the petrol.
c. The carbon dioxide was dissolved under high pressure. Opening the cap drops the pressure above the liquid, the solubility of the gas falls with it, and the excess gas leaves as bubbles.
Question 8 — a mass from a concentration
The problem
500 cm³ of an aqueous solution of magnesium chloride (MgCl₂) of composition 0.5 mol dm⁻³ has to be prepared. Find the mass of magnesium chloride to be taken. (Mg = 24, Cl = 35.5)
The answer
molar mass of MgCl2 = {24 + (35.5 × 2)} g mol-1 = 95 g mol-1
amount needed = 0.5 mol dm-3 × 0.5 dm3 = 0.25 mol
mass needed = 95 g mol-1 × 0.25 mol
= 47.5 g
Question 10 — ordering the saltern salts
The problem
In salt production several salts are deposited in each of the ponds. Arrange CaCO₃, CaSO₄, NaCl and MgCl₂, which are deposited in that way, in order of decreasing solubility.
The answer
A salt crystallises when its saturation point is passed, so the least soluble salt comes out first and the most soluble comes out last. The order they are deposited in is therefore the order of increasing solubility.
In decreasing solubility: MgCl2, NaCl, CaSO4, CaCO3.
Question 18 — how the condenser is plumbed
The problem
When setting up a Liebig condenser for distillation in a school laboratory it is fixed sloping to the vertical, and vapour is admitted to it at the upper end. Water is admitted at the lower end. Explain the importance of (a) admitting vapour from above and (b) admitting water from below.
The answer
a. Vapour entering at the top means the liquid that condenses runs downhill under gravity and drains out of the far end, rather than collecting in the tube and blocking the vapour behind it.
b. Water entering at the bottom keeps the outer jacket full — water fed in at the top would run out leaving air pockets. It also makes the water flow against the vapour, so the vapour meets colder water the further it travels and condenses completely.