Solutions, Solubility and Dissolving — Answers
Answer sheet with model answers and success criteria for the Year 8 mixtures & chemical reactions worksheet.
Success TutoringWorksheetsAnswer sheetYear 8 · Science · Mixtures & Chemical ReactionsSolutions, Solubility and Dissolving — Answers
Part A: The language of solutions
Read the passage, then answer the questions.
A solution forms when a solute dissolves in a solvent. Water is called the universal solvent because it dissolves so many substances. Substances that dissolve are soluble; those that do not are insoluble.
In the particle model, dissolving happens when the solute particles separate and spread out evenly between the solvent particles. The solute does not disappear, so the total mass stays the same.
A concentrated solution has a lot of solute in a given volume; a dilute solution has only a little. A saturated solution cannot dissolve any more solute at that temperature.
- solute
- solvent
- solution
- insoluble
- saturated
- the liquid that does the dissolving
- holds as much solute as it can at that temperature
- the substance that dissolves
- the mixture formed when a solute dissolves in a solvent
- does not dissolve
Answer: solute → the substance that dissolves; solvent → the liquid that does the dissolving; solution → the mixture formed when a solute dissolves in a solvent; insoluble → does not dissolve; saturated → holds as much solute as it can at that temperature
- AIt disappears and no longer exists.
- BIts particles spread out between the water particles.
- CIt turns into water.
- DIt becomes a gas.
Answer: Its particles spread out between the water particles.
The sugar particles separate and spread evenly between the water particles. The sugar is still there — the water tastes sweet.
- Asalt
- Bwater
- Cboth
- Dneither
Answer: water
Water dissolves the salt, so water is the solvent and salt is the solute.
Answer: True
Sand does not dissolve; it settles to the bottom and can be removed by filtering.
Answer: True
Mass is conserved when a substance dissolves: 100 g + 10 g = 110 g.
Part B: How fast does it dissolve?
Three factors affect how quickly a solid dissolves: the temperature of the solvent (hotter is usually faster), stirring (faster) and the size of the pieces (smaller pieces have a larger surface area, so they dissolve faster).
- Astirring the water
- Busing hotter water
- Ccrushing the cube into powder
- Dusing colder water
Answer: using colder water
Cold water particles move more slowly, so they break the sugar apart more slowly. The other three all speed up dissolving.
- APowder is a different substance.
- BPowder has a larger surface area in contact with the water.
- CPowder is heavier.
- DPowder is colder.
Answer: Powder has a larger surface area in contact with the water.
Smaller pieces have more surface exposed, so more water particles can reach the sugar at once.
Answer: solvent
Also accept: water.
Part C: Solubility data and concentration
Show your working.
Solubility of substance X (the greatest mass that dissolves in 100 mL of water)
0 °C: 15 g · 20 °C: 30 g · 40 °C: 50 g · 60 °C: 75 g · 80 °C: 100 g
Concentration tells us how much solute is in a volume of solution: concentration (g/L) = mass of solute (g) ÷ volume of solution (L).
Answer: 50 g
Answer: 10 g
- Only 30 g dissolves at 20 °C.
- 40 − 30 = 10 g undissolved
Answer: 45 g
- At 60 °C it holds 75 g; at 20 °C it can hold only 30 g.
- 75 − 30 = 45 g crystallises out
Answer: 100 g/L
- 250 mL = 0.25 L
- 25 ÷ 0.25 = 100 g/L
Answer: False
Solubility rises from 15 g at 0 °C to 100 g at 80 °C, so X is more soluble at higher temperatures.
Part D: Plan an investigation
Model answer: Independent variable: water temperature (for example 10 °C, 30 °C, 50 °C and 70 °C, using warm tap water rather than boiling water). Dependent variable: the time taken for the sugar cube to dissolve completely. Controlled variables: the same volume of water (100 mL), the same size and type of sugar cube, the same beaker and no stirring (or the same stirring). Method: measure 100 mL of water at each temperature, drop in one sugar cube, start a stopwatch and stop it when no sugar can be seen; repeat three times at each temperature and calculate the average. Prediction: the cube will dissolve fastest in the hottest water, because the water particles have more energy and move faster, so they collide with the sugar more often and pull its particles apart more quickly.
Success criteria:
- Correct independent and dependent variables
- At least two controlled variables
- Clear method with repeats and averaging
- Prediction explained with particle movement
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.