Solutions, Solubility and Concentration — 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 Concentration — Answers
Part A: Dissolving
A solution forms when a solute dissolves in a solvent. Water is the most common solvent. The solute particles break away from the solid and spread out evenly between the solvent particles.
Dissolving is faster when the solvent is warmer, the mixture is stirred and the solute is in smaller pieces (more surface area).
Answer: solute
Answer: insoluble
Answer: A solution in which no more solute can dissolve at that temperature; any extra solid stays undissolved.
Answer: False
The sugar particles spread out between the water particles. The total mass of the solution equals the mass of water plus the mass of sugar.
- AStirring the mixture
- BUsing warmer water
- CUsing sugar cubes instead of fine sugar
- DCrushing the sugar into smaller pieces
Answer: Using sugar cubes instead of fine sugar
Cubes have less surface area exposed to the water than fine sugar, so they dissolve more slowly.
Answer: Stirring moves fresh solvent particles into contact with the solid and carries dissolved solute particles away, so solute particles are pulled away from the solid more quickly.
Part B: Solubility and temperature
Solubility is the greatest mass of a solute that dissolves in 100 g of water at a given temperature.
Solubility (g per 100 g of water) — Potassium nitrate: 20 °C 32 g, 40 °C 64 g, 60 °C 110 g. Sodium chloride: 20 °C 36 g, 40 °C 37 g, 60 °C 37 g.
Answer: The solubility of potassium nitrate increases a lot as temperature rises (from 32 g to 110 g between 20 °C and 60 °C). The solubility of sodium chloride hardly changes with temperature.
Answer: Not saturated; 14 g more could dissolve
- Solubility at 40 °C = 64 g per 100 g water
- 64 − 50 = 14 g more
Answer: 78 g
- At 60 °C: 110 g dissolved
- At 20 °C: only 32 g can stay dissolved
- 110 − 32 = 78 g of crystals
Answer: 16 g
- 32 g dissolves in 100 g water
- Half as much water dissolves half as much: 32 ÷ 2 = 16 g
Part C: Concentration
Concentration tells you how much solute is dissolved in a certain volume of solution. Concentration (g/L) = mass of solute (g) ÷ volume of solution (L). Remember 1000 mL = 1 L.
Answer: 40 g/L
- 500 mL = 0.5 L
- Concentration = 20 ÷ 0.5 = 40 g/L
Answer: 60 g/L
- 250 mL = 0.25 L
- 15 ÷ 0.25 = 60 g/L
Answer: 60 g
- Mass = concentration × volume
- 30 × 2 = 60 g
Answer: The concentration decreases (the solution is diluted), because the same mass of solute is now spread through a larger volume.
Part D: Plan an investigation
Model answer: Independent variable: the temperature of the water (for example 20 °C, 40 °C, 60 °C). Dependent variable: the time taken for the salt to dissolve completely. Controlled variables: the mass of salt (5 g), the volume of water (100 mL), the type of salt and grain size, the amount of stirring and the type of beaker. Method: heat water to each temperature and measure it with a thermometer, add 5 g of salt, stir at the same rate and time how long until no crystals can be seen. Repeat each temperature three times and calculate the mean. Prediction: the salt will dissolve faster in hotter water, because the water particles move faster and collide with the salt more often and with more energy.
Success criteria:
- Correctly identifies the independent and dependent variables
- Lists at least two relevant controlled variables
- Describes a clear, safe method with repeats
- Makes a prediction with a particle-model reason
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.