Pupil sheet · Crystallisation
Why do crystals appear as it cools? — questions
Name ______________________ Date ____________
Try every question before you use a nudge. Write in full sentences where you are asked to explain.
1. explain
Match water and copper(II) sulfate to their destinations during gentle partial evaporation. Explain why the remaining solution becomes more concentrated.
Write the number of the matching option.
- Some leaves as vapour
- Stays in the basin
- Water: ______
- Copper(II) sulfate: ______
Why does concentration increase?
Nudge (only if you are stuck): Which component leaves as vapour? Did you add any extra salt?
2. explain
Order the four preparation actions before collecting crystals. Explain why heating stops before the basin becomes dry.
Write the number of each step in order.
- Let the main solution cool and stand
- Stop heating: test drop has crystals
- Gently evaporate some water
- Test a cooled drop
- First: ______
- Second: ______
- Third: ______
- Fourth: ______
Why keep liquid in the basin?
Nudge (only if you are stuck): Which small sample tells you when the main solution is ready to cool? What do crystals need to grow from?
3. explain
A student cools dilute copper(II) sulfate solution and sees no crystals. Explain a possible reason. Describe how to try again without replacing the salt sample.
Why no crystals?
How would you repair the attempt, and why?
Nudge (only if you are stuck): Can the remaining water still keep the salt dissolved? How could you change that without heating everything dry?
4. explain
A sufficiently concentrated solution of this salt cools. Choose every correct statement and explain what forms the solid and what remains in the liquid.
Tick every correct statement.
- Some dissolved salt forms crystals.
- The water has frozen into blue crystals.
- Some salt stays dissolved in liquid.
- All salt must crystallise.
What are the crystals and the remaining liquid?
Nudge (only if you are stuck): Are the crystals frozen water? Does every bit of salt have to leave the solution?
5. explain
You want crystals of one of the two soluble solids in the observations. Choose one for cooling crystallisation. Explain how partial evaporation helps, and why the same cooling claim cannot be applied to both solids.
| Solid | Warm water | Cool water |
|---|---|---|
| A | Much more can dissolve | Much less can dissolve |
| B | About the same can dissolve | About the same can dissolve |
Choice and evidence
Role of partial evaporation
Why B may need a different approach
Nudge (only if you are stuck): For which solid does much less stay dissolved when cool? What does removing some solvent change first?