# 2.3–2.4 Dissolving and Water Purification

F.1 · Integrated Science · Water · Lesson 6

Read solubility curves and choose a water-purification method.

## Figure

Invented solute: practise units and scaling. Paper filtration: sand stays; dissolved salt passes through.

## Key points

- **Solute + solvent → solution:** sugar + water → sugar solution. Sugar particles remain, spread throughout the water.
- **Dissolving rate:** warmer water, smaller pieces and stirring usually dissolve the same sugar amount faster. Fair stirring test: fix sugar amount, water volume and temperature.
- **Solubility (溶解度):** maximum solute dissolving in a given amount of solvent at a specified temperature; here, $\mathrm{g/cm^3}$ of **solvent**.
- **Concentration:** amount actually dissolved per volume of **solution**. Dilute/concentrated compares amounts; saturated means the maximum is reached.
- **Saturated solution (飽和溶液):** no more solute dissolves at that temperature. Stirring raises rate, not the maximum.
- **Curve reading:** check axes/units; maximum mass = solubility × solvent volume: $0.20\,\mathrm{g/cm^3}\times50\,\mathrm{cm^3}=10\,\mathrm{g}$.
- **Temperature:** solubility may rise or fall; use that solute's curve, not another substance's trend.
- **Sedimentation:** gravity settles denser insoluble solids; alum forms larger flocs from fine particles, helping them settle.
- **Filtration (過濾法):** paper traps particles larger than its pores as **residue**; **filtrate** contains water and dissolved salt.
- **Distillation (蒸餾法):** boiling → vapour → condensation → distillate. Non-volatile impurities stay; some volatile contaminants pass over.

## Must know

| Impurity / task | Method and product | What can remain? |
|---|---|---|
| Settleable insoluble solids | Sedimentation: sediment settles | Fine suspended particles; dissolved substances |
| Insoluble particles larger than paper pores | Filtration: residue on paper; filtrate below | Dissolved substances; microorganisms |
| Collect water from salt solution | Distillation: water boils, then condenses as distillate | Non-volatile salt stays in flask; some volatile contaminants may pass over |

Clear water is not necessarily safe to drink; never taste laboratory water. During distillation, wear goggles, keep the outlet open and never boil the flask dry; work under teacher supervision.

## Common mistakes

- ✗ Dissolving faster means higher solubility. ✓ Rate is how quickly it dissolves; solubility is the maximum amount at a stated temperature.
- ✗ Stirring makes saturated water hold more sugar. ✓ At the same temperature, stirring does not increase solubility.
- ✗ $0.20\,\mathrm{g/cm^3}$ means only 0.20 g dissolves. ✓ It is **per cm³ of solvent**: in 50 cm³ of water the maximum is $0.20\times50=10$ g.
- ✗ Filter paper removes dissolved salt. ✓ Salt passes into the filtrate; distillation collects water while non-volatile salt stays behind.
- ✗ Clear water must be safe to drink. ✓ Dissolved substances or microorganisms can remain; never taste laboratory water.

## Quick check

1. Sugar dissolves in water. Name the solute and solvent. Has the sugar disappeared?
2. From the curve, find the maximum mass dissolving in 50 cm³ of water at 20 °C. If 14 g is added and allowed to reach saturation, how much remains undissolved?
3. Two samples at the same temperature have equal sugar amounts and water volumes; only one is stirred. What changes: dissolving rate, solubility, or both?
4. Sand and salt water are filtered as shown. What is the residue, and what does the filtrate contain?
5. How would you collect water from the filtrate in Q4? Name the two state-change processes and explain where the salt goes.

## Answers

1. Sugar: solute; water: solvent. No; dissolved sugar remains in the solution.
2. $0.20\times50=10$ g maximum; $14-10=4$ g remains undissolved.
3. Stirring increases dissolving rate; solubility is unchanged at that temperature.
4. Sand residue; water and dissolved salt in the filtrate.
5. Distillation: boiling then condensation; non-volatile salt stays in the flask.

## Did you know?

Water treatment needs more than a filter: after clarification and filtration, a disinfection step controls disease-causing microorganisms.