Science 7–10 · Year 7

How much dissolves: a solubility curve for ammonium chloride

Solutions and mixtures

PracticalLow risk

The idea

The mass of a solid that a fixed mass of water can hold in solution rises with temperature, and a saturated solution gives back crystals as it cools.

Safety card

Low riskAn adult supervises

Hazards

ammonium chloride is harmful if swallowed and an eye irritant
hot water scalds

Controls

eye protection
hot-water beaker handled by an adult or with tongs
solutions washed down the sink with plenty of water

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.

What you need

ammonium chloride 2.6 g, weighed
boiling tube, stirring thermometer reading -10 to 110 °C (as in the RSC method), 10 mL measuring cylinder
deionised water: 4 mL to start, then 1 mL additions (the RSC method runs to 10 mL)
250 mL beaker of hot water (about 85 °C, adult-prepared), a beaker of iced water, and salt to make an ice-salt bath for the last points

How to do it

Put 2.6 g of ammonium chloride and 4 mL of water in the boiling tube and warm it in the hot-water beaker, stirring with the thermometer, until all the solid dissolves.
Move the tube to the iced water, keep stirring, and record the temperature at which crystals first appear.
Add 1 mL of water, redissolve in the hot water, cool again and record the new crystallisation temperature. Repeat to 8 mL of water; for 9 and 10 mL cool in an ice-salt bath.
Convert each point to grams of ammonium chloride per 100 g of water (2.6 g in V mL is 260/V g per 100 g, taking 1 mL of water as 1 g) and plot solubility against temperature.

What you should see

Each dilution crystallises at a lower temperature than the last, so the plotted points form a rising curve: solubility increases with temperature. Interpolating the CRC Handbook solubility table for ammonium chloride (29.4 g per 100 g of water at 0 °C to 65.6 g at 80 °C) predicts first crystals at about 79 °C for 4 mL (65 g per 100 g), 53 °C for 5 mL, 34 °C for 6 mL, 20 °C for 7 mL and 8 °C for 8 mL (32.5 g per 100 g); the 9 and 10 mL solutions (28.9 and 26 g per 100 g) are below the 0 °C solubility, so they crystallise only below 0 °C, where the table gives no values; they are cooled in the ice-salt bath and, if their first crystals appear within the thermometer's range, plotted without a prediction to compare against. Readings often come a few degrees low because a stirred solution can cool slightly past saturation before the first crystal appears. The learner knows it worked when the points fall on one smooth rising curve close to the predicted temperatures and a repeat of any single point agrees within 2 °C.

What changes

What you change: temperature at which the solution is saturated
What you measure: solubility in g per 100 g of water
What you keep the same: mass of ammonium chloride
stirring
same thermometer
reading taken at first crystal

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

A solution that looks clear cannot hold any more solute.
Heating makes the solute disappear rather than dissolve.
Solubility is the same at every temperature.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-SOL-01 SC4-WS-05
Australian Curriculum v9AC9S7U06AC9S7I04

Sources

The pages the author read to write this activity.

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