Science 7–10 · Year 10

Salt, sugar and wax: properties that reveal ionic and covalent substances

Materials

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Ionic substances such as sodium chloride have high melting points and conduct only when their ions are free to move, while covalent molecular substances such as sugar and wax melt easily and never conduct.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

hot glass; molten wax is flammable and burns skin
caramelising sugar gives off irritating smoke
low-voltage electricity near water

Controls

wax is melted in hot water, never in the flame
salt and sugar heated gently for no more than 1 minute; eye protection; ventilated room
tester at 6 V or less; electrodes rinsed between tests

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. The RSC version melts zinc chloride and potassium iodide; this version keeps to solutions, so no molten salts are heated.

What you need

sodium chloride, sucrose and paraffin wax, about 2 g of each: the melting test uses about 0.5 g and the dissolving test a further 1.00 g
three borosilicate test tubes, test-tube holder, Bunsen burner, heat-resistant mat, 250 mL beaker of just-boiled water (adult-prepared) for the wax
conductivity tester (6 V supply or battery pack, LED or bulb, two graphite or stainless-steel electrodes), deionised water, three 50 mL beakers
balance reading to 0.01 g, stirring rod

How to do it

Test each dry solid with the conductivity tester and record the result.
Stand a tube holding about 0.5 g of wax in a beaker of just-boiled water; heat tubes of about 0.5 g of salt and of sugar gently in the Bunsen flame for up to 1 minute. Record whether each melts.
Dissolve 1.00 g of each in 20 mL of deionised water (the wax will not dissolve) and test each liquid with the tester; test deionised water alone as the control.
Tabulate melting behaviour, solubility and conductivity, and classify each substance as ionic or covalent molecular.

What you should see

No solid lights the tester. Wax melts within seconds (paraffin waxes melt at about 46 to 68 °C depending on grade); sugar melts and browns as it decomposes near 186 °C; salt does not melt in a test tube (sodium chloride melts at 801 °C). The salt solution (0.86 mol/L) lights the tester, the sugar solution and deionised water do not, and the wax does not dissolve. The learner knows it worked when only the salt solution conducts and the melting order is wax, then sugar, with salt unmelted.

What changes

What you change: substance (sodium chloride, sucrose, paraffin wax)
What you measure: melting behaviour and electrical conductivity as solid and in water
What you keep the same: mass of each substance
volume of deionised water
same tester and electrodes
heating time

Common misconceptions

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

Solid salt conducts electricity because salt water does.
Sugar solution conducts because it is a solution.
All compounds have high melting points.

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)SC5-MAT-01 SC5-WS-06
Australian Curriculum v9AC9S10I05

Sources

The pages the author read to write this activity.

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