Chemistry 11–12 · Year 11

Classifying substances as ionic, covalent molecular, covalent network or metallic by conductivity and melting behaviour

Module 1: Properties and Structure of Matter

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

Whether a substance conducts as a solid, as a melt and in water, and how readily it melts, follows from the kind of particles and bonds it contains.

Safety card

Medium riskLearners carry it out, with a teacher supervising

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

Hazards

zinc chloride is corrosive, and current passed through the melt electrolyses it and releases chlorine
hot crucibles and hot melts
wax overheated in an open crucible gives off flammable vapour that can catch fire; sucrose chars and smokes if heated past melting
electrical circuit near liquids
potassium iodide irritant

Controls

teacher melts the zinc chloride in a fume cupboard and switches the current off as soon as the lamp lights
wax and sucrose heated gently and the flame removed as soon as the solid has melted; a crucible lid and a fire blanket within reach to smother any flame
6 V supply only
eye protection
heat-resistant mat
let crucibles cool before handling

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions.

What you need

Conductivity tester: 6 V DC power pack, 6 V lamp in holder, two graphite electrodes in a holder, leads with crocodile clips (or a low-voltage conductivity probe and data logger)
Sodium chloride, zinc chloride, sucrose, paraffin wax and silicon dioxide (sand), about 4 g each, because each is used once in the 1 g solution test and once in a 2 g melting or strong-heating test; potassium iodide, about 2 g; a graphite rod, a copper strip and a zinc strip
Porcelain crucibles, three; pipe-clay triangle, tripod, Bunsen burner, heat-resistant mat, crucible tongs
Deionised water, 200 mL (20 mL for each of the six solution tests and 20 mL for the control, the rest for rinsing); 50 mL beakers, seven
Thermometer or temperature probe reading to 110 degrees Celsius (for the wax)

How to do it

Test each solid for conductivity by pressing both electrodes onto it; record whether the lamp lights.
Dissolve or attempt to dissolve 1 g of each of the six non-metal solids (sodium chloride, potassium iodide, zinc chloride, sucrose, wax and sand) in 20 mL deionised water; test each liquid for conductivity; test deionised water alone as the control.
Teacher, in a fume cupboard: melt 2 g of zinc chloride in a crucible with the two graphite electrodes clamped in it, switch the current on only once the solid has melted and off as soon as the lamp lights, because current through the melt electrolyses it and releases chlorine at the positive electrode. Learners: melt 2 g of sucrose and then 2 g of wax in separate crucibles with the electrodes in the melt, heating gently with a small flame and removing it as soon as each solid has melted, record whether each melt conducts, and record the temperature at which the wax melts.
Heat 2 g of sodium chloride and 2 g of sand in separate crucibles in the Bunsen flame; record whether either melts within one minute.
Tabulate solid, melt and solution conductivity and melting behaviour for each sample; assign each to ionic, covalent molecular, covalent network or metallic and justify the assignment.

What you should see

Copper, zinc and graphite conduct as solids. Sodium chloride, potassium iodide and zinc chloride do not conduct as dry solids; their solutions conduct, and molten zinc chloride conducts. Sodium chloride does not melt in a minute of Bunsen heating (its melting point is 801 degrees Celsius; the NIST WebBook data for solid sodium chloride end at 1074 K). Sucrose melts with browning near 188 degrees Celsius (the NIST WebBook gives 461 to 462 K) and wax melts well below 100 degrees Celsius; neither melt conducts, sucrose solution does not conduct and wax does not dissolve. Sand neither melts nor conducts. Deionised water alone does not light the lamp.

What changes

What you change: substance tested and its state (solid, melt, solution)
What you measure: conductivity (lamp on or off, or probe reading) and melting behaviour
What you keep the same: voltage
electrode spacing
sample mass
water volume

Common misconceptions

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

Ionic compounds conduct because they contain ions (they conduct only when the ions can move, in a melt or a solution).
Anything that dissolves in water will conduct (sucrose dissolves but gives no ions).
Metals conduct because electrons move freely between atoms, so molten metals stop conducting (they still conduct; delocalised electrons remain).

Curriculum references

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

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH11-8 CH11/12-3 CH11/12-4
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-11-01 CH-11WS-03
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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