Quick answer: A covalent bond forms when two non-metal atoms share a pair of electrons, so each atom gains a full outer shell. This Chemistry 9-10 lab on covalent bonding gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Covalent substances look very different: wax melts in seconds, while sand and graphite hardly change in a flame. This lab tests four solids with simple apparatus to link melting, conduction and solubility to structure, so you can predict whether a substance is made of small molecules or a giant network.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C2
Aim
To compare the melting behaviour, electrical conductivity and solubility of four covalent solids and decide which have simple molecular structures and which are giant.
Equipment
- Samples of candle wax, sugar, dry sand and a graphite rod (pencil lead)
- Metal bottle lid or deflagrating spoon, tripod, gauze and small burner (or hot plate)
- Tongs, stopwatch, 3.0 V battery, ammeter, two crocodile clips, leads
- Four beakers with 50 cm³ of water, glass rod, goggles
Safety
Wear goggles. Hot metal looks cool, so lift lids only with tongs. Keep hair tied back. Do not smell the burning sugar closely and keep the room ventilated.
Method
- Place 0.5 g of the first sample on a metal lid over the flame and start the stopwatch. Record the time to melt, or what happens in 180 s. Repeat for each sample.
- Connect the circuit shown, with each solid sample between the clips. Record the current for each, repeating the graphite reading three times.
- Stir 1 g of each sample in 50 cm³ of water for one minute and record whether it dissolves.
Results
Sample data from Aarav’s group:
| Sample | Time to melt / s | Current / mA | Dissolves in water? |
|---|---|---|---|
| Candle wax | 25 | 0 | no |
| Sugar | 95 (turns brown) | 0 | yes |
| Dry sand | no change in 180 s | 0 | no |
| Graphite rod | no change in 180 s | 118, 122, 120 | no |
Analysis questions
Question 1 Classify the samples
Use the results to classify each sample as simple molecular or giant covalent.Question 2 Mean current and resistance
Calculate the mean current for graphite in mA, then use the 3.0 V supply to calculate its resistance in ohms.Question 3 Why graphite conducts
Explain why graphite conducts when the other three samples do not.Question 4 Wax and sand
Explain why wax melts in seconds but sand does not change.Question 5 The browning sugar
The sugar turned brown as well as melting. Suggest what this shows.Question 6 Solubility
Sugar dissolved but wax did not, although both are simple molecular. State what this shows.Question 7 Fair test and improvement
State two variables you would control, and suggest one improvement to the melting test.
