Properties of Materials, Grade 7: Melting Point, Boiling Point and Conductivity

Water does not boil at 100 °C in Johannesburg. It boils at about 94.

You will see 96 °C printed. At 1 753 metres the real figure is closer to 94, and 96 corresponds to roughly 1 100 m, which is nowhere in Gauteng.

The remarkable thing is that the next page hands the class the apparatus to check it. A beaker, a burner and a thermometer, heating water and reading the temperature every two minutes. A Johannesburg class can measure the printed number and find it wrong, which is the best lesson in the topic.

Where this fits in the curriculum

Subject Natural Sciences
Grade 7
Term 2
Strand Matter and Materials
Topic Properties of materials
Status Class activities. Nothing here is formally assessed
Marks The topic revision is 30. Our worksheet is 40

Boiling point changes with altitude, and South Africa is high

Place Altitude Water boils at about
Durban, Cape Town Sea level 100 °C
Bloemfontein 1 395 m About 95 °C
Johannesburg, Pretoria 1 753 m About 94 °C
Lesotho highlands Over 2 500 m About 91 °C

The rule of thumb is roughly 1 °C lower for every 300 m of altitude. Air pressure falls as you go up, so the water does not have to be pushed as hard to turn into steam, and it boils sooner.

This is also why food takes longer to cook on the Highveld. The water is boiling, but it is boiling cooler, so the cooking is slower. It is a real thing every Gauteng household already knows without knowing why.

The practical that measures it

  1. Put about 150 ml of water in a beaker on a gauze on a tripod
  2. Stand a thermometer in the water, not touching the bottom
  3. Record the temperature every two minutes, from the start
  4. Keep going for several minutes after it starts boiling
  5. Plot temperature against time

The two things that make this work:

Keep the thermometer bulb off the bottom of the beaker. The glass is hotter than the water and a bulb resting on it reads high.

And keep recording after it boils. The flat line is the point of the whole graph: once it is boiling, the temperature stops rising however hard you heat it. That flat line is where you read your local boiling point off the graph.

Melting and boiling points worth knowing

Substance Melts at Boils at
Ethanol -114 °C 78 °C
Water 0 °C 100 °C at sea level
Lead 327 °C 1 749 °C
Table salt 801 °C 1 465 °C
Silver 962 °C 2 162 °C
Gold 1 064 °C 2 856 °C
Copper 1 085 °C 2 562 °C
Iron 1 538 °C 2 862 °C

A pure substance has a sharp melting point. A mixture melts over a range, because its parts do not all change state at once. That is one of the two real tests for purity, and the reason this table is in the topic at all.

Testing thermal conductivity: use ONE heat source

You will see this set up with a separate burner under each metal rod, and then be asked to keep all the other variables the same.

Three flames cannot be matched by eye, and the size of the flame is the variable that matters most. The test is unfair before it starts.

The fix is to use one heat source for all the rods at once. Curiously, the correct design appears later in the same book, in the heat transfer topic, so it is worth borrowing from there.

How to run it properly:

  1. Take rods of copper, aluminium and iron, the same length and thickness
  2. Stick a blob of Vaseline or candle wax at the SAME distance along each one
  3. Arrange all three so one heat source warms all their ends together, or stand all three in one beaker of boiling water
  4. Time how long each blob takes to melt or slide

Copper first, then aluminium, then iron. That order is the result, and it is also why pots are made of the first two and not the third.

The graph in the skills section is wrong twice

Worth checking before a class copies the format, because they are told to copy it exactly.

There is a worked example titled "acceleration of a trolley down a hill", with a straight line rising from the origin, and the vertical axis labelled "Acceleration (metres per second)".

Problem Correction
The unit is wrong Acceleration is metres per second squared. Metres per second is velocity
The graph is the wrong quantity A trolley rolling down a hill has constant acceleration. A line rising steadily from the origin is a velocity-time graph

Relabel the axis "Velocity (metres per second)" and everything is correct. As printed, the one place a learner is told to copy a format exactly contains a unit error and a concept error together.

Fair testing, and a force you cannot actually control

A fair test changes one thing, measures one thing, and keeps everything else the same.

The flexibility test in this topic breaks its own rule. It lists "the force you apply" as something to keep constant, and then tells learners to bend each strip by hand.

Hands cannot apply equal force, so the one variable that matters is the one left uncontrolled.

The fix is to let gravity do it. Clamp each strip so the same length sticks out over the edge of a bench, hang the same mass on the end, and measure how far the tip drops. Same force every time, and a number instead of an opinion.

Insulation: the one activity that is properly designed

Three identical containers, the same volume of hot water in each, wrapped in different materials, temperature read every few minutes for ten.

This one is a genuinely fair test as written, and it is the rehearsal for the Term 3 project on designing an energy-efficient home. Worth running carefully for that reason alone.

If it does not work

Problem Cause Fix
The thermometer reads over 100 °C The bulb is touching the bottom of the beaker Lift it clear. The glass is hotter than the water
It never reaches 100 °C and the class thinks it failed Nothing. You are on the Highveld That is the result. Read the flat line and work out your altitude from it
The temperature keeps climbing past boiling The thermometer is in the steam, not the water Push it back under the surface
All three metal rods seem the same Three separate flames, or blobs at different distances One heat source, blobs at equal distances
The flexibility results are all over the place Bending by hand Hang the same mass on each and measure the droop
The water takes forever to boil Too much water, or a small flame 150 ml is plenty. A lid speeds it up but take it off before you read

How the 40 marks are made up

Part Marks
Physical properties and what they are for 8
Melting and boiling point, including altitude 12
Thermal and electrical conductivity 8
Fair testing and graphing 8
Choosing a material for a job 4

Nothing in this topic is formally assessed. The topic revision is 30 marks. Our worksheet is 40 and the split above is ours.

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

Related practicals

What you need to run it

Item Price What it is for
Thermometer, spirit filled R35 The one thing this topic really needs. Buy a class set
Beaker, borosilicate, low form R19 One per group
Tripod stand R205 Shared
Gauze, wire with ceramic centre R250 Shared, so the beaker is not in the flame
Spirit burner R260 Shared
Retort clamp with bosshead R315 For the flexibility test, to hold the strip while a mass hangs on it

At R35 a thermometer, a class set of ten is R350, and it is used again in the insulation activity here, twice more in the heat transfer topic, and in the Term 3 project. It is the most re-used instrument in the Grade 7 year, so it is the sensible thing to buy properly.

Thermometers are in thermometers, the glassware is in laboratory glassware, and the stands are in retort stands and supports.