Science and Technology K–6 · Year 3

Melting ice: the thermometer stops at zero

Science understanding: Chemical sciences

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Adding heat to ice makes it melt, but while ice and water are together the temperature stays at 0 degrees Celsius because the energy is being used to change state, not to warm the water.

What you need

crushed ice, about 30 g, weighed (100 g on a bench takes about 3 hours to melt, which no lesson holds)
1 foam cup and 1 clear cup
digital probe thermometer (minus 10 to 110 degrees Celsius)
stirring rod
timer
graph paper

How to do it

Put the weighed crushed ice into the clear cup and push the probe into the middle. Read the temperature at once.
Leave the cup on the bench at room temperature. Stir gently and read the temperature every 10 minutes, and every 2 minutes once a little ice is left. Allow one to two hours for 30 g, and write the clock time beside each reading so the readings can be shared across a morning instead of watched without a break.
Record the minute at which the last piece of ice disappears.
Keep reading for 10 more minutes after the ice has gone.
Plot temperature against time and mark the flat part of the graph and the point where it starts to rise.
Explain why the flat part happens even though heat kept flowing in from the room.

What you should see

Ice straight from a freezer at about minus 18 degrees Celsius gives first readings below zero. Warming 30 g of ice from minus 18 to 0 degrees takes 30 x 2.09 x 18 = 1.1 kJ, about a ninth of the energy needed to melt it, so the reading climbs to 0 degrees early in the run. From then on it sits at 0 degrees Celsius, within the thermometer's accuracy, for as long as ice and water are both present, then climbs toward room temperature once the last ice has melted. The energy needed to melt 30 g of ice is 30 x 334 J = 10.0 kJ. A cup on a bench takes in heat from the room at only a few watts: at 2 W the plateau lasts 5,010 s (about 84 minutes) and at 3 W about 56 minutes, during which the temperature does not change. This is why the mass is 30 g and not 100 g, which needs 33.4 kJ and holds the plateau for more than 3 hours at 3 W; a heater delivering a steady 50 W would melt the 30 g in 200 s. The 10.0 kJ that melts the ice would warm the resulting 30 g of water by 80 degrees, which is why the plateau is long and the rise afterwards is fast at first.

What changes

What you change: time (an observation over time, not a fair test)
What you measure: temperature of the ice-water mixture
What you keep the same: mass of ice
cup
room temperature
stirring

Common misconceptions

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

Ice is always colder than 0 degrees Celsius.
The temperature rises as soon as heat goes in.
Melting ice is wet because water is stuck to it (the water is the ice, changed state).

Safety card

Low riskLearners carry it out

Hazards

cold burns from prolonged ice contact

Controls

handle ice with a scoop
wipe spills

Note

Risk assessment before the lesson using Primary RiskAssess or the school's own template.

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 and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST2-6MW-S ST2-1WS-S
Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST2-DAT-01
Australian Curriculum v9AC9S3U04AC9S3I03AC9S3I04

Sources

The pages the author read to write this activity.

All Concept Studio activities