TutorForAll

Science and Technology K–6 · Year 3

Mixing hot and cold water: where the heat goes

Science understanding: Physical 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

When hot and cold water mix, heat energy moves from the hotter to the colder until both share one temperature, and that temperature can be predicted from the masses and starting temperatures.

Safety card

Low riskAn adult supervises

Hazards

  • warm water

Controls

  • teacher pours water above 45 degrees Celsius
  • foam cups on a tray

Note

Water above 45 degrees Celsius is poured by the teacher. Risk assessment before the lesson using the NSW Department of Education Chemical Safety in Schools package (CSIS 1.7 Risk Assessment) or Primary RiskAssess.

What you need

  • 2 foam cups and 1 larger foam cup with lid
  • 2 thermometers or 1 digital probe thermometer (0 to 100 degrees Celsius)
  • 300 mL warm water at about 50 degrees Celsius, mixed by the teacher from kettle and tap water
  • 300 mL cold water at about 20 degrees Celsius
  • measuring cylinder 100 mL
  • kitchen balance

How to do it

  1. Measure 100 mL of cold water into cup A and 100 mL of warm water into cup B. Read and record both temperatures.
  2. Predict the temperature after mixing and write it down.
  3. Pour both into the large cup, put on the lid, stir gently with the thermometer for 20 seconds and read the temperature.
  4. Record the result beside the prediction.
  5. Repeat with 50 mL warm and 150 mL cold, then 150 mL warm and 50 mL cold.
  6. Compare each result with the prediction rule: final temperature = (mass1 x T1 + mass2 x T2) / (mass1 + mass2).

What you should see

Equal masses of 50 and 20 degree water give 35 degrees Celsius (a little lower in practice, because the cup and the air take some heat). 50 mL at 50 degrees with 150 mL at 20 degrees gives 27.5 degrees Celsius; 150 mL at 50 with 50 mL at 20 gives 42.5 degrees Celsius. The temperature never lands outside the range between the two starting temperatures.

What changes

What you change
mass of warm water compared with cold
What you measure
final temperature
What you keep the same
  • starting temperatures
  • cup type
  • stirring time

Common misconceptions

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

  • Temperatures add up: 50 and 20 make 70.
  • Cold moves into the warm water (heat moves from warm to cold; cold is not a substance).
  • The mixture ends at the temperature of whichever cup had more water.

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 2026ST2-8PW-STST2-1WS-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-PQU-01
  • Australian Curriculum v9AC9S3U03AC9S3I01AC9S3I03

Sources

The pages the author read to write this activity.

  1. www.scootle.edu.au/ec/search?accContentId=AC9S3U03
  2. curriculum.nsw.edu.au/learning-areas/science/science-and-technology-k-6-2024/outcomes
  3. www.nsw.gov.au/education-and-training/nesa/curriculum/science/science-and-technology-k-6-2017
  4. primaryconnections.org.au/teaching-sequences/year-3/scorching-swings-and-slides
  5. primaryconnections.org.au/v84-sequences/heating

All Concept Studio activities