Mathematics K–6 · Years 5–6
Below zero: integers on a thermometer and in a lift
Number and algebra
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The idea
Numbers below zero sit on the same line as numbers above it, so a thermometer and a lift shaft show where negative numbers are and how far apart two integers lie.
Safety card
Hazards
- salt water
- glass thermometer
- salted ice can fall to about minus 21 degrees Celsius and can cause a cold burn if it or the cold cup is held against skin
Controls
- no drinking; an adult handles breakages; spirit thermometer only
- the cup stays in its tray and is moved only with the cloth around it; no hands or fingers in the salt and ice; wipe off any splash on skin straight away
Note
Only table salt, ice and water are used and nothing is heated, so the NSW Department of Education Chemical Safety in Schools package is not needed; ordinary classroom supervision under the school's usual risk management.
What you need
- a large vertical number line from minus 25 to 55 taped to the wall, marked at every degree with every fifth mark labelled, and a sliding marker, so that the salted-ice reading of about minus 21 degrees Celsius and the coldest and hottest station readings on the cards all sit on the line
- a spirit thermometer whose scale goes down to at least minus 25 degrees Celsius (salt and ice can reach about minus 21 degrees Celsius), a cup of crushed ice with salt stirred in, standing in a tray, and a cloth
- a set of blank temperature cards the class fills in from the Bureau of Meteorology climate data pages: the coldest morning and the hottest afternoon recorded at two Australian stations of their choice, with the station, date and page address written on each card
- a lift-shaft diagram with levels from minus 3 (car park) to 8
How to do it
- Place the marker at 0 on the wall line and move it to each temperature card in turn, reading the position aloud.
- Put the thermometer in the iced salt water and watch the reading fall below zero. Record the lowest reading.
- Find the difference between minus 5 and 3 on the line by counting the steps, then between minus 3 and minus 8.
- On the lift diagram, start at level 2, go down 5 levels and read the level. Then from minus 3 go up 7 and read.
- Order all the temperature cards from coldest to warmest and check against the line.
What you should see
Iced salt water takes the thermometer below zero, lower the more salt is stirred in but not below about minus 21 degrees Celsius, and the reading is counted down from 0. From minus 5 to 3 is 8 steps and from minus 3 to minus 8 is 5 steps. Level 2 down 5 is level minus 3 and minus 3 up 7 is level 4. The cards order from the most negative to the largest positive, and a reader who puts minus 8 above minus 3 because 8 is bigger corrects it by walking the marker down the line.
What changes
This activity lists no variables to change, measure and keep the same.
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Minus 8 is warmer than minus 3 because 8 is bigger than 3.
- Zero is the lowest number a thermometer can show.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
Sources
The pages the author read to write this activity.