Science 7–10 · Year 8
Convection: watching a coloured current circulate in heated water
Physical sciences — Change, content group Energy transfers (NSW Stage 4 focus area)
School laboratory, not for home
In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
The idea
Heated liquid expands, becomes less dense and rises while cooler liquid sinks to replace it, setting up a circulating current that carries thermal energy through the fluid.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- hot glass and water
- naked flame
- beaker cracking if heated hard on one spot
Controls
- gentle flame, never boiling
- eye protection
- beaker on gauze and mat
- let glass cool before moving
Note
Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.
What you need
- 600 mL beaker of room-temperature tap water, 1
- ice cubes made with a few drops of food colouring, 2
- Bunsen burner, tripod, gauze and heat-resistant mat, 1 set (or a small candle under one corner)
- tongs, 1
- torch to side-light the beaker, 1
- eye protection for all
How to do it
- Fill the beaker to 500 mL and let it stand until the water is still.
- Predict where the coloured water will go when a coloured ice cube is placed at the surface on one side; sketch the prediction.
- Using tongs, lower a coloured ice cube gently onto the surface near one wall and watch the coloured melt-water for one minute. Sketch what it does.
- Remove the cube, wait for the water to settle, then heat one edge of the beaker base gently with a small flame (or candle) and drop a fresh coloured cube at the surface on the opposite side.
- Watch and sketch the path of the colour over two minutes, marking rising and sinking regions and the direction of the loop.
- Turn off the flame and record how long the circulation continues.
What you should see
Cold coloured melt-water sinks in a plume down the wall and spreads across the base. With one side heated, the colour is carried across the base to the heated side, rises there in a visible column, spreads along the surface and sinks again on the cool side: a closed loop. The loop slows and stops some time after the flame is removed, and the learner records how long.
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.
- Heat rises (rather than hot fluid rising).
- The colour moves because the flame pushes it.
- Convection happens in solids.
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 7–10 Syllabus (2023)SC4-CHG-01SC4-WS-01SC4-WS-02
- Australian Curriculum v9AC9S8U05AC9S8I01AC9S8I04
Sources
The pages the author read to write this activity.