You will need
- 2 glass bottles with wide openings (we used milk bottles)
- 2 different food colours (we used yellow and blue)
- Hot water from the tap
- Cold water
- A tray
- An index card
- Optional: a freezer, thermometer and a stopwatch
Fill one of the glass bottles to the top with water and place into the freezer. You don’t want to freeze the water, you just want to chill it. Wait roughly 5 to 10 minutes. This step isn’t absolutely essential but it certainly makes the temperature difference between the bottles more pronounced.
When the cold water bottle is chilled, fill the other bottle with hot water to the top from the tap. Caution – if your tap is very hot use an adult’s help.
Place both bottles onto the tray. Add blue food colouring to the hot water and yellow food colouring to the cold water.
Keeping the index card as flat as possible, place the index card over the top of the yellow cold water bottle. Don’t squish down on the card, you just want to seal the bottle.
Carefully turn the cold yellow water bottle upside-down whilst gently pushing the index card upwards to keep the water sealed by the index card. If the seal is intact, you should be able to take your hand away from underneath the card and the water shouldn’t fall out. This works due to air pressure in the room pushing the water upwards. Full explanation in the upside-down water cup experiment here.
Carefully place the cold yellow water bottle directly over the top of the hot blue water bottle. Make sure that the openings of the bottles line up as close as possible!
Moment of truth! Holding the top water bottle, gently pull the index out of between the two glass bottles. You should see currents of hot blue water rise into the cold water and currents of cold yellow water fall down into the blue water.
As the experiment progresses you should find that the colours mix to form green. The currents will stop moving once both water bottles are the same temperature. How long did this take?
Get the Unit of Work on Heat Energy here!
- What actually is heat?
- How does heat move through different materials?
- How does heat change the properties of materials and more!
Includes cross-curricular teaching ideas, student quizzes, a sample marking rubric, scope & sequences & more
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What is going on?
Hot water rises and cold water falls! More specifically, hot water is less dense than cold water. When you removed the index card from between the two bottles, the dense cold water in the top bottle dropped downwards and the less dense hot water rose to the top of the bottles.
Why does this matter?
Liquids and gases both will move due to density differences. Because of this, water will move throughout the ocean due to uneven heating and different salinity levels causing density differences which then drives the water currents. The atmosphere also moves due to uneven heating, helping to cause changes in weather patterns. When we see the movement of liquids or gases due to uneven heating it is known as convection.
Variables to change in this experiment
- The temperature differences between the two bottles of water
- The opening size of the bottles… does this matter?
- Does this experiment work with salty vs. freshwater?
- Does this experiment work with different liquids?
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good stuff!
I did this for my science fair project.
Great!
on step #4 you referred to yellow as hot water bottle when it had been referred to as cold water bottle before and after that 4th step.
Thank you – fixed!