Materials needed per group
- 1 round balloon
- Food colouring
- Washing-up liquid
- Large plastic tub (that the ice balloon can fit in to), preferably transparent or translucent, e.g. a deep school drawer tray or a storage box
- Magnifying glass
Materials available per group or for the whole class
- Blow into the balloon to partially inflate it and then let it deflate.
- Put a couple of drops of food colouring into the balloon.
- Put the end of the balloon over a tap and fill it up with water – the balloon will stretch, so hold it carefully. Tie it when it is approximately the size of two adult-sized clenched fists.
- Put the balloon into a plastic bag (just to ensure there are no spills) and place it in the freezer for at least 24 hours.
- Cut and peel the balloon skin from the frozen ice balloon.
- Allow your students free investigation of the frozen balloon. Prompt them with questions about its appearance and the way it behaves. Allow anywhere between 15 and 45 minutes.
- Try the activities on the downloadable pdf. Ensure that you or any adult helpers record the questions the students ask and help them to follow their lines of investigation.
- Observe how the ice balloon melts in the water and the movements it makes as it melts. The part of the balloon in contact with air and the part submerged under water melt at different rates. This changes the shape of the balloon and shifts its centre of gravity, causing it to move.
- When the ice balloon is a lot smaller ensure that you go back to it and do some further investigation.
- Encourage students to think of and try other investigations using the ice balloons.
- Use strong balloons as they will get heavy when full of water!
- If you think your students would benefit from more guidance, you can create mini investigations focusing on specific questions, e.g. what happens if they put salt on the balloon, or what can they see if they cut it in half or shine a torch on it?
- Running the ice balloon under water or adding salt to it will drastically alter its shape, so bear this in mind if you still have a lot of investigation to do.
- If you are investigating the ice balloon outside of water the tray will come in very useful for keeping things clean.
- What happens if you add salt to the ice balloon? Salt lowers the freezing point of the water and has an almost instant effect.
- Why do you think there are air bubbles in your ice balloon? This is caused by air dissolved in the water, which becomes visible when the water freezes.
- Are there any cracks in the ice balloon? Why?
- What happens if you crack your ice balloon in half?
- What does the ice balloon look like through a magnifying lens?
- Why do you think the balloon moves as it melts?
- Is there anything else that you want to try out on your ice balloon?
- What happens if you put the ice balloon under running water?
- If you add more food colouring to your ice balloon do you notice it behaving differently if salt has already been added?
- Does the weight of the additional food colouring affect the balance of the ice balloon?
- For younger children this investigation can be used in partnership with learning about floating and sinking. You could begin to look at other objects and test whether they float or sink. The density of materials changes depending on whether they are solid or liquid. Water is one of the few materials that is less dense when it is a solid.
- For older children use ice balloons to investigate insulation. Test different materials to see which best slows melting (measure the amount of melt water or weigh the remaining ice and record your findings). Consider if water or air is a better insulator by seeing how much ice balloons melt inside and outside the water. Look out for the tide mark which shows which part of the ice balloon has been floating above the surface.
- Try dropping small objects (such as paper clips or small toys) into the balloon before freezing. Where in the balloon do the objects freeze (jn the middle or near the edge)? What is the best way to excavate them from the ice?
- Try making ice balloons with different liquids such as sparkling water, lemonade, cooled boiled water, fast-flowing cold tap water or slow flowing warm tap water. If you use only clear, colourless liquids can you tell which is which by their behaviour in the water?
- Tie weights to each end of piece of thread and lay it over your ice balloon. The thread pulling down will melt where it touches the ice and cut the balloon in half, but as it passes through the ice will refreeze behind it.
Links to everyday life
An iceberg is a frozen body of fresh water that is floating in salt water. Although icebergs may look quite small above the water line, they are much larger below the surface because of the differing densities of salt water and ice. As an iceberg melts it makes a fizzing sound as trapped air is released, which is sometimes jokily referred to as ‘bergy-seltzer’.
Students can see a real-life link to this investigation and the way that ice behaves when it is floating in water, e.g. when they have a drink with an ice cube in it.
The science – an introduction
Water is one of the few materials that is less dense when it is a solid, which means that solid water (ice) will float. In much the same way that you only see the tip of an iceberg, when you put an ice balloon into water you can only see the top section of it. The part of it in contact with air melts at a different rate to the part submerged under water. This changes the shape of the balloon and shifts its centre of gravity. As the force acting on the centre of gravity shifts position this causes the balloon to move.
As you try out different things with your ice balloon you can see the effect on the melting process. Adding salt to ice will lower its melting point, making it melt a lot faster than ice where no salt has been added.
The bubbles of air within the ice balloon are formed when the water freezes and the air which is dissolved within the water is trapped within the ice balloon and takes the form of a bubble. This in turn changes the position at which gravity pulls on the balloon and causes it to move.