This is my ideal practical: cheap, easy to set up and impressive. Just make sure that all is done well in advance and nobody touches the set-up
You Will Need:
|Bottled mineral water||£0.95||6x500ml bottles Still. Not tap water. Unopened|
|2 x bags of ice||£2.00||You can get crushed if you are short of time|
|1-2kg table salt||25p/kg||Goggles on when pouring this amount|
|Thermometer||n/a||Must measure to -20°C|
|Metal Bucket||n/a||A repurposed fire bucket works well|
|Hammer||n/a||If not using crushed ice. Remember goggles|
You can either set this up before the lesson, or set it up at the start of a 2 hour lesson and demonstrate at the end. This is only practical if you have somewhere where the bucket will not be knocked or touched.
- Crush your ice with your hammer. Remember goggles and watch your fingers. Pre-crushed ice makes this step easier.
- Add around 2kg (1 bag) of crushed ice to your metal bucket.
- Add water to make a slush.
- Remove the labels from your bottles.
- Place at least three bottles into the slush, as vertical as possible. Spread them evenly around the bucket The lids of the bottles should be sticking out of the slush for easy access.
- Add salt! You need at least 1kg for 6 bottles. Make sure to dust salt off the lids of the bottles. (If you watch the temperature as you add the salt, you will see it plummet. Try and get your pupils to explain how/why this happens).
- Wait! This needs around an hour for the water to cool. Rotate the bottles very gently every 15 minutes.
- After an hour, check your thermometer. You are looking for a temperature below -5°C. You should also see ice formed on the outside of your bucket.
- Gather the class around the bucket. Ask them what the freezing point of water is. Ask the class what freezing is (you may want a recap on particle diagrams – you could ask a pupil to draw liquid water and solid water particle diagrams on the board at this point)
- Very gently, remove one of the bottles. Tip it (gently) to show it is still liquid. Ask your students how it can be liquid when the thermometer shows such a low temperature.
- (Gently) swing the bottle back and hit it on the wall.
- You should see the water instantly freeze.
- You can get pillar of ice by pouring your supercooled water over an icecube. (This is my favourite part – much ooooo-ing and ahhhhh-ing).
- Discuss why this happens – this is a good place to bring up specific latent heat and the associated graphs.
What is happening?
When you add the salt to your slush, the salt melts the ice. This lowers the temperature of the mixture as far as -10°C
It is actually very hard for water to freeze on it’s own. It needs something to help line up all the water molecules in a regular structure: an ice crystal. Usually, these crystals need to form around impurities in the water, other bits of ice, or bits of grit. These are called nucleation sites. (This is why a pond freezes from the outside in towards the middle.)
The water in your bottles is very pure, and the sides of the bottle are very smooth. This makes it harder for the bottled water to freeze. When you bang the container, you make microscopic bubbles at the surface, that ice crystals can form around. You can make towers of ice because the best nucleation site for ice crystal formation is actually other bits of ice.