Today we are looking at the science behind curly potato fries. First, let’s talk about how we make them.
- Carefully chop up a potato into straight thick chips.
- Boil around 250ml of water and stir salt into this water until no more salt will dissolve.
- Fill a bowl with tap water and place half of your chips into this bowl.
- When the salty water has cooled pour it into another bowl and add the rest of your chips to this.
- Leave both bowls of chips out overnight.
- The next day you should have one bowl of chips that are still hard and straight and the other bowl (with salty water in) will be full of chips that are more flexible, that you can shape into curls.
The addition of salt to the water allows you to make curly fries due to osmosis. Osmosis is the movement of water from an area that has few molecules in the water to an area that has more molecules in it to try to even things out and create a balance.
Plants like our potato here are made up of millions of cells that have a cell membrane around its edge which allows some things in and not others. Water can easily flow through this but the salt we dissolved in it can’t. Cells are filled with lots of little molecules so water usually flows into the cells and fills them to dilute the liquid. But when we have lots of salt in the water, there are more particles in the water outside of the potato cells than inside so the water leaves the cells.
When cells are filled with water they are quite rigid and packed closely together making a fairly sturdy chip. When the cells are dehydrated, they are smaller leaving space between cells, allowing the chip to bend without snapping.
Osmosis is used in all plants – not just when you cut them up and put them in a bowl of water! Plants use osmosis in their roots to allow water to move from the soil into their roots.
This week we’re making ice cream but instead of using an ice cream machine, we’re going to make it using science!
You will need:
- Two Ziploc bags – one small, one large
- 100ml double cream
- 50ml milk
- 40g sugar
- Vanilla extract
- Measure out the milk, cream and sugar and place them into the smaller Ziploc bag.
- Add a dash of vanilla extract then zip up the bag.
- Fill the larger bag 2/3 full with ice.
- Pour a generous amount of salt onto the ice.
- Making sure the small bag is tightly zipped up, place it inside the bigger bag with the salt and ice.
- Gently shake the bag for 5-10 minutes, be careful not to rip the bag!
- Leave the ice cream to sit inside the ice and salt bag for another 10 minutes
- Open up your bag and enjoy!
Try making different flavours of ice cream by swapping the vanilla extract for strawberry or mint extract or even cocoa powder for chocolate ice cream. You could also try adding chocolate chips.
How does this work?
Water, as I’m sure you know, freezes to make ice at 0oC. But your freezer at home is around -18oC, so how are we making the ice cold enough to freeze your creamy mixture? The secret is in the salt.
Ice is in a constant state of melting and refreezing and melting and refreezing. When we add salt, the salt particles block the path of the melted ice, stopping it from freezing back on to the rest of the ice but ice can still melt. Therefore more ice is melting that freezing.
Now you may be thinking that surely if the ice is melting that means it is getting warmer? It’s actually the opposite. For ice to melt it needs to break the bonds that are formed between the H2O molecules. This breaking requires energy which it gets in the form of heat. When a molecule melts away a bond is broken, taking heat away from the surrounding, causing the temperature to drop.
This is also the reason that salt is put on icy roads – it stops water forming ice.