I love treacle toffee, it always reminds me of bonfire nights where we’d go out wrapped up warm in hats and scarves with everything smelling a bit smoky. This treacle toffee recipe is a great way to explore changes of state, as you have to melt the sugar first and then leave it harden.
500g brown sugar
1/4 tsp cream of tartar
100g black treacle
100g golden syrup
- Place the sugar and water in a pan and heat, stirring gently. The sugar will melt and the two liquids combine.
- Add the rest of the ingredients and heat gently for about 10 minutes. You need to get the mixture to 143 C, which is known as the soft crack stage. If you don’t have a sugar thermometer, carefully drop a little of the mixture into cold water, when it separates into threads which are hard but not brittle it is ready.
- Pour into an oiled tin, and leave to set. You can either mark the mixture with a knife before it sets, or break with a little hammer later on.
As we made the toffee I showed the children the sugar in its solid state and asked them how they thought we could melt it. My 4 year old wanted to leave it out in the sun, but we decided heating on the hob might be faster.
As the sugar melted we could smell a toffee like smell and as it got hotter and hotter we saw more and more bubbles.
The science part
This experiment is a great way to demonstrate the process of changing state from solid to liquid and back again.
Everything we know exists in three states: A solid, A liquid or a Gas!
Substances can change from one state to another by different processes.
SOLID ––> LIQUID = MELTING
LIQUID ––> GAS = EVAPORATING/BOILING
GAS ––> LIQUID = CONDENSING
LIQUID ––> SOLID = COOLING/FREEZING
When the sugar was heated it changed from solid to liquid which is an example of melting and when we move back from a liquid into a solid it is an example of cooling/freezing.
The reason this happens is because when you provide heat the particles that make up the solid are given energy which cause them to vibrate which breaks the bonds holding them together. As they cool they lose this energy and so forms bonds again but not in the same shape.
At the soft crack stage the amount of water left in the mixture is very low. This gives a hard, but still chewy toffee. For harder toffee’s and lollipops you need to heat to the hard crack stage when there is almost no water left.
A much simpler way to demonstrate changes of state would be to melt chocolate and make rice krispie cakes!
Please be careful with the hot sugar solution.