This week we decided to study chemistry by growing rock candy crystals made of sugar. This is also an experiment of transformation between solid and liquid states of matter. I discussed this while the sugar melted and noted upon it as we took the crystals out of the remaining liquid. My daughter chose this experiment probably because I told her she could eat the end results. This is a good experiment with kids, even though it does require thorough parental guidance as you will be working with hot liquids. It also requires patience as the rock candy can take up to a week to form. Well, at least we decided to end our experiment there as daughter was most insistent on eating it at that point.
Concepts in this experiment:
- Saturation point (of sugar in this case) = the point where there is as much sugar dissolved in the water as will possibly dissolve in the water. Saturation point is reached when it is impossible to dissolve more sugar into the water. After this the water is saturated with sugar. The concept of saturation applies with other substances as well.
- Viscosity = Viscosity is a state of liquid being sticky and thick due to internal friction resisting the flow and movement of the liquid. Water has very little viscosity while honey has much more viscosity.
- States of matter: liquid and solid = In liquid, the atoms that make up the liquid are close to each other, but can flow. In solid, atoms stay put and can only vibrate. These states of matter will be explored in other posts.
- Crystallisation = Sugar is solid at room temperature. When liquid sugar cools down, it begins to form crystals as that is the form sugar takes when solid.
I used the guidelines by Thoughtco and Spruce. However, I chose to use only 3dl (1,5 cups) sugar and 1dl water (0,5 cups), because I did not want to make too much as this was my first attempt. I read all the directions carefully. I chose to use sticks of wood as the the base of the growing crystals because I had nothing I could, or wanted to use, to weigh a string down. I measured closely how long they need to be to be immersed in the liquid but not touching the bottom. These preparations are best done beforehand and invite the child at this point to come along, at least if dealing with a young child like mine.
- Measure the water and sugar into the kettle
- Heat until all sugar is dissolved into the water. You can add sugar into the liquid until it reaches it saturation point. If there is sugar in the liquid that will not dissolve as saturation point is reached, be careful not to pour them in the containers where the sugar crystallisation is to take place. If any crystals come along, they will act as an easy point for crystallisation to begin from, so less will be on any stick or string you want to grow the crystals in.
- Dip the sticks into the viscous liquid and then sprinkle some sugar on top of the stick and allow to dry. This is called seeding. The sugar crystals stick to the liquid and form a good base for new crystallisation to take place.
- Pour the sugar-water fluid into containers and place those containers in hot water. Leave to cool. Cooling the liquid slowly helps with the crystallisation. Add food colouring at this point if you want to.
- Place the seeded sticks into the liquid as it is cooling.
- Once room temperature, place in the refrigerator and allow to stay there until the crystals are ready.
If there is crystallisation everywhere else and not really on the stick, you can reheat the liquid until all the excess crystals have melted and then repeat instructions 4-6. Or do as I did which was to let them be and allow daughter to eat them separately.
How we faired:
This was a fairly simple experiment. I followed the instructions I had read. However, I dumped the entire sugar amount into the liquid and melted it without expressively adding more, but we talked about the saturation point. I basically just said: “There is a concept known as saturation point, which means that there is as much sugar dissolved into the water as can possibly be”. When all the sugar had melted, I poured the liquid into two containers. My daughter had added quite a liberal amount of food colouring into those two containers. Less would have been sufficient, but she was faster than I was. We then dipped the sticks in the viscous liquid and allowed to dry off a bit. I added the sugar onto the red one and not onto the blue one as I wanted to try if crystallisation would happen anyway (It did, but not as much). Once the sticks had dried off a bit and the liquid was less hot, we added the sticks into the liquid and allowed to cool. I then placed them into the fridge and then we waited. The red acted just as it was supposed to and crystals started to form onto the stick. The blue one (the one not seeded with sugar) instead started to form crystals everywhere. We followed their growth for a week and then daughter ate the end result. The crystals were not very big, but quite sufficient in any case. We then used the rest of the liquid to study viscosity (more on this later).
If you try this at home and your child asks questions, please let me know what they asked in the comment section so I can further develop these instructions. Or if you have any questions, please let me know. Also, please remember like and share if you find this useful.
- Thoughtco: How to grow sugar crystals – Make your own rock candy
- The Spruce: How to make homemade rock candy
Copyright text and images: Satu Korhonen. You are free to try these experiments out, use them in your teaching. But instead of copying the text or images, link back to this page.