In this demonstration, students will observe how to make rock candy in order to understand how sugar crystals form. They will be able to explain what a supersaturated solution is and how it is relevant to sugar crystallization.
This demonstration will help prepare your students to meet the performance expectations in the following standards:
- 5-PS1-4: Conduct an investigation to determine whether the mixing of two or more substances results in new substances.
- Scientific and Engineering Practices:
- Analyzing and Interpreting Data
By the end of this demonstration, students should be able to
- Understand that dissolving sugar in water is a physical change.
- Define saturated and supersaturated solution.
- Explain the scientific process associated with the creation of rock candy.
This demonstration supports students’ understanding of
- Supersaturated Solution
- Solute and Solvent
- Physical Change
Teacher Preparation: 15 minutes
Lesson: 45 minutes
- 3 cups of sugar
- 1 cup of water
- Large microwave-safe container to mix sugar and water (taller is better than wider)
- 2 clothespins (to secure the skewer in the cup)
- 1 skewer or popsicle stick
- 1 cup measuring cups
- Oven mitts
- Food coloring
- Wax paper
- Plastic wrap
- Spoon or stirrer (metal is recommended)
- Paper or plastic plate (optional)
- Store bought rock candy (for comparison)
- Computer/projector to show video
- Always wear safety goggles when handling chemicals in the lab.
- Students should wash their hands thoroughly before leaving the lab.
- Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
- Ask for volunteers to help you with the demonstration to keep students engaged.
- After one week, when you are ready to remove the popsicle stick from the solution, use a spoon or your stirrer to gently break the sugar crust that formed along the top of the solution. Once this crust is broken, the popsicle stick should be easy to take out.
- When cleaning your microwave-safe container after the lab is complete, you may find that sugar crystals are difficult to remove from the sides and bottom of the container. If this is the case, add hot water to your container to let the sugar crystals dissolve. You may need to empty and refill your container with hot water a few times before all the sugar crystals come out.
- Teachers may want to consider doing this activity on a smaller scale, using small student groups to each create their own solution—depending on the age and ability of students.
- Additionally, a similar experiment from the AACT library that may be of interest to teachers is, Winter Crystals.
- Gauge background knowledge by asking the following questions:
- How many students have had rock candy before?
- What do you think are the ingredients in rock candy?
- Pass around store bought rock candy (without the label) and ask students to infer which ingredients are in rock candy. You may let students have a small taste of the rock candy if you choose.
- When sugar is added to water, the sugar crystals dissolve. When you add sugar to the point it can no longer be dissolved, the solution is called saturated.
- Saturation points can be different depending on the temperature of the solution; higher temperatures can hold higher amounts of sugar.
- When candy is done cooking and begins to cool, there is more sugar in the solution than typically possible and the solution is supersaturated with sugar. In a supersaturated state, sugar will begin to crystallize into a solid state.
- Understanding how sugar crystallizes is important in the candy making business; if recipes are not followed correctly, sugar crystals can form and give candy a grainy texture.
- One way to prevent crystals from forming is to add an acid like lemon juice to break up sucrose into fructose and glucose. Fatty ingredients like butter also help to block the crystallization process.
- [Optional] Watch a that describes how to make rock candy, which is the crystallization of sugar.
- Explanation of the lab:
- A supersaturated solution was created by heating the saturated solution and letting it cool down. Supersaturated means the solution contains more sugar (or other solute) than it can hold. The excess sugar precipitates out and forms a solid. This method is called precipitation.
- Another method is evaporation. When the water evaporates from the solution, it becomes more saturated. As the water evaporates, sugar crystals form.
- Day zero (Monday, when the experiment is started): Solution should be clear, the only sugar present is what may have fallen off of the Popsicle stick.
- Day 1 (Tuesday): Some crystallization may be forming a crust on the top of the solution, but the popsicle stick is largely unchanged.
- Day 2 (Wednesday): Crust on top of the solution is present. Some sugar crystals begin forming on the popsicle stick and on the bottom of the container.
- Day 3 (Thursday): Sugar crystals on the Popsicle stick are larger and more obvious. They should be growing where the rolled sugar was on the popsicle stick.
- Day 4 (Friday): Crystals are starting to form on the sides of the container, crystals on the popsicle stick have noticeably grown in size.
- Day 7 (Monday, final day of the experiment): Really large crystals have formed on the popsicle stick (see photos below), lots of crystals are present along the bottom of the jar.
For the Student
Today you will observe how rock candy is made in order to understand how sugar crystals form. You will learn about a supersaturated solution and how it is relevant to sugar crystallization.
- Always wear safety goggles when handling chemicals in the lab.
- Wash your hands thoroughly before leaving the lab.
- Wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
- What do you think are the ingredients in rock candy? Write down your ideas below:
- What is a physical change? What is a chemical change? Give an example of each.
Why is it important to understand how sugar is crystallized? What would happen if we were making candy and didn’t understand how crystals are formed?
- You will watch the teacher demonstration.
- Check the solution every day and record your observations. Carefully look to see if anything in the container has changed from the day before!
- After one week, compare your results with store-bought rock candy.
|Day||Observations of Solution (Record any changes!)|
- What do you think would happen if we didn’t heat up the solution? Would the results be the same?
- Why do you think we rolled the Popsicle stick in sugar before adding it to the solution?
- What type of change(s) happened in the container? Were there any physical changes? Were there any chemical changes? Explain.
Based on your observations, what do you think would happen if we left the solution there for 2 weeks?