You’re watching an action movie, and suddenly the hero dives through a glass window! Or a car window shatters as the hero navigates an exciting car chase! The glass looks so real, but believe it or not, movie sets rarely use real glass for those scenes. Can you guess what they use instead? If you guessed candy, you’re right! Instead of using real glass, which is expensive, difficult to replace for multiple shots and dangerous, movie sets use sugar glass! This cheap, easy-to-make substitute looks like glass—and tastes like candy! In this activity you’ll make your own beautiful version—a stained sugar glass window that looks good enough to eat!
You’re probably pretty familiar with table sugar. You know what it tastes like—and probably what it looks and feels like too. You might even know that table sugar comes from sugar cane or sugar beets (that’s right, 20 percent of the world’s sugar comes from beets!). There are several different types of sugar, which are distinguished by their chemical structure. The chemical name for table sugar is sucrose. Sucrose is a relatively large sugar molecule, which can be broken down into smaller, simple sugars called glucose and fructose. For today’s activity, when we say sugar we'll mean sucrose.
The table sugar crystals we can see and feel are called sugar granules. Table sugar granules are made up of millions of tiny sucrose molecules, all clustered together. One sugar cube has at least one billion trillion sucrose molecules!
If you’ve ever put a sugar cube or teaspoon of sugar into a cup of tea or lemonade, you know that sugar usually dissolves in water. When this happens you can’t see the sugar in the liquid (even though you can taste it!).
When sugar is added to water (or water-based drinks), those big sugar granules break up into individual sucrose molecules that are too small to see. This happens because the individual sucrose molecules are attracted to the water molecules. Because sucrose and water molecules are attracted to each other, you can dissolve a lot of sugar in water. However, to get candy to be sweet, we need lots of sugar. This is tricky because just like a sponge can only hold so much water, water itself can only hold a limited amount of sugar. At a certain point there are too many sucrose molecules and not enough water molecules to pull them away from each other. When this happens we say the solution is saturated—it can’t hold any more sugar. When the solution becomes saturated the sugar granules can’t be pulled apart. When this happens we can see those undissolved sugar crystals in the water. So how do we get the leftover sugar to dissolve in the water, and make our candy extra sweet? We’ll find out in this activity!
- One-eighth teaspoon cream of tartar
- One cup of water
- One and three-quarters cups of granulated sugar
- Half cup of light corn syrup
- Aluminum baking pan
- Non-stick cooking spray or parchment paper
- Mixing bowl
- Cold water and a few ice cubes
- Oven or hot plate
- Food coloring
- Wooden spoon
- Metal fork (at least one)
- Candy thermometer (it’s possible to do this activity without a thermometer, but we recommend using one if its available. If you don’t have a thermometer, refer to this online guide to judge the temperature of the sugar syrup: Stages of Sugar Cooking)
- Cooking mitt
- Clock or timer
- Piece of paper (or lab notebook) and pencil
- Adult helper to assist with handling materials at very high temperatures
- Combine the cream of tartar, water, sugar and corn syrup in the sauce pan.
- Put several cups of cold water into your mixing bowl (until it is at least halfway full). Add a few ice cubes.
- Place the candy thermometer in the saucepan—make sure it’s not touching the bottom of the pan.
- Spray the aluminum baking pan with cooking spray or cover with parchment paper. If using parchment paper, fold the edges so the paper fits into the pan.
- On your paper make a table with columns for temperature, time, appearance, feel, and other observations. Draw eight rows. Fill out each row of the temperature column as follows:
Row 1 “Room Temperature 1”
Row 2 “230°F”
Row 3 “235°F”
Row 4 “245°F”
Row 5 “255°F”
Row 6 “275°F”
Row 7 “300°F”
Row 8 “Room Temperature 2”
Note: Sugar syrup is extremely hot and sticky! The steps below should be done with the assistance and direct supervision of an adult helper.
- In Row 1 of your table, fill out the other four columns based on your observations. Use the wooden spoon (or your clean hands) to make note of the appearance and feel of the contents of the pot. Use the other observations column to note anything else that seems interesting. How thick is the liquid in the pot? Would you compare it to water, or maple syrup? Is the solution clear? Can you see the separate ingredients, such as the sugar, or did it dissolve in the water? What do you predict will happen as you heat the solution?
- Place your saucepan on the stove, and set to medium heat. Write the time that you started heating the liquid in the time column of Row 1. Be careful while heating the solution: to make a clear window you need to heat the sugar slowly, otherwise it will caramelize (turn brown). Use the wooden spoon to slowly stir the ingredients, and notice their consistency and appearance. Continue to stir the solution until it comes to a slow boil.
- When the temperature on the thermometer reaches 230°F, have your adult helper use the wooden spoon to carefully drop a small amount of the solution from the saucepan into your bowl of ice water. Record the time that the temperature reached 230°F in Row 2 of your table.
- Wait until the sugar cools, and then use your hands to explore the drop of sugar solution in the water. Write your observations in Row 2 of your table. What does it feel like? Does it dissolve in your fingers or keep a shape? Is it hard or soft? What does it look like? What does the solution in the pot look like? How has it changed since you started heating it?
- Repeat these steps when the temperature on the thermometer reaches 235°F. Have your adult helper use the wooden spoon to carefully drop a small amount of the solution from the saucepan into your bowl of ice water. Record the time that the temperature reached 235°F in Row 3 of your table.
- Wait until the sugar cools, and then use your hands to explore the drop of sugar solution in the water. Write your observations in Row 3 of your table. How has the feel and appearance of the sugar changed? What does it feel like? Does it dissolve in your fingers or keep a shape? Is it hard or soft? Can you mold it to form different shapes? What happens if you try to remove it from the cold water? What does it look like? What does the solution in the pot look like? How has it changed since you started heating it?
- Repeat these steps when the temperature reaches 245°F, 255°F, 275°F and 300°F. If the temperature of the sugar syrup is not increasing, gradually increase the heat of the burner. As the solution heats up, allow sufficient time for the sugar to cool in the ice water before handling it. You can add ice to the bowl as needed to keep it cold.
- When the temperature is between 300°F and 310°F have your adult helper carefully pour the sugar mixture into the prepared foil pan. Spread the syrup evenly in the pan with a spoon.
- Drop food coloring across the surface of the hot syrup. Use your fork to swirl the colors – have fun and be creative! But don’t lick the fork—the syrup is very hot!
- Allow the syrup to cool completely.
- Carefully remove your sugar glass from the pan. Hold it up to a light source or window so you can see the colors clearly. Beautiful!
- Fill out the final row (Row 8) in your table. Note the difference in the appearance and feel of the sugar solution at each different temperature. At what temperature did you notice the biggest changes in appearance? At what temperature did you notice the biggest changes in the feel of the syrup? Did the sugar heat up faster at certain points than it did at others?
- Extra: Different types of candy can be made by varying the temperature of the sugar and the rate that it heats and cools. You can try different candy making by removing the syrup at lower temperatures and observing what types of candy are made at each temperature stage. You can test higher temperatures but be careful, sugar burns at 350°F.
Observations and results
In this activity you observed the physical changes that sucrose goes through as it is dissolved in water and heated. When you began the activity at Room Temperature 1 the solution was saturated, and you were able to see the sugar crystals in the water. However, as you heated the sugar syrup, the sugar dissolved and the solution became clear so that you could no longer see the individual sugar crystals.
Why did heating the solution allow more sugar to dissolve in the water when it couldn’t dissolve before? Heating up the solution causes the molecules to move around more and at greater speeds. As a result, the sugar and water molecules come in contact with each other more often, and with greater force. Imagine building three Lego cars, and putting the cars in a jar together. If you gently shake the jar, a few Lego pieces might fall off, but the cars will stay largely intact. However, if you shake the jar harder, the cars will bump into each other more often and with more force, and more pieces will break off. This is similar to what happens when you heat your sugar solution. Water molecules bump into the sugar molecules, causing them to break away from the larger sugar granule.
As the sugar dissolved in the solution you should have observed other changes in the appearance and feel of the sugar syrup. At each recorded temperature point, the sugar in your solution was at a different stage. These stages are marked by changes in the physical appearance and form of the sugar. These changes take place as the water in the solution boils away and the sugar concentration of the solution increases. At 230°F the sugar syrup was at the thread stage. At this stage your sugar syrup is approximately 80 percent sugar and 20 percent water, which is still too much water for the syrup to form a shape. When you dropped the syrup in cold water, it formed a very loose, liquid thread that probably didn’t hold a shape very well. Even though you can’t make candy at this stage, you can make a sugar syrup.
The next stage of sugar cooking is known as soft ball stage when the temperature of the syrup reaches at 235°F. At this point the sugar concentration is the syrup is approximately 85 percent and the name of this stage probably makes sense to you. The higher sugar concentration makes the syrup thicker, allowing it to hold a shape. When you dropped the sugar syrup into cold water you were able to mold it into a soft, flexible ball. If you removed the ball from the water, you should have observed that it would lose its shape and flatten out. If you removed the sugar from the heat at this point, you could make fudge or pralines!
The next time you tested the syrup, at 245°F, it had reached the firm ball stage. Can you guess why this stage has that name? Hopefully you can because when you dropped the syrup in cold water at this stage, you could form it into a firm ball that held its shape when removed from the water. At this point the syrup is approximately 87 percent sugar, and just that small difference in the sugar concentration results in a dramatic change in the firmness of the syrup.
The next stage is called hard ball stage, reached when the syrup heats to 255°F. At this point the sugar concentration is 92 percent, and as you dropped the syrup in cold water you might have observed that it formed thick, ropelike threads as it dripped from the spoon. Because it has less moisture at this stage, once the syrup reached the cold water it should have formed a hard ball that was more difficult to mold into different shapes. Marshmallows and gummies are made from sugar at the hard ball stage.
When the syrup reached 275°F you might have noticed the syrup in the saucepan had smaller, thicker bubbles. This stage is known as soft crack stage, and the sugar concentration is very high, approximately 95 percent. As a result, when you dropped the syrup into cold water it should have formed threads that could be bent slightly before they would break. If you wanted to make taffy, you would take the sugar off the heat at this point.
The final stage in this activity is hard crack, reached when your syrup heated to 300°F. At this temperature, almost all of the water has boiled off, so the solution is 99 percent sugar. When you dropped it in the cold water, it should have formed hard, brittle threads that would break easily. This is the stage used to make toffee and hard candies like lollipops—or sugar glass like they use in the movies!
You might be wondering why we added the other ingredients: corn syrup and cream of tartar. Adding these ingredients helps to create the clear, glasslike structure we want in this activity. The corn syrup helps prevent the sugar molecules from joining back together and forming large granules (or crystals) as the syrup cools. The cream of tartar helps break the sucrose molecules up even father, into the simple sugars glucose and fructose. And the food dye makes the whole thing a work of art!
More to explore
Solubility Science: How to Grown the Best Crystals, from Scientific American
Sweet Science: Making Marshmallows, from Scientific American
Candy Chromatography: What Makes Those Colors?, from Science Buddies
Science Activities for All Ages! from Science Buddies http://www.sciencebuddies.org/science-activities
This activity brought to you in partnership with Science Buddies