Making a Sugar Thermometer
Bakers (and those who help bakers!) know that at some point in every baking recipe, the instructions will tell you to preheat your oven to a certain temperature. But if you’ve ever tried to bake cookies and they come out flat, or take too long, it’s possible that your oven is to blame. You might set your oven to 350°F, but how do you know that the inside of your oven actually reaches that temperature? You could use a thermometer…. or you could use sugar! Because we know that table sugar (known as sucrose) melts at a very specific temperature, we can test the accuracy of an oven by experimenting to see if sugar melts when the oven is set to the correct melting point. In this activity you will use table sugar to make a homemade oven thermometer, and put your oven to the test!
If someone asked you to name foods containing sugar, you could probably name quite a few (and some of them might even be your favorite foods!). But would beets be one of the foods you listed? If not, then you might be surprised to learn that about 50% of the sugar in our foods comes from sugar beets! The vast majority of the sugar we eat in processed food comes from either sugar beets or sugar cane. But a lot of work goes intro extracting and purifying the sugar from these plants. In fact, processing sugar was so expensive and difficult that even just a few hundred years ago refined sugar was considered a fine and very expensive spice, and wealthiest people could afford to eat it!
To extract sugar from sugarcane or sugar beet plants, the plants are soaked in very hot water and squeezed through a press to extract a sugar syrup. When this syrup cools, it forms the sugar crystals (sucrose) that we buy at the grocery store.
Because the process of making sugar involves heating and cooling it, sugar manufacturers have learned that the melting point of sucrose (table sugar) is 367°F. Knowing this information, we can use sugar to test the accuracy of your oven’s temperature. In this activity you’ll use sugar to test whether your oven is as hot as it says it is!
- Aluminum foil, cut into two ~6in x 6in squares
- White Sugar (at least 3 tablespoons)
- A baking tray
- Oven mitts
- 3 plates
- An oven
- An adult to help
In this activity you will be working with materials heated to very high temperatures. This should only be done with the help and under the direct supervision of an adult.
- Preheat your oven to 350°F
- Fold the outer ~1/2 inch edge of each aluminum foil square up to make a miniature dish that is about 5in x 5in square and about ½ inch tall.
- Add a tablespoon of sugar to each sample dish.
- Add a tablespoon of sugar to each aluminum foil sample dish. Notice the consistency of the sugar. What words would you use to describe what the sugar looks and feels like?
- Once the oven temperature has reached 350°F, put the first foil sample dish into the oven.
- Set a timer for 20 minutes and wait. While you’re waiting, put 1 tablespoon of sugar on one of your plates. As you look at this sugar, make predictions about what you think will happen to the sugar being heated in the oven. Will heating the sugar affect how it looks? What about how it smells, feels or tastes? Can you think of other things that look or feel different when they’re heated?
- After 20 minutes, remove the sugar from the oven and transfer it to a different plate. Remember, the sugar is hot! Observe the sugar in sample dish #1. Does it look any different after being in the hot oven? Is this what you predicted, or did you expect something different?
- Set your oven to 375°F degrees. When it reaches that temperature, put the second sample dish into the oven, and set a timer for 20 minutes. What do you think will happen to the sugar in the 2nd sample dish? Do you think it will be the same as the first experiment, or something different?
- After 20 minutes, remove the second sample dish and transfer it to your third plate. Remember, the sugar is hot!
- Carefully set the plates side by side. Compare the three samples. What differences do you see between the two baked samples? Are they different from each other? If so, what is different about them? What do you think accounts for the differences between the samples? What differences do you notice between the unbaked sample, and the other two samples? Is the uncooked sample similar to either of the cooked ones? Why might this be?
- Once the samples have cooled down, taste them! Notice if there are any differences in how each sample tastes.
Note: If you did not see any changes to the appearance of the sugar in either test, try repeating the experiment with the oven temperature set to 400°F.
Extra: Try repeating this activity using other temperatures. For each test, try getting a little closer to sugar’s melting point (367°F). If your sugar doesn’t melt when your oven is set to 365°F, but does melt when it’s set to 370°F, you’ll know that you have a very accurate oven!
Extra: Repeat the test using oven safe bowls of water, to test how accurate your oven is at lower temperatures. The boiling point of water is 212°F, so you could try repeating the test with your oven set at 200°F, and then 225°F. Does the water boil at one temperature but not the other? If so, you have a very accurate oven! Remember, only do these tests with the help and under the direct supervision of an adult!
Observations and Results
In this activity you baked two different samples of sugar at two different temperatures. Knowing that sucrose (table sugar) melts at exactly 367°F allows us to use sugar as a very simple thermometer. If your oven is accurate, baking the sugar at 350°F should not have significantly changed the appearance of the sugar, because 350°F is 17°F below the melting point of sugar. However, in the second part of the activity, baking the sugar at 375°F (which is 7°F higher than the melting point of sugar) should have caused the sugar to melt, making it look almost like glass. If this is what you saw during your experiment, you know you have a very accurate oven!
However, don’t worry if the sugar didn’t melt when you tested it at 375°F, this means that you’ve found a very interesting result! This result tells you that your oven temperature isn’t correctly calibrated (your oven thinks it’s heating up to 375°F, but it’s actually not getting that hot!). Hopefully you repeated the test with the oven set to 400°F. If your sugar still didn’t melt at 400°F, you know that your oven’s temperature is off by over 30°F! Make sure to keep this in mind next time you’re baking cookies!
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