AbstractHave you ever broken a candy bar in half to share with someone? Some might snap in half quite cleanly, but others might be gooey and flexible. If you stick a candy bar in the freezer, will this change how the materials break? Try this sweet project to find out!
- Snickers® is a registered trademark of Mars, Inc.
- Airheads® is a registered trademark of Perfetti Van Melle Inc.
Find out whether or not freezing candy bars or other snack foods makes them more brittle.
Have you ever noticed that different types of food have different textures in your mouth? Some might be hard and crunchy, while others might be soft and chewy. The study of different materials and their properties—like how flexible they are, and how they break—is called materials science. Real-world materials scientists and engineers try to improve materials that we use to build things, like making stronger metals for buildings and cars, or creating plastics that are easier to recycle and reuse. In this project, you will learn the basics about materials science using tasty (but not necessarily healthy) snacks, like the ones shown in Figure 1!
Figure 1. Some examples of the types of candy you can use for this project.
You have probably noticed that some types of food, like a cooked piece of spaghetti or a gummy worm, are very flexible. They are very easy to bend with your hands, and some of them will even bend under their own weight. Other types of food, like some candy bars, are somewhat flexible, but they will hold their shape after you bend them. Materials that maintain their bent shape are called ductile. Other foods, like pretzel sticks or uncooked pasta, tend to snap or crack very easily when you try to bend them; these materials are called brittle.
All of the examples in the video were at room temperature. What do you think will happen if you put all those food items in your freezer? Will they become more brittle, or more ductile? Try this project to find out!
This note contains more-advanced materials science terms for students interested in a deeper understanding of the subject. You do not need to understand the terminology in this note in order to do the project.
While ductile and brittle have specific definitions in materials science, flexible does not. Flexible is an everyday word we use to describe materials that are easy to bend or twist. In materials science, technically, materials are categorized depending on how they deform (what happens before they break) and how they fracture (what happens when they break).
There are two types of deformation, elastic deformation and plastic deformation.
- Elastic deformation occurs when a material is "springy." That means when you bend, stretch, or compress it, it will spring back to its original shape, without any permanent deformation. Think about what happens when you flex a ruler; as long as you do not bend it too much, it will spring back to its original shape.
- Plastic deformation occurs when you bend, stretch, or compress a material so much that it deforms permanently. This is what happens when you bend a paper clip too much. If you only bend the paper clip a very small amount, it will spring back to its original shape (elastic deformation). If you bend it past a certain point (referred to as the material's yield strength), it will deform permanently and hold the new shape (plastic deformation).
- Note: the name "plastic deformation" can be a bit confusing at first. Do not get it confused with plastic materials, like plastic bottles. Non-plastic materials (like a metal paper clip) can undergo plastic deformation.
Remember that the two types of deformation describe what happens before a material breaks. There are two types of fracture that describe how a material breaks: ductile fracture and brittle fracture.
- Ductile fracture occurs when a material undergoes a lot of plastic deformation (meaning, it stretches out or bends permanently) before it breaks. Ductile fracture occurs slowly as the material deforms more and more. Imagine bending a paper clip again; how far do you have to keep slowly bending the paper clip before it finally breaks? This is ductile fracture.
- Brittle fracture occurs when a material undergoes very little, if any, plastic deformation before fracture. Imagine flexing a wooden ruler or pencil. You can bend them slightly and they will spring back to their original shape (elastic deformation). However, if you bend them too much, they will instantly break in half (brittle fracture) before they experience any permanent bending (plastic deformation).
The 1–5 scale described in the Procedure of this project is an easier way to rate how "flexible" materials are. For an advanced project, you can classify what type(s) of behavior you see when you bend each material (keep in mind that not all of the items you test will fracture; some might just show elastic or plastic deformation).
Terms and Concepts
- Materials science
- Elastic deformation
- Plastic deformation
- Yield strength
- Brittle fracture
- Ductile fracture
- What is the difference between ductile and brittle materials?
- What do you think will happen to the flexibility of your food items when you put them in the freezer?
This video shows what happens when you freeze a certain type of candy, then bend it:
- FormationStation. (2012, November 15). Brittle vs Ductile Deformation. Retrieved April 14, 2015.
This page describes how cold temperatures can affect whether some metals are ductile or brittle (the same concepts apply to food):
- UNSW Australia. (2014, October 24). Ductile to Brittle Transition. Retrieved April 9, 2015.
This page describes what an average is and how to calculate one:
- MathIsFun. (n.d.). How to Find the Mean. Retrieved April 14, 2015.
Materials and Equipment
- Chocolate-coated candy bar, like Snickers® (6)
- Note: Full-size candy bars are recommended, as "fun-size" candy bars might be difficult for young children to break in half.
- Chewy, semi-hard candy, like Airheads® (6)
- Soft, flexible candy, like gummy worms (6)
- Clock or stopwatch
- Access to a freezer
- Lab notebook
- Remove any wrappers from all of your candy items.
- Place three of each type of your candy items in the freezer so they have time to get cold. Make a note of the time. In one hour, these candies will be ready to be tested.
- Create a data table like Table 1 in your lab notebook. If you bought different types of food, fill in the names for what you are using instead.
|Food||Trial 1||Trial 2||Trial 3||Average||Trial 1||Trial 2||Trial 3||Average|
- You will fill in Table 1 using a 1–5 scale to rate how flexible the foods are:
- 1 = Food is totally flexible, very easy to bend with your hands, or bends under its own weight, but does not permanently hold its bent shape.
- 2 = Food is somewhat ductile, you can bend it and it will only partially return to its original shape, and retain some of its bent shape.
- 3 = Food is ductile, you can bend it and it holds the bent shape.
- 4 = Food is somewhat brittle, you can bend it a little bit and it will hold the bent shape. If you bend it too much, it snaps.
- 5 = Food is totally brittle. If you bend it, it will not retain its bent shape at all, and if you bend it too far, it snaps suddenly.
- Take your room-temperature Snickers bar (or other chocolate-covered bar), hold it with both hands, and slowly bend it in half (as shown in Figure 2) while watching it very closely.
Figure 2. Hold your candy bar with both hands near the ends and slowly bend it in half.
- Pay close attention to how it bends and if it breaks. Assign it a number based on the scale in step 4. Note that some materials might not break.
- Write down your results (a number between 1 and 5) in Table 1.
- Repeat steps 5–7 two more times for your two other room-temperature, chocolate-covered candy bars. Each test is called a trial. Scientists always do multiple trials for an experiment, to make sure their results are repeatable and not just a fluke.
- Repeat steps 5–8 for your room-temperature Airheads (or other semi-hard candy) and your gummy worms (or other soft candy).
- Check the time. Wait until it has been one hour since you put your candy in the freezer.
- Repeat steps 5–9 for your frozen items. Only remove them from the freezer one at a time, and test them right after you remove them from the freezer, so they do not have time to warm up again.
- Calculate average values for each food item at each temperature and write them down in Table 1.
- An average helps you combine measurements from multiple trials into a single measurement. The average of a set of numbers gives you the "central" value of those numbers.
- To calculate an average, add up the three numbers for the three trials, then divide the result by the number of trials (in this case, 3). Ask an adult for help if you need help with division.
- For example, if you have values of 3, 4, and 5, your sum would be 3 + 4 + 5 = 12. Then divide the sum by 3, and you get 12 ÷ 3 = 4. The average of 3, 4, and 5 is 4.
- Sometimes your result might include a number with a decimal point. For example, if your values are 1, 2, and 2, the sum is 1 + 2 + 2 = 5. The average is then 5 ÷ 3 = 1.67.
- Remember to ask an adult, an older sibling, or a teacher for help, if necessary.
- Analyze your results.
- Look at the numbers you filled in for Table 1.
- What happened to each individual type of food after you put it in the freezer? Did the number get higher (more brittle) or lower (more flexible)?
- Do you see an overall trend in your results? Did all the foods get more brittle or more flexible as their temperatures dropped? Or were your results mixed?
- How do your results compare to your predictions?
- Which item had the biggest change in flexibility? Which had the smallest?
- Optional: Make a bar graph of your data.
- Put the type of food on the horizontal axis.
- Put the 1–5 flexibility scale on the vertical axis.
- Plot the average values for each food item.
- Make two sets of bars, each with its own color: one for room temperature, and one for frozen.
- You can make the graph by hand or use the Create a Graph website if you need help making a graph.
Ask an Expert
- Try expanding the project to test more foods. What if you test non-candy items, like pasta, pretzel sticks, or slices of bread? Can you find food items whose flexibility does not appear to depend on temperature? For example, a food that is always ductile, or a food that is always brittle?
- Try testing more temperatures. What happens if you cool foods less, by putting them in the refrigerator instead of the freezer? What about if you heat the foods, by leaving them in direct sunlight, microwaving them, or putting them in the oven (adult supervision is required for using the microwave or oven)?
- Look up the terms elastic deformation and plastic deformation (Hint: See the technical note in the Introduction). How are they related to ductile and brittle fracture of materials? Did you observe any elastic or plastic deformation in this project?
- Try pulling both ends of the snack foods apart instead of bending them. This is called a tensile test. Are the items harder to break by pulling on them? How does temperature affect your results?
- Try pushing on the ends of the items instead of bending them. This is called a compressive test. Are the items harder to break by pushing on them? How does temperature affect your results?
- What happens if you cycle a material through different temperatures; for example, freeze it for one hour, take it out of the freezer and let it come back to room temperature, then do a test? Do materials retain their original room-temperature properties, or does freezing them once cause a permanent change?
If you like this project, you might enjoy exploring these related careers: