What's better than a fun science experiment? One that is mixed with ice cream! Ice cream is tasty, but did you know that it also has all sorts of science learning potential? From super cooling to chemistry, there's a lot of science sitting on top of that cone.
Root Beer Float Bubbles
There's no better excuse to have a root beer float than in the name of science. That cup of frothy goodness holds a host of scientific opportunities waiting to be discovered. If you've ever wondered why a root beer float has so many bubbles and what you could do to change that, this experiment is for you! It's suitable for elementary-aged students and will take about 30 minutes to an hour if you follow up with one or more of the variations. (Note: Measuring volume by displacement is appropriate for students who are at least in middle school.)
- Clear glass
- Root beer
- Vanilla ice cream with and without sugar
- One cup measuring cup for solids, and a measuring cup for liquids
- Tape measure (Tip: For younger kids, get masking tape that has a ruler on it, and then tape the side of the glass or cup before you start the experiment.)
The following procedure is for young students who can measure with a measuring tape, but could not do the math to figure out the volume of the substances.
- Measure one cup of root beer in a measuring cup. At the end of the experiment, you'll want to see how much root beer you were able to add to the ice cream.
- Slowly add the root beer to the ice cream. You want to measure the amount of foam produced, but you do not want the foam to go higher than the top of the glass.
- When you've added as much root beer as you can without the foam going to the top, measure your glass.
- How much foam is there? (Use the measuring tape to measure.)
- How much root beer could you add? (Subtract how much there is in your measuring cup from the one cup you started out with. This is how much you added.)
Everyone knows that the bubbles in root beer are carbon dioxide bubbles. However, have you ever wondered what is happening to make the foam that... foamy?
- Supersaturated solutions: Root beer (and all carbonated beverages for that matter) is supersaturated with carbon dioxide. Supersaturated is the scientific way of saying that there is more solute (in this case carbon dioxide) than the solution (in this case root beer liquid) can hold. Supersaturated solutions are described as 'unstable.' This means they seek to release their solutes as soon as possible, and this is why you hear the hissing sound of the gas escaping and see bubbles start to form when you open up a bottle of carbonated beverage.
- Nucleation sites: When you add root beer to the ice cream, you are essentially knocking the carbon dioxide bubbles out of the root beer. However, that's not the only place that your foamy bubbles are coming from. Ice cream also has air in it. Those bubbles of air in your ice cream provide nucleation sites that allow the carbon dioxide bubbles to form and grow.
- Surface Tension: Ever notice that some of the bubbles in a root beer float get really big? That's because some ingredients in the ice cream lower the surface tension of the soda so the gas bubbles can expand, while other ingredients trap the bubbles and leave them suspended in your frothy drink. It is much like how protein traps sea foam in the ocean.
Making root beer floats isn't just child's play. If you want to have more fun with your food, try these variations which range from relatively easy to more difficult.
- Switch the Procedure: Instead of adding the ice cream and then the root beer, add the root beer first and then add ice cream. Does it change the reaction at all?
- The Effect of Your Ingredients: Try your experiment a couple more times, but use a brand of ice cream with more sugar this time and then a brand of ice cream with less sugar for the next one. Which one produces more foam as measured by your tape measure? What happens when you use frozen treats with less air such as gelato, or sorbet which doesn't have milk? Make sure you use the same amount of frozen treat for each experiment to get more reliable results.
The easiest way to figure out the volume of the foam that is produced is to do the experiment in scientific beakers that have the volume marked on the side. However, if you are feeling particularly mathematical, and don't have access to a science beaker, there is a way to figure out the volume of your foam by using a little math and some deductive reasoning.
- Look on the package to see how many fluid ounces your cup holds. This is the total volume when your cup is completely filled.
- Next, you're going to measure the volume of one cup of ice cream using the displacement method. The idea of the displacement method is simple. If you can fill the volume of your cup with 10 ounces of water, how much water can you pour into your cup after you've added one scoop of ice cream? That answer is the volume of your ice cream.
- You should already know how many ounces of water your cup holds. (Use water since it will not react with ice cream.)
- Add a scoop of ice cream to your cup. You do not need to measure it in advance.
- Using another measuring cup, add water to the cup with the ice cream until it is filled all the way up. (Make sure you keep track of how many fluid ounces you're using.)
- Subtract the total of the fluid ounces you added from the total fluid ounces your cup can hold. For example, if your cup can hold 10 fluid ounces, and you were able to pour eight fluid ounce into the cup, your ice cream has a volume of two fluid ounces.
Freezing Point Depression Ice Cream
Freezing point depression sounds complicated, but a middle school student who is taking at least Algebra and is comfortable working with variables should be able to do the calculations. Making the ice cream takes about 30 to 45 minutes.
Essentially, to get ice cream, you have to make the cream-milk-sugar concoction cold enough to freeze into a solid state. A lot of recipes call for the ice cream mixture to be cooled in ice cubes with salt added to them. By adding salt to the ice cubes, you are lowering the temperature at which water freezes, thus making the ice cubes colder. This is also known as freezing point depression. The video below explains what's happening chemically when you depress the freezing point of water.
- Two parts cream
- One part whole milk
- Sugar and other flavorings to taste
- Electric mixer
- One small metal bowl
- One large metal bowl (the small bowl must fit comfortably inside)
- Crushed ice
- Measuring cups and spoons
- Thermometer designed for science lab use
- Various salts of differing sized granules such as table salt and rock salt
- Calcium chloride, magnesium chloride, sodium chloride and/or potassium chloride (available at hardware stores as ice melt)
- Stopwatch or timer
- Put the smaller bowl inside the larger bowl and then add ice to the larger bowl. The small bowl should be surrounded by ice.
- Carefully pour one half cup to one cup of table salt into the larger bowl with the ice. The exact amount depends on how large your bowl is, but the idea is that you want to at least cover the top of the ice with salt.
- Take note of your initial temperature.
- Start your timer.
- Begin using the electric mixer to stir the ice cream ingredients. Be careful not to over beat or you will have whipped cream before the ice cream has a chance to form.
- Take temperature measurements at one-minute intervals.
- Stop when you have ice cream that is the consistency of soft serve.
- Repeat the experiment with rock salt. If you have time, you can repeat the experiment with the type of salt that is used on a driveway during the winter. However, for safety reasons, do not eat the ice cream you make using ice melt.
Questions to Consider
- Which type of salt yielded ice cream the fastest?
- Is there a relationship between the size of the salt granules and how fast the ice cream solidifies?
- Does the amount of salt you add make a difference in how quickly the ice cream is made or in how low the temperature gets?
- How cold does the ice have to be to yield the perfect ice cream?
Freezing Point Depression Calculations
The formula for freezing point depression is as follows.
Calculate how much salt (the solute) is needed to decrease the freezing point of water (the solvent) from 0˚C to -20˚C.
Calculate how many grams of salt are required to create a -20˚C freezing point depression for 250g of ice.
If you want to do the same calculations using a different amount of ice or different types of salt instead of just NaCl, the following resources can help you find the right numbers to use:
- Van't Hoff factor - This video guides you through the process of finding the Van't Hoff factor for any compound.
- Molecular Weight - This page with a video explains how to find the molecular weight of any compound.
Ice Cream With Homemade Dry Ice
It's clear that to make ice cream, you need to put it in something really cold to change the cream and sugar mix into something solid. While most recipes call for salt and ice, the truly adventurous can try making ice cream with homemade dry ice. Dry ice is actually frozen carbon dioxide. While you can get your salt and ice combo to reach about -20°C, dry ice is around -78.5°C. The colder temperatures will help you make your ice cream much faster!
Since you're working with dry ice, this experiment is for high school students. Alternatively, an adult can do the demonstration for younger students. Although the time may vary, this experiment takes about 20 to 30 minutes to carry out.
- Carbon dioxide fire extinguisher
- Cloth sack (you can use a pillow case, sock or something similar)
- Duct tape or strong electrical tape
- Large metal bowl
- Small metal bowl or metal can that can accommodate an electric mixer and fit inside the larger metal bowl
- Two parts cream
- One part milk
- Sugar or other flavorings to taste
- Safety goggles and work gloves
- Mix the cream, milk, sugar and flavoring in the small metal bowl.
- Tape your cloth sack over the nozzle of your fire extinguisher. At this point, you'll want to start wearing your work gloves and goggles.
- Release all the carbon dioxide into the sack. This is actually dry ice!
- Add the dry ice to the large bowl, and nestle the smaller bowl inside the dry ice and begin mixing your ice cream until it reaches the soft serve stage.
Important Safety Precautions
Make sure that dry ice doesn't get into the ice cream. If it does, you can still do the dry ice experiment, but do not eat the ice cream. Also, you should not touch the dry ice with your bare hands. Always use gloves, or you can also use tongs.
Ice Cream Fun
Ice cream is like the perfect chemistry experiment waiting to happen. Whether you just want to make ice cream, add it to something else, or work to test the perfect recipe and additives, the opportunities are nearly endless. Make sure you keep a log book and remember not to eat your ice cream if any of the chemicals get into your experiment.