Safe Endothermic Reactions and Making Ice Cream

Do you care if a chemical reaction is endothermic?  Do you like ice cream?  Without an endothermic reaction, ice cream would just be soup.

First, what is an endothermic reaction?  Glad you asked.  An endothermic reaction is a chemical reaction that requires energy. Simply put, heat energy is absorbed.  Two examples of endothermic reactions in nature are photosynthesis and water evaporation. 

Photosynthesis is all about sun light, which is heat energy, being absorbed by a plant.  With this energy, the plant takes carbon dioxide and water and changes them into a simple sugar, releasing oxygen back into the atmosphere.

Water evaporation is even simpler.  Heat goes into the liquid water and the water changes into a gas.  When it is really hot outside people will put water on their arms, legs, or face and as the liquid water heats up it evaporates, cooling the skin it was on.  Feels good on those long summer days, doesn’t it.

 Let’s go back to talking about ice cream.  Making ice cream takes energy.  A fun and tasty experiment to prove this is to make some ice cream.  Get a one-quart self-closing plastic bag and add the following ingredients to it:  1/2 cup milk, 1/2 cup heavy whipping cream, 1/4 cup of sugar, and 1/2 teaspoon of flavoring like vanilla or almond extract, seal the plastic bag, make double sure it is sealed tightly.  Now get a one-gallon self-sealing plastic bag and add 2 cups of ice, and 3/4 of a cup rock salt.  Put the smaller one-quart plastic bag inside the larger one-gallon plastic bag and seal it up.

 Let the fun begin, this where the energy is put into the experiment.  Begin to shake the plastic bag for about 10 minutes.  See, lots of energy going into this.  When you can see the ice cream is at the right consistency, remove the smaller plastic bag from the larger one, and dish the ice cream.  Bon-Appétit!  Who knew endothermic reactions could taste so good.

 So, what is the science behind making ice cream in plastic bags?  The ice cream froze because the heat (energy) from the hands shaking the plastic bag, along with the shaking action (more energy), caused the ice to melt and this cold was transferred to the ice cream mixture, causing the heat of ice cream mixture to leave and go into the ice and salt.  The rock salt was used because it lowers the temperature of the ice so that the resulting temperature between the ice and ice cream mixture would be cold enough to freeze the ice cream.  The heat transfer continued until the temperature of the ice and salt in the outer plastic bag was the same temperature as the ice cream mixture in the smaller plastic bag.  This is not only an endothermic reaction, but also an example of dynamic equilibrium.

 Endothermic reactions are occurring constantly all around us.  They can make us feel cooler, and they make tasty fruits, vegetables, and ice cream.