Thursday, February 24, 2011

Chemistry for Kids, Part 5: Endothermic & Exothermic Reactions

Lesson 5/Week 5: Endothermic & Exothermic Reactions

Today we will be working with some acids and a base we learned about before: Vinegar and Citric acid (which is found in fruits like lemons and oranges), and baking soda, which we use for lots of things such as cooking, removing bad smells, and cleaning.

Remember that when you put an acid together with a base, they create something called a reaction. A reaction between chemicals happens when two chemicals are mixed together and the molecules of one chemical break their bonds and form new ones with the molecules of the other chemical to form a new chemical. A reaction does not always have to include adding chemicals together, though.

Endothermic Reactions
Endothermic Reactions are reactions that need energy in the form of heat, to happen or proceed. You put heat INTO it.
Examples of some endothermic reactions:
-Ice melting is an example of an endothermic process** (not a reaction) (corrected this oversight as per the 11/13/12 comment below)- it needs heat energy to happen.
-Trees and plants use an endothermic reaction called photosynthesis to make food. They need the heat and light from the sun to do this.

Activity: Feel an Endothermic Reaction
  • Mix Citric Acid solution in a plastic cup with baking soda solution
  • Allow kids to place two fingers into the cup to feel the reaction. They should feel it bubble more vigorously and their fingers should feel cold after a bit.
  • Ask them to describe what they feel/felt. See if they can explain where the heat in their fingers is going.
  • Explain: When you put your fingers in the solution, it will bubble and speed up because you're giving the acid and the base mixture the heat energy it needs to get going and keep going.
  • Also, it's taking the heat from your fingers so your fingers will feel cold in the solution and when you remove them.
Exothermic Reactions
Exothermic Reactions give off heat as they happen. They are the opposite of an endothermic reaction.
-When salt is made from Sodium and Chlorine (remember our periodic table), this is a reaction that gives off heat, or an exothermic reaction
-When you light a match, this is an exothermic reaction

Activity: Feel an Exothermic Reaction
Make Hot Ice ahead of time according to the recipe and method at the link below. Touch it to demonstrate how it crystallizes. Let the kids feel it giving off heat.
**I tried making this and got nothing TWICE and this did not work for me either time. I have no idea what I did wrong but I'd be interested to know if you're able get it to work.** 

Activity: See a chemical reaction between Mentos & Diet Soda (Teacher-Demonstrated, Outdoors)- An exothermic reaction
What we did: We used a bottle of Diet Coke (because there's no sugar, it easily washes away without becoming sticky) and added the Mentos to it using a rolled. I used I strongly recommend if you want real fireworks that you add at least half to three quarters of the pack. I stuck with a website's recommendation of "4 Mentos" and this was a disappointment for me, although the kids thought it was great either way. Needless to say, I will be repeating the experiment with a bigger "bang" next time. Can't wait!

Show video either before or after demonstration:

Activity: Coloring Page:

All images taken from


  1. can you give me an explaination for the mentos and soda experiment? thanks

  2. Sure, if you're looking for why it works, see this video and it will help with the visualization. Basically, there are many different spots on the candy where CO2 can be manufactured and when combined with the soda and all its CO2, this creates an eruption since the bottle can't contain all the gas. All the CO2 gas wants to do is escape and escape it does! Let me know if this answers your question or if you have more.

  3. how is this related to exothermic reactions? i want some sort of experiment/demonstration for my class to understand exothermic reactions better.

    oh and for the endothermic reaction, can orange juice be used for citric acid? i tried this myself and i couldn't really feel the reaction..maybe i used too much baking soda than citric acid? sorry for all these questions but it would be great if you answered them. :)

    thanks so much!

  4. I don't think you can use OJ because it's not pure citric acid. Vinegar will probably do if you don't have it and need a substitute. The only issue is, vinegar tends to bubble and foam A LOT whereas citric acid just bubbles mildly (if you add a little at a time) and allows the solution to remain clear so you can observe the temp change without getting all messy.

    I'm not sure why the reaction is classified as exothermic because I was not close enough to the geyser to feel any heat given off inside the bottle. If I find something,I'll let you know. I don't mind the questions;^)

  5. A way to get the sodium acetate is best done as your link suggests, but continue to reduce liquid once film starts to appear. making sure you scrape any crystals that form round edge of pan back in to liquid. once its about 50 ml and has free floating crystals in liquid seal jar and place in fridge.
    leave to cool and harvest any crystals that form.
    the crystals are acetic acid but need reconstituting with boiling water (pref distilled). it only need a little water to reconstitute. let this chill in fridge and you should have a usable form of hot ice

    1. You are awesome! Thanks so much for your helpful tips.

  6. Just a comment on your first example:
    Melting ice is an endothermic process, but not an endothermic reaction. It is not classified as a reaction because there is no change in the composition of the substance.
    Ice is water in solid state, when it melts it changes its state to liquid, but it is still water (H2O). Melting is a physical change. Melting requires energy, so it is an endothermic physical process.

    About the soda and mentos, I wouldn't say is an exothermic reaction, un less it releases energy in the form of heat (or light). In fact nobody is sure if the explosion is entirely a physical process or if there is a reaction involved. The CO2 is already in the soda. It is not manufactured. What is manufactured is the size of the bubbles, that need to grow big enough to break the surface tension of water. Its better explained here:

    The other examples are very nice examples. Congrats for homeschooling! Nothing better than the children being close to their parents! I would do it myself if I could.

    1. Thanks for your comment. It was hard to find good, consistent info on this topic so I honestly did the best with what I had! I appreciate your input and will refer readers to your comment for clarification. With regards to the Mentos, the Mythbusters video in one of my previous comments does say the reaction manufactures additional CO2 (other than what's already in the soad) and the excessive presence of CO2 causes the explosion. Maybe there's a difference of opinion on what is actually happening??

      I also appreciate your positivity, it's encouraging where I need it most;^))

  7. Listing this link strictly as an FYI, I found this recently and wanted to share:


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