This fun science trick demonstrates that hot and cold water have different densities.
The demonstration reminded my kids that lower density liquids float above higher density liquids. They learned that hot water molecules move faster and further apart from each other than cold water molecules.
The trick also allowed us to revisit the topic of surface tension and to build on the air pressure science we learned last term.
What you need
- 2 identical jars or drinking glasses (eg babyfood or salsa jars)
- hot and cold water
- red and blue food colouring
- index cards or similar
- scissors
- a large shallow dish to catch spills
What you do
1. Cut a square of card big enough to completely cover the mouth of a jar.
2. Fill one jar with cold water. Add a drop of blue colouring and stand the jar in the baking pan.
2. Fill the second jar with very hot water. Add a drop of red colouring.
3. Slowly add more hot water to the red jar until you see a bulge of water over the rim. (Take the opportunity to talk about surface tension here.)
4. Lay the square of card on top of the jar of hot water and tap it gently.
5. Ask your students to predict what will happen when you quickly turn the jar upside down. Then invert the jar, and discuss how air pressure and surface tension keep the card in place and the water in the jar.
6. Put the upside-down red jar on top of the blue jar. Ask your students to predict what will happen when you slowly pull out the card, then do so.
What happens
When you remove the card between the two jars, the hot red water stays in its jar on top and the cold blue water stays on the bottom.
Reverse the jars
7. Repeat the experiment the other way round. Turn the jar of cold water upside-down and put it on top of the hot water jar. Again, ask students to predict what will happen when you remove the card.
What happens
When you remove the card, the water quickly turns purple, as the cold and hot water mix together.
The scientific explanation
When water is heated, the water molecules move faster. They bounce off one another and move further apart. The extra space between the molecules means the same volume of water weights slightly less and is less dense than cold water.
When we put the hot water on top, the less dense hot water floats on top of the denser cold water. The red water and blue water stay in their respective jars and do not mix.
When we put the cold jar on top, the more dense cold water immediately sinks to the bottom and the two colours mix.
We talked about a previous demonstration in which we created a column of liquids with different densities, and reminded ourselves by pouring water into a glass of oil and watching the oil float to the surface.
More fun water experiments
The Tiger Chronicle recently did a bunch of fun water experiments. I especially like the supercooled water video.
All Things Beautiful explored density as part of their science investigations that all students should do before high school series.
I found the Amazing Water Trick at Exploratorium.
I’m appreciatively linking up here:
Science Ideas From After School Link Up
That’s a cool experiment and it does go well with the oil/water/layer density experiment.
Thanks Julie. Yes it’s nice when things link up and learning is consolidated, isn’t it?
That is a great way to show density! I love it!!
Thanks Phyllis!
We tried this years ago, but I’m afraid we didn’t get such wonderful results as you. I remember lots of wet going everywhere! Maybe it’s time to try again. I really do have to pull up my science socks and get sciencing again this year! (spell check does not like me!)
I was surprised it worked as well as it did! Sciencing should definitely be a verb. 😉
Love how you conceptually linked the experiments here to work you had done in the past! And C(9)’s expression as she watches the water molecules migrate is priceless! Will have to do some water experiments soon with the kids–they look like such fun!
Both children look a bit perplexed at what’s going on, don’t they?! I’ll take it as a sign the demo made them think! 🙂
This is a very cool experiment. I’ll have to give it a try. Fingers-crossed for good results — we aren’t always successful with our science experiments…. 🙂
Some unsuccessful experiments is probably a sign you’re doing a good amount of science, Hwee! Sometimes we learn the most from the ones that don’t work.
This is such a great experiment/demonstration to do with kids. I’ll have to put it on our list of things to try. I’m so glad you shared at the After School Link Up.
I liked it because it’s simple but demonstrates a number of science concepts – good value! 😉
Thank you for hosting the After School Link Up – I’m very happy to have found you!
Great ideas! I love the hot and cold water experiment – that’s really cool!
Thanks Tonia!
I really like this demo. We did a similar experiment making an underwater “volcano” by placing colored hot water in a flask then down into a tank of cold water. I like the smaller scale of this one and the addition of the paper. Nice job documenting the process, too.
Thank you, Carol. I love the sound of the underwater volcano – what a great idea! Did you post about it? I will have a look on your blog.
Fabulous! Love! 🙂
Thanks, Carol! We did too. 🙂
Love it and pinning to my science board. It looks almost like magic 🙂
It does, doesn’t it, Natalie? And magic always goes down well round here! 🙂
Coolness! (or hotness, depending on your view!). I also like the hot and cold water balloon demo (I put a drop or two of blue and red colouring in for extra special effect): http://www.abc.net.au/science/experimentals/experiments/episode17_1.htm
Ooh yes – thanks Ingi, that one look fun too! I’ve put it in the queue!
Popping in from the Weekly Wrap-Up!! This looks so cool. I think my kids will definitely want to try this.
Have fun with it, Shelly!
Ok this is just way cool! We are trying this next week!! I always love your Science posts Lucinda. You always have the most fun with your kids!
Thank you so much, Karen!
Very cool!
Thanks Mary!
Lucinda, this is cool that you could do this with just water and food coloring. I learned something new today 🙂
Isn’t homeschooling great for playing and learning alongside our children? 🙂
How neat! We’ve done the column of liquids but never tried this with hot and cold. 🙂
Thanks, Leah! I’d never seen it before, either.
This is such a cool experiment. I’m so glad you shared it a the After School Linky Party. I’m pinning this for future use with my son!!
Thank you. (And I’m glad you stopped by so I could find your blog!) Enjoy the “magic” water experiment!
I’m featuring this tomorrow on my blog at the After School Linky. Stop by, check out the feature, and share more! I’ve also pinned it to the After School pinterest board.
Thank you so much for the feature – I love your linky. Very happy to have discovered it!
This brought back memories from science class, long ago! I loved seeing the results of layering liquids with different densities. What an interesting and fun thing for kids to explore.
Thanks, Bella. Your science classes sound like they were more fun than mine! Or perhaps you just have a better memory. 😉
We like pretty, visual science round here!
This is a great post! I’m featuring it as part of a round up of science activities today.
Thank you so much, Anna!
Thanks for hosting. I can't wait to check out some of the other blogs. Everything looks yummy. I'm new to your link party and would love it if you stopped by my blog. http://www.michellestastycreations.blogspot.com. Thanks, Michelle