Rain in a jar

Following on from our water cycle in a bottle, I thought it would be fun to try a version of the popular rain in a jar experiment.  The idea is that you create a cloud, then add water until the weight of the water droplets in the cloud is too much and they start to fall.  The cloud that most people seem to use is made of shaving foam, but I thought we could give it a go with soap foam.  My son has recently enjoyed playing with and learning about soap foam which I made with his bath soap and a whisk, and he was therefore keen to give something else involving the foam a go.

Foam on top of water in the jar

We half-filled a jar with water and then added soap foam on top of this, with my son scooping it in until it sat above the level of the jar.  So that we can see the 'rain' falling within the existing water in the jar, we added a couple of drops of blue food colouring to the water we were going to add to the cloud.

Cloud nearly saturated with blue water

My son then used a children's pipette to add the blue water a little at a time to the foam.  We could see the foam turning pale blue, with no blue colouring in the water after the first few additions.  After one addition of blue water to the top, little streams of blue water started to travel down through the water in the jar, looking like rain!  It had worked with the soap foam, and we talked about how the weight of the rain drops in the cloud had become enough for it to rain.

'Rain' falling

This was a very simple and quick demonstration, and we repeated it a couple of times.  It was a nice follow-up to our weather diary and water cycle activities and he asked a few questions as we were doing it, and wanted to know if the clouds outside were getting full and it would rain soon (it didn't).  I suspect the soap foam may hold a little less 'rain' than the shaving foam others seem to use, but it held about 20ml of the blue water, which fitted my son's attention span for squirting water into the foam with very little happening before he could see rain.  It's also very easy to clean up as it just helps you wash the jar!