The science behind the EXTREME ice bucket challenge!

By Wendy Sadler

Unless you’ve been living under a rock for the last few weeks, it can’t escape your notice that a lot of people have been pouring icy water over themselves in the name of charity.

We had a thought at SMS HQ, that the water they are using might not in fact be very cold. There is a misconception that water with ice in must be pretty close to freezing, maybe 1 or 2 degrees. We did some experimenting and found that if you add ice to a glass of water the temperature stays at around 10 degrees for quite some time. It takes around 5 minutes to bring the temperature down from there, and even then it will most likely be around 3 or 4 degrees.

Remembering an old ITV Wales show I recorded some years ago, I remember using salt in a glass of ice water to bring the temperature down below freezing, without the water turning to ice. This would make a great extreme ice-bucket challenge I thought! Much to my husband’s glee I set about doing it!

You can watch the video, which demonstrates the science, here:

Why would salt make a difference to the temperature?

When you add ice to water, the water is at a higher temperature than the ice. Obviously. Otherwise it would be ice! So the warmer water creates a situation where the ice will melt. To melt ice, you need energy and this energy comes from the heat in the water. So when the ice starts to melt, the water around it will cool down.

If you have a salt solution (salt dissolved in water), then the freezing point of that solution is much lower than zero degrees.

Colder than cold ice bucket challenge! CC-BY-NC science made simple

Colder than cold ice bucket challenge! CC-BY-NC science made simple

When you add salt to the water it dissolves and this takes energy from the water. The melting ice also takes energy from the water, which means it cools down much faster and much further.

A salt-water solution won’t freeze at zero degrees you can keep taking the energy from the water much longer than if you had pure water (which would become ice at zero degrees).

If you add enough salt and ice, and wait long enough you can use this method to get a liquid solution down to – 10 degrees, that’s almost as cold as your freezer! If you fancy you can then use this cold ice-bath to make your own ice cream – as described in our ‘cool chemistry’ blog.

Why not challenge your friends to take the much colder, extreme ice-bucket challenge! (Try not to use tap water though as it is a bit wasteful – use a rain butt, or collect the water from your shower).

 

Becky with water

CC-BY science made simple

 

 

Our Water show ‘It’s only water…Or is it?’ will tell you a bit more abut the properties of water as a solid, liquid or gas and is packed with surprises that may result in you getting wet. You have been warned…

 

 

 

 

Curriculum links

This science made simple blogpost aims to enrich and give context to:

Key Stage 3:

Energetics

Chemical reactions

Key Stage 4:

AQA GCSE Chemistry

C.2.5 Exothermic and endothermic reactions
C.3.3.1 Energy from Reactions

 

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Posted in Activity, Chemistry, Exploring Science