Frozen in motion

Activity overview

Run the activity

1. You’re going to watch a short video. The aim isn't to find right answers, it's to explore ideas and find out what they know.

• Do they know what might happen based on the image?

2. After you've watched the video, lead a discussion with your class:

• What observations can they make about frozen waterfalls?
• Which change of state has occurred and why?
• Can they think of examples from everyday life where they have seen this change of state happening?
• How might it be reversed?

3. Ask the class to describe what they saw using only one word.

Background science

Water can exist in three states – solid (ice), liquid (water), and gas (water vapour). 

For a waterfall to freeze the water needs to be very cold, (at least –6 degrees Celsius) and remain at that temperature for a long time. Most people know moving water can't freeze, but if it is cold enough and there is a spot where water meets a solid surface (the rock or ice) then the liquid comes to a stop or slows for a fraction of a moment. When the water molecules slow down, they begin to stick together to form tiny, round, solid particles of ice.

The tiny particles of ice will clump together as they come into contact with one another, as well as sticking to nearby surfaces. If the waterfall is flowing down the face of a cliff, the particles of ice will accumulate against the cold rock, while for a free-falling waterfall, ice will cling to the overhang.

Eventually the ice will form a base from which it will grow and, provided the temperature of the water is sufficiently cold for long enough, it will create an icy structure that runs the length of the waterfall. Over time, the river or stream will completely freeze over, leaving an icy waterfall frozen in time and motion.

Watch out for 

Children sometimes think that ice is a different substance to water because it has a different name. Keep referring to ice as solid water.   

Take it further

Children could use recycled yoghurt pots and plastic food packaging and trays to help them create ice with different shapes. They could look for patterns in the size or shape of a piece of ice and how long it takes to melt.  Does ice with same volume but a different shape take the same amount of time to melt if it is left in the same space? Or is there a pattern to which melt first? 

This Royal Society of Chemistry activity has children investigating freezing different liquids and comparing them. 

Children could investigate the impact of the global warming and melting ice on the Earth with this European Space Agency activity. It looks at land ice and sea ice and investigates the effects of these melting.  

Linked Explorify activities- our recommendations: 

For further investigation into how ice forms see Explorify What’s Going On? Snowflake or can the children guess what this is Hold on tight? Debate What if water couldn’t freeze? 

Watch 

Have a look at these sculptures out of snow and ice. This BBC video explains changing states.