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

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.

Take it further

For further investigation into how ice forms see Explorify What’s Going On? Snowflake.

To further investigate the properties of ice, see Explorify Problem Solvers Ice-block skyscraper, and get your class thinking about how they can create a skyscraper from ice cubes.