Menu Close


Log in to personalise your experience and connect with IOP.

Video transcript of Episode 8: Hot Water Rising

Below is the full transcript of Episode 8: Hot Water Rising delivered by IOP's Imogen Small.


On-screen text reads: "Do Try This at Home from the Institute of Physics. Episode 8: Hot Water Rising."

The camera cuts to Imogen and Sam who are presenting the Hot Water Rising experiment. We see four see-through bottles filled with coloured water in a large oven dish.

The bottles have been arranged into pairs. In the first pair, a bottle filled with yellow-coloured water is sitting upright in the dish. Balanced on top of it, is another bottle filled with blue-coloured water. It has been turned upside down, and a thin piece of card sandwiched between both bottles enabling the upside down bottle to balance.

The second pair of bottles sit next to the first pair but in reverse. The bottle filled with blue-coloured water sits upright in the dish whilst the bottle balancing on top of it is filled with yellow-coloured water. As with the first 

Sam is looking to pull the card away from between two of the bottles. We hear him ask: “Quick like a tablecloth?” 

“No, not quick like a tablecloth!” replies Imogen laughing.

We hear a whooshing noise, and the camera cuts to Imogen who is standing behind a kitchen counter.

Imogen: "Hello and welcome to Do Try This at Home brought to you by the Institute of Physics."

On-screen text reads: “Imogen Small, IOP Public Engagement Officer."

Imogen: "This is an experiment I call Hot Water Rising, a beautiful way to talk about temperature with your family."

Camera cuts to a close-up shot of the kitchen counter.

Imogen: "The first thing that you’re going to need are four bottles that are see-through. They can be glass or plastic."

We can hear some clinking as Imogen sets the four empty bottles down before her.

Imogen: "The next thing is some cold water from your cold tap and some hot water from your hot tap."

Imogen gets two jugs full of water and sets them down beside the empty bottles.

Imogen: "You’re going to need some food colouring to colour my water, some teaspoons to mix it in. Something to get that water to the bottom of your bottles – I am using half a turkey baster – a tray to catch some drips and you’re going to need a couple of pieces of thin cardboard as well."

As she speaks, Imogen picks up each item in turn and shows it to the camera.

Imogen: "Choose a colour for your hot water and one for your cold water. I’m going to go with yellow for my hot and blue for my cold and then you just want to mix them in."

Imogen places the jug of cold water and blue food colouring in front of the camera. She sets aside the see-through oven dish and the bottles. We can hear them clinking as she sets them aside..

Imogen: "Let’s start off with my cold water."

Imogen opens the cap of the blue food colouring and dips the end of the teaspoon into the bottle. The end of the spoon now has blue food colouring on. She dips that into the jug of water and stirs until the water turns blue.

Imogen: "There we go. I’m going to do exactly the same with my hot water."

Imogen sets both cold water jug and the bottle of blue food colouring aside. She then places the jug of hot water, and the bottle of yellow food colouring in front of her. She repeats by dipping the end of the spoon in the yellow colouring and then into the jug of hot water, stirring until the water has turned yellow.

Camera cuts to Imogen setting up two of the empty bottles in front of her; the empty jugs have now been set aside.

Imogen: "The next thing is to fill up two bottles with your cold water and two bottles with your hot water."

Imogen has placed the turkey baster into the first bottle to act as a funnel for the water. She starts to pour the cold blue coloured water into the top of the funnel. Some of it goes into the bottle but some spills onto the counter. "Argh!" she exclaims.

The camera cuts to Imogen standing up behind the kitchen counter. She is having another go at pouring the water into the bottle but there is still some water spilling over. Imogen laughs and we hear her say: "Such a mess!"

Eventually she manages to fill the first bottle before turning her attention to the second bottle.

Imogen: "Okay that’s one. You want to get the bottles as full as possible. And now we’re going to do exactly the same with the hot water as well."

The scene cuts to Imogen pouring the yellow-coloured hot water into the remaining two bottles. She uses the turkey baster as a funnel for both bottles which have been placed in the oven dish this time to catch the spills. She now has two bottles filled with blue water and two bottles filled with yellow water.

Imogen: "Take a bottle of hot and a bottle of cold. Take your pieces of cardboard put it on top and then flip the bottle over holding the cardboard on top and balance it on top of the cold water. There we go. Is that balanced?"

Imogen places one bottle of yellow water, and one bottle of blue water in the oven dish. She then takes the remaining bottle of yellow hot water and places a piece of card on the mouth. Holding the card in place, she flips the bottle upside down. She then balances it on top of the mouth of the bottle in the oven dish that contains the blue water. She carefully aligns the bottle until it is fully balanced.

Imogen places a piece of card on the bottle filled with blue water. As before, she flips it upside down and balances it the remaining bottle containing the yellow water.  We hear her say: "Phew!"

Imogen: "So now we are ready to start the experiment."

We hear a whooshing sound. A small amount of time has lapsed. Imogen is looking around in the direction of someone else and we hear her say: "Sam, can I borrow you for a moment?"

The camera cuts to both Sam and Imogen standing behind the counter.

Imogen: "If I hold these steady, can you slide this little bit of cardboard out of the way? Okay so if I hold and you slide."

Imogen holds the first pair of bottles in place. Sam carefully pulls away the piece of card that is sandwiched between both bottles. He succeeds and we see the yellow-coloured hot water from the bottom bottle is rising up and starting to turn the water green. 

Imogen: "And then same on this side. Perfect!"

As before, Sam pulls away the card between the second pair of bottles while Imogen holds them. This time the blue-coloured water is in the bottom bottle. We see a tiny colour change where the bottles meet but that is all. 

Camera cuts to a medium shot of Imogen. We see that the yellow-coloured water in the first pair of bottles has fully mixed and become green. In the second pair of bottles, the two colours have not mixed except for a tiny amount of green at the top of the first bottle. 

Imogen: "Where the hot water was on the bottom and the cold water was on top the bottles have now almost completely mixed. On the other side where we’ve got the cold water on the bottom and the warm water on top they’ve stayed in perfect layers. But what’s going on here?"

As she speaks Imogen points to each pair of bottles in turn. 

We hear a whooshing noise accompanied by a small animation. The camera focuses on Imogen again and we see that bottles have been set to the side.

Imogen: "We’ve discovered that hot water floats just like a cork will float to the top of water. The cork floats because it is less dense than water."

Imogen picks up a cork and a 1 litre bottle of water with a wide neck. The cap has been removed. She drops the cork into the bottle of water and flips the bottle upside down. We see the cork rise to the top of the upside-down bottle.

Imogen: "Now everything is made of molecules, and density tells you how squished those molecules are together."

Imogen sets aside the bottle containing the floating cork. 

Imogen: "The more dense something is, the more that they’re squished."

As she explains we see Imogen clench her hands into fists.

Imogen: "Cork is less dense than water because there are loads of tiny gaps in there filled with air which is why it floats and rises to the top. But what about a material that’s the same made of the same molecules like the water in my experiment?"

We hear a swishing noise. We see an image of lots of H2O molecules. The camera zooms in on one molecule. We see three parts to this molecule:  One large circle that represents O, and two smaller circles that represent H. The larger circle is connected to both smaller circles using lines.

Imogen: "If we could zoom in on those molecules of H2O, we will be able to see them jiggle and move."

Camera cuts to Imogen holding a tomato. She has stuck two cocktail sticks in the tomato and threaded a black olive on the end of each. To demonstrate what she is saying, she wobbles the tomato to create a jiggling motion and moves the tomato from one side to another.

Imogen: "Now temperature’s a measure of how fast those molecules are jiggling and moving. The higher the temperature the quicker they go. In hot water, those molecules are jostling each other, they’re moving around, and they end up on average further apart than in my cold. If we had exactly the same amount of water there will be more molecules in my cold water than in my hot water, my cold water is more dense. 

"On this side I put my hot water, something less dense, on the bottom, and my cold water, something more dense, on top."

Imogen points to the pair of bottles that originally had hot water at the bottom. 

Imogen: "We know from my cork experiment that this can’t last and so the hot water rises up through the cold and the two have mixed together."

Imogen picks up the bottle with the floating cork and shows it to the camera. She sets it down.

Imogen: "There’s a special word in physics for this called convection. On the other side we’ve got something more dense on the ‘bottom, something less dense on top. There’s no convection there and my water has stayed in perfect layers."

We hear a whooshing noise accompanied by a small animation. 

Imogen: "This was Hot Water Rising and see you next time!"

Camera cuts to Sam and Imogen. We hear Imogen say to Sam: "Nobody got wet!" Both look at each other and use a thumbs-up gesture. They are smiling. We hear Imogen say: "Yay!"

The image is replaced by some on-screen text which reads: "Do Try This at Home from the Institute of Physics. For instructions and more Do Try This at Home films visit These experiments have not been specifically safety tested for home use, but we believe them to be safe if the instructions are followed. Adult supervision is recommended as appropriate. All experiments are carried out at your own risk."