Mixing two fluids of different densities

02 April 2006


The squash ends up under the water


Derek - Hello and welcome once again to the Naked Scientists' laboratory where we've got some great science lined up and an experiment that you can do at home. It's really very easy and I think the result may surprise you as well. With us to explain what's going on and how to do the experiment in the first place is Herbert Huppert.

Herbert - What we're going to do is a very basic experiment but one that occurs in many natural and industrial situations where fluid of one density flows into fluid of a different density.

Derek - Ok, so we're going to be doing some mixing of fluids here with some surprising results. Also with us is our helper who will be doing this experiment for us. Could you please give us your name and age?

Chris - I'm Chris and I'm thirteen.

Derek - Thanks for coming along. My duty is to ask you what you like about science?

Chris - I like chemistry and doing experiments and seeing what happens.

Derek - Well we'll be doing experiments today and seeing exactly what happens! At home, if you want to do this experiment, then these are the things you will need. Firstly you need to go to your bath tub or your kitchen sink, although the best effect will be seen in your bath. What you need to do is run the bath and get six or twelve inches of water in the bottom. It doesn't have to be hot water. Then you need a two litre jug and you need to put salt water in it. In two litres of water you'll need at least half a dozen tablespoons of salt. Finally you need to add some food dye to the salt water so that it looks a completely different colour. Now Herbert will instruct as to what to do next.

Herbert - What we're going to do is stand at the end of the bath that's away from the taps and we're going to pour the salty dyed water down side of the bath quite rapidly and see what happens. Chris is going to do this here. We don't quite have a bath as this wouldn't be appropriate in a departmental laboratory. But what we do have is a nice container full of water and Chris has the container with salty water ready to go.

Derek - yes it was a little bit difficult for us to get our bath tub down into the lab but we do have a similar thing which is transparent. However, this is easy to see in your bath or sink as well. Chris is all poised and ready. What do you think is going to happen?

Chris - I think the food colouring will sink to the bottom.

Derek - Ok, well we're ready here to pour the fluid into the tub but we're not going to do it yet as we want you to do it at home. We also want you to tell us your results. We'll do this at the end of the show for real in the lab and Herbert will be giving us an explanation too. So until then it's back to the studio.

Chris - Thanks Derek. Now do note that if you're doing this in your bath be sure to check that it's not porous before putting in food colouring as this may stain your bath. You might want to do it in your kitchen sink if you don't want the danger of a green bath!

LATER ....

Derek - Hello again. Welcome back to the Naked Scientists' laboratory and we are poised and ready to pour some of this salty dyed water into the big tub of just normal water that we've got. So Herbert, would you care to instruct Chris, our helper, what to do.

Herbert - Right Chris, stand at the end away from the plug and then just pour it in to the side of the bath.

Derek - And tell us what you see.

Chris - It's dispersing in the water and going towards the end.

Derek - Ok then. And if we look down from the side and try to get an idea of where it is, top or bottom, what do you see?

Chris - It's at the bottom.

Derek - This tub is totally transparent and if we look from the side we can see that from the top, the water still looks kind of transparent, doesn't it?

Chris - Yeah, it's dark at the bottom and light and then transparent.

Derek - Ok and so we've got a nice layer of blue liquid which is on the base and even from the top as in a bath tub we can still see this effect. So it didn't actually mix in did it? Why do you think that it didn't mix in?

Chris - I think it's because the food colouring is more dense.

Derek - So Herbert then, what we all really want to know is why this happened.

Herbert - Chris made the water heavier by adding the salt. The food colouring hardly changed the density at all. The salt makes it heavy. As he poured it in, the relatively more dense salty water sunk to the bottom and displaced the relatively light clear water. It then ran along the bottom really driven by gravity. Gravity is making it run horizontally along the bottom of the container or the bath and leaves the dyed salty water at the bottom. The heaviest bit of the fluid is at the bottom.

Derek - So the thing is that we did pour it in there quite quickly and yet it's remained at the bottom and it hasn't mixed in. It's not just become one big slightly blue mixture. Why is that?

Herbert - It doesn't mix because the salty water is heavier and separate. In fact, if we waited here Derek, I think we might have to wait maybe about ten or twenty years before it would really mix in effectively. That's a long time for this show.

Derek - It is indeed! We've got live radio to run, we can't wait that long The thing is, it's all water isn't it. It's either water with salt or water without, so it really does strike me as very strange that they don't mix together of their own volition.

Herbert - If I took a relatively heavy tennis ball and I released it in the air and it stayed there, you'd get a bit of a shock. What you'd expect to happen is because it's heavier than the air, it drops to the bottom. The same is true here that you have relatively heavy salty water that goes to the bottom. It can be mixed but you have to give the energy that's needed in order to lift the heavy fluid up and mix it in.

Derek - And of course you at home can actually try that if you like, because you'll notice it won't really mix in. There are ways to add that energy and mix it in. Just quickly, what are the especially good ways to try and mix that in Herbert?

Herbert - There are two different experiments that you might like to do here. One is to take a ruler or your hand and just swirl the ruler round keeping it vertical. Then what you're doing is giving it horizontal motion and there'll be rather little mixing. The other thing you could do is take a spoon and actually move the fluid vertically to lift the heavy fluid up and then it will mix very much better.

Derek - Now then of course we do see this effect around the world in nature. Where do we see this Herbert?

Herbert - Well it happens in lots of different situations but let me just tell you that in the Antarctic when ice forms, it leaves behind the salt and so you get salty water just as Chris has made. It then sinks to the bottom. When this happens at the South Pole, where can it go but north? It's the only direction. In fact we now know that that current ,which is called the Antarctic bottom water current, goes probably as far as fifty degrees north. So it travels a very long way indeed, driven by that excess of salinity.

Derek - Excellent. Thanks very much Herbert and Chris for helping us with this one. What did you think of it?

Chris - Very interesting.

Derek - And are you going to be going home and doing this and preventing everyone in your family from having a wash?

Chris - Definitely.

Derek - Good stuff. We have a convert once again, that's great. Well that's all here from the Naked Scientists' laboratory and we'll be back next time with some more kitchen science for you to do. Until then, goodbye.


A bowlA bowl with fairly smooth sidesSome coloured squashSome brightly coloured squash - preferably with sugar.


Fill the bowl with water

Gently pour squash down the side of the bowl

watch what happens.


As you pour the squash down the side of the bowl it will run down to the bottom, slosh around and end up settling at the bottom. While it is moving it does tend to swirl and mix the squash and water slightly. 

Once the fluid has stopped moving there is hardly any mixing at all.

The Bowl at the End


Squash has a lot of dissolved sugar in it so it is considerably more dense than water, so it will sink in the same way that a dense stone will sink. Once it has sunk there is nothing but molecular vibrations to mix the two liquids, it would probably take literally years for the two liquids to mix together. This is why it never works very well if you add the squash after the water - there is nothing to make them mix.

However when the squash is moving you can see it swirling as in moves next to the water.

The Flow

This is because the squash next to the water is slowed down which tends to twist the water in the same way that a car will twist if you drive one side into sand because one side will be slowed down.

A car in sand

 This random swirling is known as turbulence very good at mixing fluids and is actually what you are creating when you stir your coffee or when you pour water on top of your squash.


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