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Graduate Trainee Engineers STEM Experiments

WMG Graduate Trainee Engineers Demonstrate STEM Experiments to Try at Home

An image of a pattern made by placing coloured sweets in a plate of water.

Rainbow on a Plate

How to do it

Take coloured sweets and place them around the outside of the plate. Add some warm water to the middle of the plate until it touches all the sweets and then just wait!

The colour on the outside of the sweets is picked up by the water and carried away from the sweets. When two colours meet, the water is already ‘carrying’ as much colour as it can so the colours don’t mix! Eventually, if you wait long enough, the whole thing will turn a murky brown as all the colours mix together.

The dye on the sweets is soluble in the water. It dissolves into the water like salt would and then starts to spread out. Water can only dissolve so much dye until it reaches a maximum capacity, and water that has nothing dissolved in it yet can dissolve things really readily – so the colour will move away from the sweets towards the water with no colour dissolved into it yet. When the two colours meet, they form a line and don’t mix straight away because the colour more obviously shows up when it’s in the colourless liquid than when there are two colours meeting. They are mixing slowly… as you’ll see if you leave it for a while longer!

The dye molecules are polar and will dissolve in any polar solvent – like water! There is a high concentration of dissolved molecules near the sweets and a low concentration in the middle. Wherever there is a difference in the concentration of molecules in a solvent you will end up with diffusion and movement from the high concentration area towards the low concentration. Because of the Second Law of Thermodynamics – “Everything turns brown eventually, thanks Entropy” ­– the colours spread out into the colourless water and then slowly start to mix until the molecules of each dye are evenly spread out across the plate.

An image of a jar of water being turned upside down without water pouring out.

Gravity-defying Water

How to do it

The version in the video uses a metal lid from a jar that has had the top cut out of it and then a piece of mesh cut from a sieve. This method can leave sharp metal edges as well as a hole in a sieve which parents won’t thank me for when it comes to dinner time and the peas end up in the sink! The recommended way to do this is to use a fabric mesh (you can either use an old pair of tights or one of the reusable fruit bags supermarkets are now selling to reduce plastic waste) and stretch this fabric mesh over the opening of the jar. You can then secure the mesh in place using several elastic bands – make sure it covers the whole hole and is securely held in place. Pour water through the mesh until the jar is full, place a piece of paper over the top and then, holding the paper in place, turn the jar upside down so that the bottom of the jar is now pointing directly up to the ceiling. Hopefully, when you now take the piece of paper away, the water stays in place and doesn’t all come splashing out! When you’re ready for the big reveal and to get the water out, simply tilt the jar a little to one side.

The water wants to get out and pour downwards, the air wants to bubble its way upwards into the jar. Both the water and the air trying to move past each other through the tiny holes in the mesh causes a crush just like when a crowd of people try to get through a doorway at the same time – they all get in each other’s way! When you tilt the jar you get the water to move towards the lowest side of the jar and the air to move to the highest side of the jar. This is like telling everyone to walk on the left as they go through a corridor – suddenly everyone can get past each other, so the water comes pouring out and the air bubbles up into the jar – SPLASH!

Remember that when the water has poured out of the jar it isn’t empty – it’s filled with air. If it was totally empty it would be called a vacuum and we all know what vacuums do – they suck. That’s how a vacuum cleaner works – it sucks on all the rubbish on the floor to pull it up into the cylinder of the machine. When you turn the jar upside down and the water starts to move downwards, until some air can get in and replace any water that spills out, we have a vacuum starting to form and that sucks on the water, pulling it back into the jar. The mesh stops the air forming into bubbles that could move into the jar and stop this vacuum forming because the bubbles would have to be so tiny and the surface tension of water is really strong.

You remembered to put a bucket down…. Right?

The water is pulled down by gravity and this pushes against your mesh, stretching it slightly. This increase the volume inside the jar but water cannot expand or be stretched. The increase in volume, then, has to be made up by the air bubbles that are trapped inside the jar and as they expand their pressure drops hugely. This vacuation pressure creates a force that pulls on the water to try and pull it back into the jar and return the pressure to normal. Combined with the air pressure outside the jar pushing up on the water surface, these forces cancel out the gravitational force pulling the water downwards. Since water surfaces really want to minimise their area, they tend to form the flattest surface possible with the least area exposed to air molecules.

For bubbles to form on the inside of the mesh and start to replace the water pouring out of the jar, the water surface would need to curve inwards to surround the bubble – and the surface tension really opposes this curvature. Since it is hard for bubbles to form inside the jar and the vacuation and air pressures cancel out gravity, nothing happens until you reduce the difficulty for air bubbles to form. When you tilt the jar the water is pulled to the lower side and so, at the higher end of the jar the water molecules are trying to move downwards towards the other end anyway and this disruption lowers the barrier to forming bubbles. Once the first tiny bubble starts, the vacuation pressure starts to drop and a feedback loop is created ending with all the water pouring out of the jar and into the bucket that you carefully placed at the start of the experiment.

An image of a boat being pushed along by soap across a water surface

Surface Tension Boats

How to do it

We used a laser cutter to cut our boats out of wood or strong plastic but paper will work just as well. Cut out a rectangle that is about 5 cm by 3 cm. Make one end rounded and then at the other end cut a V shape out by making a cut from each corner towards the middle of the rectangle. Pour some clean water from the tap into a clean dish – there can’t have been any soap in either of these or this won’t work! Make sure you rinse the dish thoroughly with water before this. Float your boat on the water and then take a cotton bud or roll up a piece of tissue and use it to dab a tiny bit of soap just behind your boat – if all has worked out well it should zoom off across the water!

Note: this will probably only work once before you’ll need to clean the dish and replace the water for fresh, soap-free water.

The soap spreads out over the water and covers the whole surface really quickly and as it goes it appears to push the boat along! It isn’t really pushing the boat though, what is really happening is that the soap is changing how the surface of the water behaves. Water without soap in it really doesn’t like having air in contact with its surface so every bit of water is fighting to pull the boat over itself to have some cover from the air. Water with soap in it has soap to get in between the water and the air and this means it isn’t fighting so hard to pull on the boat – which means the water farther from the soap wins the fight and pulls the boat towards it.

The force that is going to pull the boat across the water is called surface tension. Tension happens when something is pulled and stretched – like an elastic band. If you’ve got something under tension – you’ve stretched out your elastic band – and the force on it is all very nice and stable, the object can stay exactly where it is. If you were to cut the elastic band while you had it stretched though – ping! It would snap and fly off. Releasing tension can cause things to move. The surface tension of the water pulls the surface flat to try and keep as little water in contact with the air as possible. This is because water and air don’t interact very well – water is polar and air is not, so they just don’t get along with one another.

Soap is interesting though because its molecules are actually polar in some parts and not polar in others which means that the molecules can arrange themselves so that the polar parts are pointing at the water and the non-polar parts are pointing at the air. This reduces the need for the water to pull so tightly to try and be flat and keep the air away! The force pulling on the surface all over, nice and evenly before the soap is added, is suddenly unequal and is a lot less where the soap has been added. Now that the forces all around the boat are unequal the boat will move in the direction of the strongest force – like having one team that is much stronger in a tug of war!

Water is a small molecule made up of an oxygen atom that can become negatively polarised (it hoards and has more than its fair share of the electron density – that is, how often the electron is around the oxygen atom) and two hydrogen atoms that are far more likely to be positively polarised (they don’t pull too strongly on the electrons in the molecule). The difference in polarisation across the molecule makes it a bit like a magnet – the negative and positive sides of the molecule are like a north and south face of a magnet. Because the molecule is also really small it can form into networks easily that have the negative and positive parts of the molecules all pointing at each other so that we end up with strong interactions all over the place. This makes the whole liquid pull together and try to keep out anything that would break up those networks – like air. The surface of water is always the smallest possible area – hence round droplets and flat water surfaces – thanks to this pulling tension force across it.

A molecule that can interact with both polar and non-polar molecules is called an amphiphile. They can mitigate the poor ability for water and air phases to interact. The surface tension, that normally pulls equally across the whole surface – cancelling out any force on the boat, is broken wherever there are these amphiphiles and so there is a greater force on the side of the boat that is away from you and this pulls the boat along.