Hands-on science: The Magic of Science

Levitating Ice

At the surface of the ice cube because it is in contact with warm air there are always some molecules of water melting. When you add the salt it dissolves in the surface water and the resulting salt solution has a lower melting temperature than the pure water . However, in our experiment as more ice melts the salt solution becomes more diluted and eventually it reaches a point where the melting temperature is no longer so far below zero and because the temperature of the bulk ice is below zero the solution solidifies again around the string and so it can be lifted up.

Salt and ice make incredible science-magic tricks. Check out Dr Yan using salt to turn water into ice with a single bang.

Psychedelic Milk

Water molecules bond to one another creating a tight surface, known as surface tension.

The colours whizz away from the soap due to the reduction in surface tension. Since milk is mostly water, it has surface tension like water. This means the positive end of one water molecule is attracted to the negative end of another molecule and so on, which creates a tense surface (see diagram). The food dye, when poured in, sits on top. The milk is more dense than the dye (it has lots of fats and proteins in it) so the dye doesn't change anything. However, when you introduce soap to the milk, the soap molecules (surfactants) break the bonds between the water molecules. This lowers and surface tension so the dye is free to flow throughout the milk. Think of it as stretching a rubber band (so it is very tense). You can imagine what would happen to the band if you took a pair of scissors and cut it in the middle. That is like what is happening to our food dye when the surface tension is broken with the soap.

A soapy mess

So how does the soap break the surface tension, you ask. Well, the surfactants in the soap have polar heads that are water loving (hydrophilic) and tails that are repelled by water (hydrophobic). When the soap is added to the milk these surfactant molecules arrange with the hydrophilic heads in the milk (where the water is) and the hydrophobic tails in the air. The surfactants spread out on the top of the surface, interrupting the surface tension, which causes the colour to whizz away from where the soap is added.

Blue is Top

Why full-fat milk? Below the surface there are more surfactant molecules which form into structures called micelles. These micelles interact with the fat globules in the milk (hence blue top) which enter into the centre of the micelles. There is movement of the surfactant molecules through the milk which results in the currents that can be seen by adding the food colouring.

Electric Balloon

Materials are made up of atoms and molecules that contain positively charged protons and negatively charged electrons. When there are equal numbers of each in an object there is neutral charge on the object. But when the balloon is rubbed against your hair/jumper electrons move from the jumper to the balloon. So the balloon becomes more negatively charged. The negative charge builds up on the surface of the balloon and this charge is static electricity.

There are also lots of negative and positive charges in the cornflour, evenly distributed in each particle, resulting in it being neutrally charged. When the balloon is brought near to the cornflour the negative charges within each starch particle are repelled by the negatively charged balloon. But the positive charges are attracted. So each particle has one side that is attracted and one that is repulsed. They line up so that the positive sides are nearest the balloon and this combined attraction is strong enough to allow the whole stream of cornflour-oil mix to move towards the balloon. Adding oil gives the slime the perfect consistency for the experiment to work.

Balloon Kebab

Balloons are made of rubber/latex, which is made up of long polymer chains which form cross links with each other. The chains can stretch up to a point and the cross links between chains allow them to return to their original formation – the balloons are made up of an elastomer. Where the inflated balloon is at its widest the rubber is under the most tension. Piercing causes the tension in the rubber to pull on the hole making the balloon burst. The polymer strands are stretched more tightly here and the cross links are more likely to break.

But at either end of the balloon is an area where the rubber is not as stretched and under less tension. Here there is a greater concentration of polymer chains forming cross links. The skewer can pass through the rubber making a hole and pushing the surrounding polymer chains aside. The polymer chains have enough elasticity to stretch around the skewer. The oil helps to seal the hole. Removing the skewer will allow the air to escape slowly though the hole.