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 The Crazy Custard advert that you may have seen at the cinema is one of a series of five zippy experiments with perhaps not-so-obvious explanations for how they work. You can catch them all on Channel 4, in between the regular-length schools programmes. Youth presenter Misha Badr-Safdari is the super-speedy Experimenter, who zooms around her kitchen showing what's needed for the experiments, how they’re done and what the outcome is in each case. She also poses the question ‘WHY?’ – and tells pupils to PUT THEIR TEACHER ON THE SPOT for the explanations. So here they are. To play the videos you’ll need QuickTime. If you don't have it download it free by clicking here.  CUSTARD GONE CRAZY  What happens: Mix 8 eggcups of custard powder with 4 eggcups of water, making sure to stir in the water slowly so you don’t get any lumps. (Don’t cook it!) The result is a strange yellow substance that can either acts like a solid (if you punch it quickly, or roll it into a ball) OR it can act like liquid, if you touch it gently. If you increase the amounts of custard and powder, you could in theory fill a bathtub with the mixture, and run across the top without falling in. (You’d have to be very quick though!) Why Does it Behave Like That? The custard powder + water mixture is a mixture of solid particles suspended in a liquid, called a colloid. When you move the mix around slowly the custard powder particles can move around in the water quite freely, and so it acts as a liquid. When you move the mix faster or hold it in your hand tightly, the solid particles rub against each other causing friction. This makes them stick together and act like a solid. LIGHT SHOW What happens: Misha almost-severs a grape in half … then puts it in the microwave, switches it on for 40 seconds … and the result is a sparking frenzy within the oven. SAFETY FIRST : Please do not do this experiment at home. 40seconds is plenty of time for the grape to be in the microwave, any longer can cause damage. Microwaves are not toys and should be treated with respect. If you do this experiment in the classroom, beware, once the experiment is over allow the grape to cool before removing it. Why? The light you see is plasma, a very hot ionised gas, similar to lightning. The grape halves act as a tiny antenna, attracting the microwaves to the bit of skin left between them. This grape skin gets very hot and burns, leaving a smell of burnt sugar and causing the plasma. This can happen repeatedly, causing the flashing effect. CHEATING AT ROUNDERS What happens: Misha puts a rubber ball in the freezer … and substitutes it for an identical ball that’s being used to play rounders with. It looks the same, but the frozen rubber ball doesn’t bounce as well as the garden-temperature one… Why? The rubber in the ball is made up of long stretchy polymers. Normally when you drop the ball some of the kinetic energy released is converted to heat and the rest is absorbed by the rubber. The ball is slightly deformed. The molecules are stretched for a moment, after which they return to their original positions, causing the ball to bounce upwards. When a ball is frozen the polymers are less pliable and more of the energy is converted to heat. The ball is, therefore, less bouncy. SACHET DIVER What happens: Misha fills a 2 litre plastic bottle full of water. She then puts a sachet of ketchup in the top and puts the lid on. What she discovers is that when she squeezes the bottle the sachet dives down into the bottle… Why? An object will only sink if it is denser that the fluid around it, otherwise it floats. The ketchup is denser than water, so should sink. However the sachet also contains an air bubble, so overall the sachet floats. Squeezing the bottle causes this air bubble to shrink, become denser and thereby decrease the buoyancy of the sachet, causing it to sink. Letting go of the bottle allows the air bubble to return to its usual density and so the sachet floats again. YEAST EXTRACT What happens: Misha spoons out some yeast extract and taps it with the knife. And keeps tapping. And keeps tapping. She taps for 40 minutes in total (NB this makes it a much better activity for a team of people to do, so they can take turns). After 40 minutes the yeast extract has changed in colour to pale brown. Why? As you repeatedly hit the yeast extract you are adding energy and air. These two things change the structure of the marmite, making it appear lighter. Get QuickTime You’ll need QuickTime which can be downloaded by clicking here. Go back |