Are these the world's greatest DIY experiments?

The Naked Scientists (aka Chris Smith and Dave Ansell) think they've found the best experiments to try at home, and the maverick duo have set up a website to get us all hooked


Instant ice

Take an unopened bottle of pop and cool it to about minus 2 celsius (use a thermometer or trial and error) on an "ice and salt sandwich".

This consists of layers of salt and crushed ice, which will create an instant freezer capable of lowering the temperature to -18 degrees C. Then open the bottle and let the fizz out. In less than 10 seconds it will freeze.

Why? Because in the same way that adding salt to a road lowers the freezing point of water and prevents ice forming, carbon dioxide dissolved in a fizzy drink lowers the freezing point of the drink.

But when you let some of the gas out, by opening the bottle, the dissolved carbon dioxide escapes and the freezing point of the liquid becomes higher than its temperature. Hey presto, instant ice.

The chemistry of coppers

Take four or five well-tarnished copper coins. Place them in an egg-cup-full of white-wine vinegar or lemon juice for 10 minutes. Then take them out and dry them. What's changed? The coins have gained a shiny new lease of life and shed all of their tarnish. Why? Because when coins are left in your pocket for a long time, the copper in them reacts with oxygen in the air and turns into copper oxide. That's the dark coating on the surface. Vinegar contains acetic acid (or citric acid if you used lemon juice), so when you immerse the tarnished coin, the acid dissolves the copper oxide, uncovering the shiny metal beneath.

What's the chemistry here? The acid has hydrogen in it, which will react with the oxygen in the oxide and turn into water.

If you soak large numbers of coins, you'll notice that your vinegar takes on a greenish tinge. This is a chemical called copper acetate and it's produced when the acetic acid reacts with the copper oxide. If you evaporate all of the vinegar away by placing the liquid on a saucer on a hot surface, you'll be left with some pretty green crystals - but don't eat them!

Create your own submarine

Take a large (2-litre) plastic bottle and fill it to the brim with water. Add an unopened sachet of ketchup (like the ones given out at fish and chip shop). The sachet will just about float, but if it's too buoyant add a little piece of Blu-Tack to the outside so that it is sitting just at the surface of the water. Now screw on the cap tightly and squeeze the bottle hard. The sauce submarine will dive to the bottom of the bottle.

Why? It's all down to a piece of physics worked out by the famous Greek thinker Archimedes. When you squeeze the capped bottle, you apply pressure to the water it contains. Liquids are incompressible, so the pressure is transmitted to the sauce sachet, which contains small amounts of nitrogen gas (to keep the sauce fresh). The gas is compressed by the squeezing effects of the water, and because it shrinks, it reduces the amount of space taken up by the sauce sachet as a whole. This means that the density of the sachet becomes greater so it can no longer float, and it dives. This is, in effect, how a submarine controls its depth, by increasing or decreasing the air volume in buoyancy tanks.

Fool your senses

Fill three bowls with water at different temperatures. Into the first add ice-cold water; to the second tepid water and to the third hot (but not scalding) water. Put one hand into the ice water and the other hand into the hot water - at the same time - for one minute. Then put both hands into the tepid water. What happens?

Despite the fact that both hands are now in water at the same temperature, one hand (the one that was in the hot water) is telling you that the water feels cold, but the other hand that the water feels hot. After a short while they begin to feel the same.

Why? This is because our senses are relative. They don't measure an absolute temperature. Instead they measure how things change, and when something ceases to change we stop noticing it.

This is a process called adaptation and it is intended to prevent sensory overload. It explains why we get used to smells that are around for a while, and why we are not continuously aware that we are wearing clothes.

Matchstick boats

Take a matchstick and ask an adult to make a cut some way along the length of it using a knife, turning the matchstick into a Y-shape. Float the matchstick on the still surface of a bowl of water. Add a drop of washing up liquid to the split end of the matchstick. It zips away across the water surface, but why?

This is down to surface tension. Water molecules link together; each being pulled equally in all directions, except at the surface where the water meets the air. The water surface is under "tension", like a piece of clingfilm stretched over a jar. But anyone who has used clingfilm knows only too well that when a tiny split appears, it rips rapidly and the two sides of the tear move apart from each other. This is what happens when the detergent is added. It breaks apart the linkages between the molecules. As the tear enlarges, the surface tension pulls the matchstick towards the sides of the container.

Electric slime

Mix some cornflour with cooking oil until you get a consistency like thick cream. Blow up a balloon and rub it on your hair to charge it up with "static" electricity. Now move the balloon close to the cornflour, and what as the "slime" begins to behave strangely, moving towards the balloon and becoming much thicker.

Why? The cornflour is made up of tiny particles of starch, each less than 10,000th of a millimetre across. The balloon rubbed up on your hair has a positive charge. So when the particles get near to the balloon, negatively charged electrons inside the particles will move towards the balloon, making the side of the particle nearest to the balloon more negative; the side farther from the balloon will, therefore, become slightly more positive. Because the negative end of the particle is nearer to the balloon, it will be attracted more strongly than the more distant positive end is being repelled, which is why dust is attracted to charged objects.

But in our slime, because there are billions of tiny particles, each surrounded by an insulating liquid (the oil), the changes cannot move between the particles, so each one of them develops a more positive end and a more negative end. This makes each particle try to stick to the particle next to it, so the thickness of the slime increases greatly.

Chew your way to sweet bread

Take a slice of cheap white bread, put it in your mouth and chew it. Whatever you do, don't swallow it, just keep chewing, while paying attention to the flavour of the bread. If you keep chewing for long enough, you'll notice the bread begins to taste very sweet, but why?

It is because bread, which is made from flour, contains a plant polymer called starch or amylose. This consists of lots of glucose (sugar) molecules linked end to end. When we eat starch (either as bread, cakes or potatoes), enzymes in our digestive tract chop up the starch releasing individual glucose molecules we can absorb. So what's happening in your mouth?

Well, saliva contains a starch-busting enzyme, called ptyalin. If you chew the bread for a sufficiently long time, enough ptyalin is produced to break down the starch and release sugar in your mouth.

Why do we have this enzyme in our mouths? Some scientists think that that ptyalin might work a bit like a chemical toothpick, dissolving pieces of starch that are left stuck between our teeth.

Ping-pong levitation

Take a hairdryer set to blow cool air (or take a bendy drinking straw and blow through it hard). Point it vertically upwards and position a ping-pong ball in the air stream.

Mysteriously, the ball bobs around in the air flow, without falling off. But what keeps it there? Why doesn't it fly off and fall to the floor?

This is an example of what is called the Coanda effect. Air, when it flows near a curved surface, tends to follow that surface and stick to it. So if our ping-pong ball moves to the left, the air around the right of the ball will want to follow the ball and so the air will also tend to move to the left.

There's a very important law called Newton's third law of motion, which says that every action has an equal and opposite reaction. So, if air moves around the ball to the left, the ball must be being pushed to the right.

So the Coanda effect holds the ball in place by pulling it left if it moves right and right if it moves left. In fact, the effect is so strong that you can even tilt the hairdryer and the ball will stay levitated in the angled air stream!

See sugar light up

Crushing sugar cubes in the dark with a pair of pliers creates an awesome light show. This is a phenomenon called triboluminescence. When pressure is applied to the cube, it causes a charge to accumulate in parts of the sugar crystals. When each charge becomes big enough, it is discharged as a tiny bolt of lightning. You can pull the same stunt with sugary sweets, by biting them hard.

Make a whirlpool

Take two empty lemonade bottles and fill one about two-thirds full of water. Invert the empty bottle over the full one and tape the necks of the two together. Then, turn the bottles upside down so that the full one is at the top. Give them a swirl, and you should see a whirlpool.

What's going on? When you give the bottle a swirl, the water starts to move in a circle. As the water passes through the neck connecting the bottles, it's forced to spin in ever smaller circles, and is also accelerated because it is falling downwards. Together these two effects cause the water to spin so fast that it is flung against the sides of the bottle, creating a whirlpool with a hollow centre.

For more experiments to try at home, or to download the Naked Scientists' podcast, visit www.thenakedscientists.com. Cambridge Science Week runs until 25 March, www.cambridgescience.org

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