Science experiments: do try these at home

How are clouds formed? And why do oranges float? These Science Museum experiments will answer every child's questions - and amaze adults too

Salt and marble mystery

You will need: A plastic test tube with an end cap or cork, salt and a marble.

What you do: Put the marble in the test tube and fill the test tube three-quarters full with salt, then seal the end of the test tube with a cork. Shake the tube and see what happens.

What happens next: The marble travels through the salt until it is at the top of the test tube. Each time the tube is shaken, the marble and the salt move up at the same speed. Because salt particles are lighter and smaller, they experience greater relative friction than the marble when rubbing against each other. This causes the salt to slow down faster. After each shake, more salt is packed under the marble, until it emerges from beneath the salt.

Soda fountain

This experiment should be done outside.

You will need: A packet of Mentos mints, a two-litre bottle of diet fizzy drink and a plastic box.

What you do: Stand an open bottle of pop in a plastic container. Put the Mentos into a paper tube and add them to the bottle all at the same time. Stand back!

What happens next: Carbon dioxide is pumped into fizzy drink bottles using lots of pressure. Water molecules, which are strongly attracted to each other, cling together, surrounding the bubbles of gas. Because the surface tension is so strong, most of the gas stays suspended in the liquid, and cannot expand to form more bubbles. Mentos contain gum arabic, the sap from an African tree. The proteins in gum arabic break the surface tension of the water molecules, releasing the CO2 gas. As well as this, as the sweets dissolve, tiny pits form on their surface, where CO2 bubbles can form. As the gas is released so quickly, it takes a lot of the bottle's contents with it.

Clouds in a bottle

You will need: A large glass pickle jar, a balloon, a large rubber band, chalk dust or talcum powder, cold water.

What you do: Wash the jar and put an inch (25mm) of water in the bottom. Cut the balloon to form a sheet of rubber, cover the mouth of the jar with it, and hold a book on top to keep it in place. After 10-15 minutes, remove the book and rubber. Drop in a spoonful of chalk dust or talc and quickly replace the rubber. Wrap the band tightly around the rim to keep the rubber in place. Push on the rubber until it is depressed a little way into the jar. After 15 seconds quickly remove your fist.

What happens next: When you push down the air is compressed and warmed, so it can hold more water vapour. When you release the rubber, the pressure drops, cooling the air, and the excess vapour condenses on particles of powder in the air, clouds are created.

Make your own lava lamp

You will need: A clear plastic bottle with a tight lid, a dropper, cooking oil, food colouring and salt.

What you do: Pour water into the bottle until it is two-thirds full. Add a few drops of food colouring. Slowly pour in enough oil to form a layer on top of the water. Sprinkle a few pinches of salt on top of the oil and see what happens! Keep adding more salt to make your experiment last longer.

What happens next: The water and oil do not mix. The molecules that make up water are made up of even smaller building blocks called atoms. All atoms have an electrical charge which can be positive, negative or neutral. One part of the water molecule has a mostly positive charge, and the other part has a mostly negative charge. Molecules like this are called polar molecules - they love to stick together. Oil molecules are different. Their positive and negative charges are spread out quite evenly. They're called nonpolar molecules. When you try to mix polar molecules like water and nonpolar molecules like oil together, the polar water molecules stick together so well that the oil molecules get left out! When you sprinkle salt on top of the oil, it sinks to the bottom carrying globs of oil with it. This happens because the salt is of a greater density than the water. However, unlike oil, salt is soluble in water so as soon as it has dissolved, the oil floats back up to the surface.

Quicksand science

You will need: One 450g box of cornstarch, a large mixing bowl, a baking sheet, a square cake pan, a jug of water, a spoon, newspaper or cloth to cover the floor, water.

What you do: Pour a quarter of the cornstarch into the bowl and slowly add about a half a cup of water. Stir. Add cornstarch and water in small amounts until you get a thick mixture. Try to grab the fluid and pull it up. That's the sensation of sinking in quicksand! Drop a plastic toy into the bowl then try to get it out. It's not easy!

What happens next: The faster you move your hand around, the more like a solid it becomes. The cornstarch and water mixture acts like a solid sometimes, and a liquid at other times. It is a non-Newtonian fluid - which defies Isaac Newton's law of viscosity.

The 'egg in the bottle' trick

You will need: A wide-mouthed glass bottle, hardboiled eggs and matches.

What you do: Make sure your egg is wider than the opening of the bottle. Light a match and put it inside the bottle. Place the egg on top of the bottle.

What happens next: The egg will get sucked into the bottle. The match heats air molecules in the bottle, causing them to move away from each other, and some air escapes from the bottle. When the flame goes out, the air molecules cool down and move closer together. Normally air outside the bottle would rush in to fill the bottle, but the egg is in the way! The pressure of air outside the bottle is so great that it pushes the egg into the bottle.

Floating fizzy drinks

You will need: A can each of regular and diet fizzy drink, and a sink or bucket full of water.

What you do: Put both cans in the water (making sure there are no air bubbles under them).

What happens next: The regular drink sinks, the diet drink floats! The reason is all about density. The can that sinks must be heavier than the can that floats. The diet drink has less mass because it has less sugar.

Gravity in action

You will need: A large paper cup and water - do this in the garden - NOT indoors.

What you do:

Using a pen, poke a hole in the side of the cup, about halfway up. Cover the hole with your thumb and fill the cup with water. Hold the cup up high and uncover the hole. You'll see that the water gushes out quickly. Now fill the cup up again, hold it up high with your finger covering the hole but this time let go of the cup.

What happens next:

When you let go of the cup you'll notice that the water stays inside until the cup reaches the ground. When you're holding the cup, gravity pulls down on both the cup and the water, but the only thing that can move is the water as you're holding the cup in place. Gravity pushes the water towards the bottom of the cup and the weight of the water forces it through the hole. However, when you drop the full cup from a height, gravity pulls equally on the cup and the water so they both fall at the same speed. As they fall together, there is no force pushing the water through the hole.

Orange life jackets

You will need: Two large oranges and a large bowl or basin filled with water.

What you do: Peel one of the oranges and submerge both of them in the water.

What happens next: The peeled orange sinks because the water cannot support it - it is heavier than the volume of water it displaces. The peel of the orange, meanwhile, is made of a spongy material that traps air. This reduces the overall density of the orange as a whole, which allows it to float in water. Density describes how closely packed together all the atoms are in an object. The peel gives the fruit buoyancy in the same way that an air-filled life jacket allows people to float in water.

Invisible ink

You will need: A lemon, plain paper and an iron.

What you do: Using lemon juice, write your message on a piece of paper and let it dry. Then heat the paper with the iron.

What happens next: When the paper dries it is clear, but the iron reveals the message. This is because the acidic juice weakens paper. When you heat paper it discolours, and weakened paper discolours quicker.