Number 22: Burning steel

What do you need to make stuff burn? Just three things, oxygen, a energy source to get it all started and some fuel. These three things are there every time you light a candle or start the engine of a car. The candle wax (or petrol) is the fuel, the match (or a spark) gets is all started and the whole thing is kept going with the oxygen in the air.


So what else can we get to burn? How about steel?

Steel is mostly iron mixed with some carbon and sometimes, other metals (depending on what the steel will be used for) and its not something you'd normally think burns.

You'll need

• Steelwool
• 9 Volt Battery
• Some old tiles or something similar that you don't mind getting scorched. 

Safety

This reaction generates a lot of heat (chemists call it an exothermic reaction) and can throw out sparks so make sure there is nothing near by that might catch fire. Have a bucket of water or fire extinguisher handy.  You can also end up with some small particles of steel wool being chucked up and you don't want to get them in your eyes, so wear safety goggles. Make sure there is a responsible adult supervising. Finally, because you get some smoke and sparks produced you should do this outside, you don't want to set fire to the kitchen.

What to do:

1. Fluff up the steel wool a bit. This is to make sure there is plenty of air in amongst it all.
2. Put the wool on the tiles (or whatever it it you are using)

3. Touch the terminals of the battery to the wool.

Almost instantly you'll see part of the wool glowing red hot, very quickly this spreads through the whole clump of wool, consuming it all. Like this:

What's going on:

When you burn things with carbon in them (these are known as organic compounds), like candle wax or fuel in the car, you are reacting the carbon with oxygen to make carbon dioxide gas (which has the chemical formula CO₂ meaning 1 carbon and 2 oxygens). But in this case there isn't any carbon to burn nor are we lighting anything with a flame. Instead the electricity from the battery runs through the steel wool and heats it up. This happens because the electrons and ions that form the electricity collide with other particles that make up the steel wool making them move around, and heat is just the result of particles (like atoms) moving.

The heat speeds up the reaction between the iron in the wool and the oxygen in the air. This would happen anyway, without your help, just much slower (that's why things rust). And this reaction produces heat (its exothermic) which kept the reaction going until you run our of fuel (i.e. the steel wool).

When you burn candles or wood you don't end up with much left over. That's because carbon dioxide is a gas, so it floats away. But when you burn steel wool you end up with iron oxide which is a solid, hence the black stuff that's left over. One more thing,  the chemical formula the iron oxide is Fe₂O₃ ie. 2 iron atoms (which have the chemical symbol Fe) react with 3 oxygens.

Number 16: Cinder toffee

No, I haven't turn this into a cookery blog. There's actually loads of cool science involved in cooking and cinder toffee is a particularly interesting example, plus as an added bonus it tastes great.

You'll need:

• Sugar
• Golden syrup
• A jam/jelly thermometer
• Bicarbonate of soda
• Grease proof paper
• A baking tray
• A saucepan

Safety:
The toffee mix gets very hot, be careful when handling in and make sure there's an adult helping.


What to do:

1. Weigh out 100grams (3.5 oz) of sugar into the saucepan.

2. Add 3 tablespoons of syrup

3. Heat the mixture on a stove whilst stirring it.

4. Check the temperature of the mixture.
5. Carry on heating until it reaches 145-150^oC (293-302).

6. Quickly stir in 1 teaspoon of bicarb. It will suddenly bubble up. 

7. Now pour it into the baking tray, lined with grease proof paper.

8. Leave it to cool. 

9. Break it all up (best done with a hammer) and enjoy!

What's going on?


So that's a nice simple recipe for a tasty treat but where is the science?

First off there's the sugar and syrup. There are actually loads of different types of sugars, the stuff you put in your coffee and the granulated sugar used here is sucrose. It looks like this:

Sucrose

Golden syrup is a mixture of water, sucrose and two other sugars called fructose and glucose. They look like this:

Fructose

Glucose

Sucrose is actually made up of a fructose and glucose molecule that have been joined together.

So why do we need these three sugars to make the toffee? Well, when they are mixed all together they interfere with crystal formation. To explain how this works let's represent each of the sugars with a different shape. 

If we have one type of sugar then the molecules can pack together nice and neatly, like in the diagram. And that is exactly what happens in a crystal. But if you mix them all together they can't form ordered patterns and so you don't get crystals forming.

So if we tried to make the toffee with just one type of sugar then we'd end up with crystals forming which make for hard dense toffee (more like a boiled sweet). But by using 3 different sugars the crystals don't form and instead you end up with a brittle, crunchy, glass like toffee.

Then there's the bicarbonate of soda. You normally put this in cakes to make them rise. That's because when you heat up the bicarb it turns to carbon dioxide gas (hence the bubbles in your cakes). The same thing happens here (and its also almost the same reaction as in the film canister rockets). When you spoon the bicarb into the hot sugar it almost instantly gets converted to carbon dioxide and causes the mixture to foam up.

Hope you enjoy the toffee and whilst you do you can find out more about the science of cinder toffer here.

Number 14: Elephant's toothpaste

Here's a great example of biochemistry in action.

You'll need:

• A bottle of hydrogen peroxide. You can get this from most pharmacists. Try and get 9%, but 6% will do.
• Dried baker's yeast.
• Washing up liquid.
• An empty plastic bottle (500ml or so).
• A glass with a 20ml of water in it.
• A teaspoon.

Safety: 

Hydrogen peroxide is a mild bleach (which is why its used to make your hair go blond). Be careful with it and don't get it in yours eyes. Best get some adult supervision for this one. And it makes a bit of a mess so do it outside.

What to do.

1. Put about 2 teaspoons of yeast into the glass with the water. Mix it about until it looks like brown muddy water.

2. Squirt about 2 teaspoons of washing-up liquid into the plastic bottle.

3. Pour 50ml of hydrogen peroxide onto the washing-up liquid.

4. Pour the yeasty water into the plastic bottle. Stand back and ...

What's going on?

Remember the chemical formula for water? H₂O, meaning water is made from two hydrogens and one oxygen. Well hydrogen peroxide is almost the same except its got an extra oxygen, so its formula is H₂O₂. But whilst water is very stable (remember how we needed to put electricity through water to break it down in the hydrogen and oxygen), hydrogen peroxide is unstable, it slowly decomposes to water and oxygen. However when the yeast is added to the hydrogen peroxide that extra oxygen gets released really quickly and you end up with loads of oxygen gas (the washing up liquid is just there so that the gas gets trapped in all that foam). 

So why does adding yeast result in the oxygen being released so quickly? Well yeast contains an protein called catalase. It looks like this. 

Catalase is a type of protein called an enzyme. Enzymes¹ speed up specific chemical reactions. In this case the reaction is:

2 H₂O₂ → 2 H₂O + O₂

Which is just a way of saying that 2 hydrogen peroxide molecules turn into two water molecules plus 1 oxygen molecule. The really astonishing thing about catalase is how fast it works. One catalase molecule can break down 40 million molecules of hyrodgen peroxide every second!

¹ The word enzyme comes from the greek ενζυμον, meaning "in yeast". Plus things that speed up chemical reactions are called catalysts, hence the name catalase. So if you follow the etymology of these things you end up going around in circles.

Number 10: Polishing the silver

There's all sort of potions and polishes that you can buy to clean up the silver. Don't buy any of them. Because you can find everything you need to get the best cutlery sparking again in your kitchen cupboards.

What you'll need:

• Bicarbonate of soda
• Aluminium foil 
• A cup or glass
• Some tarnished silver spoon (its best if its solid silver, it will work with silver plated stuff, but you run the risk of removing the plating)

What to do:

1. Put a heaped teaspoon of bicarb into the glass.

2. Tear up 10 to 20 bits of foil and add them to the glass.

3. Put the spoon in the cup

4. Pour on hot water. Water from the hot tap will do fine. 

5.Stir and leave for 5 minutes.

6.Take the spoon out and inspect it. All the tarnish will have disappeared. 

Before

After

What's going on:

There's some pretty niffy chemistry going on here.  First lets dispel a little myth, the tarnish is not due to a reaction with silver and oxygen like a lot of people claim. Iron reacts with oxygen to make rust, but silver tarnish is something different. Silver tarnishes when it reacts with sulfur containing chemicals (usually hydrogen sulfide, which smells like rotten eggs) to produce silver sulfide. Eggs are particularly high in sulfur which is why they make your silver tarnish so quickly.

That's the easy bit. How the aluminium and bicarb work to clean the silver is a little more complicated.

Lets take a look at the chemicals that we start off with. You've got silver sulfide (the tarnish), sodium bicarbonate (the bicarbonate of soda), aluminium  hydroxide (the foil is actually covering in a layer of this, the aluminium itself is underneath and we need to get at it).

Step 1: The bicarbonate reacts with the aluminium hydroxide and turns is back to aluminium.

Step 2: The aluminium reacts with the silver sulfide to make silver and aluminium sulfide.

and that leaves you with nice clean silver.