 Okay, I'm going to look specifically at balancing combustion reactions because they can occasionally throw up a few difficulties, but as long as you follow this basic method of balancing them you should always be okay. The basic method relies on the order of balancing and you can remember it by CHO, CHO. First balance the carbon, then balance the hydrogen, then balance the oxygen. So this is specifically for combustion of carbon based fuels. So let's try methane and burning in oxygen. So methane is CH4 plus oxygen, gives carbon dioxide plus water. Okay, so balance the carbon first, one carbon on the left, one carbon on the right. That's fine. So next the hydrogens, H4. So four hydrogens on the left and over here in the water two hydrogens. So we need four hydrogens on the right. So we're going to put a two in front of the water, that will give us four hydrogens. Okay, now we do the oxygen. We've got O2, so two on the left and on the right we've got two oxygens in the carbon dioxide, one oxygen in the water but there are two waters, so two there. So a total of four on the right. So two on the left, four on the right. We just need to double our number of oxygen atoms on the left. Okay, let's try butane. Oxygen is the fuel that is used in lighters and sometimes to fire barbecues. And its formula is C4H10, again we burn it in oxygen and it also produces carbon dioxide and water. Okay, so CHO, carbon's first, four carbons on the left and only one on the right. So we need four carbon dioxide to use up those four carbons. H next, we've got 10 hydrogens on the left and we have only two on the right. So we're going to need five water molecules in order to use up those 10 hydrogens. And the last thing to do is the oxygen, so we've got two on the left and we've got four times two, it calls eight oxygens from the carbon dioxide and five oxygens from the water, that's a total of 13. Okay, so this is where it might seem tricky. We've got a total of 13 oxygens on the right and we've got two on the left. Now how many oxygen molecules do we need to provide 13 oxygen atoms? What we need is seven and a half molecules, or 13 on two. But we're not really allowed to have fractional numbers of molecules. So how can we turn this into a whole molecule? Well, the easiest thing is to multiply through the entire equation by two. If you double everything, then the oxygen will become a whole number. So I'll put them underneath here. We'll double our butane, double our oxygen, that gives us 13, double our carbon dioxide, that gives us eight and double our water, that gives us 10. So I'll just write out the formulae again and we'll just go through and do a double check that it's all balanced. Carbon's first, two times four on the left, that's eight, and eight carbons on the right. Hydrogens, two times 10, that's 20 hydrogens on the left, and 10 times two, that's 20 hydrogens on the right. And then oxygens, 13 times two is 26 oxygens on the left. And then we've got eight times two is 16, plus 10 is 26, so we're all good. And this frequently happens with combustion reactions. If you ever run into a situation where your oxygen looks a bit weird, that you've got four and a half, or six and a half, or 10 and a half oxygen molecules, all you need to do is multiply through the entire thing by two. Okay, last one, ethanol. Ethanol has the formula C2H6O. Ethanol is the alcohol that's in spirits and beer and things like this. And it's also blended with our petrol in cars. Burn it in oxygen, and it also gives carbon dioxide and water. Okay, carbons first, two carbons on the left, so we need two on the right. Six hydrogens on the left, so we need three water molecules to use up those six. And then for oxygens, on the left, we have the two from the oxygen molecule. But don't forget there's also one in the ethanol. So we actually have three over on the left. On the right, we've got two times two is four, plus three is seven altogether. So somehow we need to change the left hand side to get seven oxygen atoms. Now of those seven, one of them is going to come from the ethanol. So that only leaves six, and we can get those six from three oxygen molecules. So just a final double check, two carbons on the left, two carbons on the right. Six hydrogens on the left, three times two is six hydrogens on the right. Three times two oxygens plus one is seven. And over here we've got two times two is four, plus three is also seven. So.