 In this video, you're going to learn how to balance chemical equations. The law of conservation of mass states that no atoms are lost or made during a chemical reaction. They cannot be made from nothing, and they cannot vanish into nothing. If you don't believe me, you try making an atom from nothing. The number of each kind of atom remains the same during a chemical reaction, even though they might arrange themselves differently in compounds. So here there are six oxygen atoms. And here. And here. But these are three different ways of arranging the atoms. Chemical reactions are all about rearranging atoms. Chemical reactions can be represented by symbol equations, so long as the number of atoms on each side of the equal sign remains the same. Often, when you write out chemical reactions, the atoms do not balance straight away. You need to balance the equations to conserve atoms. You do this by putting numbers in front. Here we start off with three O2 molecules, six oxygens overall. In the formula, you can work out how many atoms there are by doing three times the subscript or little number on the oxygen, which is two. Three times two is six. Then we make two ozone molecules. The subscript is now three, so the number at the front has to be two, so that something times three is equal to six. The big numbers at the front change everything in the formula, whereas the small subscript numbers only change the element they follow after. Until you are really familiar with using formula symbols like this, it's much easier to draw out the molecules in full, as we have done here. So Na2O has two sodium atoms and one oxygen atom, whilst SNO2 has one tin atom and two oxygen atoms. If you have two Na2O, you have two times two sodium atoms, that's four sodium atoms. But you only have two times one oxygen atoms. How many oxygen atoms are there in these compounds? Pause the video for a moment. Ready? Remember, if there is no number next to it, there's just one, not zero.