One of the most important concepts in chemistry is the idea of conservation of mass, which means that in a chemical reaction, mass is always conserved, or in other words, no atoms are created or destroyed, it's only the bonds between atoms that change. For example, if we took the reaction sodium plus chlorine goes to form sodium chloride, then because we have two chlorine atoms in each chlorine molecule, we'd have to have two sodium chlorides on the right. and so two sodiums on the left, which is the principle you use every time you balance an equation.
We can also see this concept if we compare the total relative formula masses of each side. On the left, we have 2 times 23 for sodium, plus 2 times 35.5 for chlorine, which gives us 117. Then on the right, we have 2 times 23 plus 35.5, which is also 117. so we can see that both sides do balance. Another way to think about it would be to look at the actual masses on each side. So if we reacted 2.3 grams of sodium with 3.5 grams of chlorine gas, then as long as they reacted together completely, we'd make 2.3 plus 3.5, so 5.8 grams of sodium chloride.
So if we did our experiment on a set of scales, then the mass should stay the same throughout the entire reaction. However, there is an important exception to this rule which you need to know about, which is that when your reaction involves a gas, the mass might seem to change, because the gas can come from the air, or float off into the air, in which case they won't be measured by our scales. For example, if we heat up a metal, like magnesium, it will react with oxygen to form magnesium oxide, and because the relative formula mass of magnesium oxide is higher than that of magnesium by itself, it will weigh more. For example, 1 gram of magnesium would go to form about 1.6 grams of magnesium oxide.
So if the oxygen was just coming from the air around us, it would appear as if our product weighed more than our reactants. But really, our scales just didn't weigh all of the reactants, because they didn't take into account In contrast, if we were to do the whole experiment inside a sealed container, the gas would be trapped within the container, so when we measure the reactants mass, it would take into account both the gaseous oxygen and the solid magnesium, so the weight of our reactants would balance perfectly with the products. On the other hand, if we decomposed calcium carbonate to calcium oxide and carbon dioxide, then the gaseous CO2 would float off into the air, and the weight of our products would seem like it had decreased.
Like before though, if we did it in a sealed container, then the mass would stay the same, because all the CO2 would be trapped in the container. So to quickly summarise what you should take away from this video, the first point was the conservation of mass principle, which says that mass is always conserved in a chemical reaction. regardless of whether you look at the mass in grams or you count the number of atoms. Sometimes though the mass may appear to change.
If the mass seems to increase, it's probably because one of your reactants is a gas, while if the mass decreases, it's probably because one of your products is a gas. And finally to avoid this issue, all we need to do is do our experiments in a sealed container. That's all for this one. So cheers for watching and we'll see you next time.