 Have a look at this reaction. This is wood burning and forming ash. Now if I ask you to take mass of this wood that is burning and take mass of the ash that is formed, which mass will be more? Well, if you're like me, you will say that the mass of the reactants of the wood over here is more than the mass of the ash formed, right? Ash can even fly off in the air. It's so light. It seems that burning or combustion destroys matter. Or we can say that when something burns, mass decreases. That is, the mass of the reactant, the piece of wood here, will be more than these ashes. But what if I say that the mass really remains the same? How can that be? Let's find out. Well, we'll talk about the law of conservation of mass in this video, which basically, it basically states that mass can neither be created nor destroyed in a chemical reaction. For example, if we take magnesium and heat it so that it starts burning, which will look kind of like this, look at how it burns with a really bright flame. Now the reaction involved here is magnesium plus oxygen, which forms magnesium oxide and it also releases light and heat energy. We can represent this reaction in the form of atoms. So let's say we have two atoms of magnesium. They will react with one molecule of oxygen, which already has two atoms of oxygen. And then it forms two compounds of magnesium oxide. Now you can see that no atoms were created or destroyed in this reaction. We can extend this to say that no mass is created or destroyed. So if we say 48 grams of magnesium and 32 grams of oxygen, then we will get 80 grams of magnesium oxide. Or if we take, let's say if we take 20 grams of magnesium and 10 grams of oxygen, then we will get 30 grams of magnesium oxide. No mass is created or destroyed in a chemical reaction. Now let's come back to the wood example. Over here, wood reacts with oxygen to form carbon dioxide, water vapor and some ash. If this reaction happens in the open, the gases formed will float in the air. But if we had a sealed container like this, where we can entrap or contain the gases, then if we measure the mass of the reactants, that is wood and oxygen, and if we measure the mass of the products, that is carbon dioxide, water vapor and ash, they will come out to be the same. So if there is let's say 500 grams of wood and oxygen combined, then we will get 500 grams of combined carbon dioxide, water vapor and ash. Similarly, if we carry out the reaction of magnesium with oxygen in a sealed container, we will get the products of the same mass as that of reactants. So this is the law of conservation of mass, which says that mass can neither be created nor destroyed in any chemical reaction.