1)After burning, the mass of the ash is lighter than the mass of the wood.
2)After burning, the mass of the product, magnesium oxide is heavier than the mass of the magnesium ribbon. ( Burns the ribbon)
3) In a chemical reaction, the total mass of the reactants equals the total mass of the products.
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So let see, in 3) this is because the atoms reactants and products must be the same. Then why in 2)
the mass of the product is heavier than the mass of the reactants ( ribbon)?
Shouldn’t it be the same?
And according to 3) then the mass of the wood in 1) should also be the same how come it is different? I know the mass of ash is of course lighter than wood, but if you combine all the ash together wouldn’t it be the same? Please explain 1) 2) 3) to me why it is different I don’t get it. Thank you.
Edit: I understand it now my stupid ignorant 14-year-old brain didn’t think of the external factors which caused the mass of the ash to be lighter.
In: Physics
Instead of doing these experiments in the open, let’s imagine we’re doing them in a sealed container – the container just contains the fuel (the wood or the magnesium) and enough air for the substance to burn in.
If we weigh our sealed container, with the fuel and the air sealed inside it both before and after we burn the fuel it’ll weigh the same.
So, what does this tell us? If the weight of the combustion product (the wood ash, or the magnesium oxide) **plus** the weight of the air is the same before and after combustion then we know that everything that we had at the start of the reaction is still there at the end.
Because the combustion product weighs a different amount, it must mean that something has either gone into or out of it – the only place it could have either gone to or come from is the air because there’s nothing else in our sealed container.
In the case of our magnesium making magnesium oxide, oxygen has been taken from the air. If you could weigh just the air in the container, you’d find that it was lighter than when we’d started as some of it is now in the solid magnesium oxide.
For the wood, we have the standard combustion products of burning something carbon based – usually carbon dioxide, but possibly also carbon monoxide too. Again, we’ve taken oxygen from the air but in this case it’s formed a gas. There may also be things like water vapour from the wood which have escaped from the solid and are now in the air. In this case, although the wood ash is lighter than the wood was, if we weighed just the air in the container, it would be heavier.
You’re forgetting the things that are added or removed from the “final product”.
> After burning, the mass of the ash is lighter than the mass of the wood.
Because a chunk of carbon has reacted with oxygen to form CO2 gas that floats away, in addition to water vapor, soot, or other smoke particles that no longer exist in the ash.
> After burning, the mass of the product, magnesium oxide is heavier than the mass of the magnesium ribbon
You’ve added oxygen. That’s what “Oxide” means, you take magnesium, add oxygen, and form magnesium oxide. The added oxygen is what you’re registering as heavier.
> In a chemical reaction, the total mass of the reactants equals the total mass of the products.
Correct, but you must actually identify the reactants involved. All of them. Not just the ones that are solid and you can see.
>In a chemical reaction, the total mass of the reactants equals the total mass of the products.
That’s always the case (ignoring relativistic effects).
>After burning, the mass of the ash is lighter than the mass of the wood.
But add in the CO2, soot, and water vapour that was produced by the burn and you end up with the same mass as the wood and oxygen that was used.
>After burning, the mass of the product, magnesium oxide is heavier than the mass of the magnesium ribbon.
Oxide being the operative word. You’re adding oxygen. Oxygen has mass.
You seem to be ignoring the masses of the gases in the reactants and products.
When burning wood, carbon in the wood reacts with oxygen to form carbon dioxide gas. Thus some solid mass is consumed when forming gaseous products.
The opposite holds for the magnesium example: gaseous oxygen is consumed by reacting with solid magnesium, producing a solid product. Thus the mass of the oxide should be greater than the mass of the metal reactant.
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