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What is the law for conservation of mass?
no atoms are destroyed or created and therefore there are the same type and number of atoms in the reactants and products of a reaction
How do you check if mass was conserved from a chemical reaction formula?
Add up the Mr for the products and then reactants, if mass was conserved they should be equal
Explain why mass may increase after a reaction?
One of the reactants will be a gas found in the air around the reaction
when the gas reacts to form part of the product, it becomes contained within the reaction vessel
so the total mass of the products in the reaction vessel increases
Provide an example of a reaction where the mass of the products increases because of a gas.
metal(s) + oxygen(g) → metal oxide(s)
When a metal reacts with oxygen in an unsealed container, the mass of the container increases and therefore the mass of the metal oxide produced equals the total mass of the metal and oxygen that reacted from the air
However the mass of the products will seem heavier because the mass of oxygen will not have been accounted for in the reactants
Explain why the mass may appear to decrease after a reaction?
One of the products were produced as a gas
if the vessel isn’t contained, the gas product can escape therefore causing the mass to decrease
however the same amount of products and reactants were made, just not contained and accounted for
Provide an example of a reaction where the mass may appear to decrease.
Thermal decomposition of a metal carbonate
metal carbonate(s) → metal oxide(s) + carbon dioxide(g)
Note 
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