1) The amount of product you get from a reaction is its yield.
2) percentage yield compares the amount of products chemists really get with what they expect to get.
3) A 100% yield means that all of the product was made, a 0% yield means that no product was made.
1) A reversible reaction is one where the products can react and turn back into the reactants.
A reversible reaction symbol is 2 arrows going opposite directions one below and atop another.
Three ways why percentage yields are less than 100
1) With reversible reactions some of the products are always reacting together to turn back into reactants. So the yield will be lower.
2) When you filter a liquid to remove a solid, you always lose a bit of liquid or a bit of solid. so, some of the product may be lost when it's separated from the reaction mixture.
3) Sometimes there can be other unexpected reactions happening. these will use up the reactants. This means there's not as much reactant to make the product you want.