The rate of a reaction is how quickly reactants are used up or products are formed. To find it you measure how much of a substance changes in a given time. The method you choose depends on what the reaction does.
If a gas is given off, as when magnesium or marble chips react with hydrochloric acid, you can collect the gas in a gas syringe and read the volume at regular time intervals. Another way is to stand the flask on a balance, plug it loosely with cotton wool so that gas can escape but spray cannot, and record the loss in mass as time passes.
If a solid precipitate forms and makes the mixture cloudy, as when sodium thiosulfate solution reacts with hydrochloric acid, you can use the disappearing cross method. You look down through the flask at a cross drawn on paper and time how long it takes for the cross to become invisible. A shorter time means a faster reaction, and so a higher rate. If a solid is used up, such as magnesium ribbon in acid, you can time how long it takes for the solid to disappear.
To make a fair test, change only one variable and keep the others the same, for example the temperature and the volume and concentration of the acid. Repeat the experiment and calculate a mean to make the results more reliable.
