 In this video you're going to learn what reaction rate is as well as some ways of measuring reaction rate. So your first question is what is reaction rate? Reaction rate is the measure of how quickly the reactants in a reaction change into the products of the reaction. Now let's look at how you can measure reaction rate. The rate of a chemical reaction can be measured in two ways. The first way is to measure how quickly the reactants, that's the substances on the left of the arrow in the equation, decrease. The second way is to measure how quickly the products, that's the substances on the right of the arrow in the equation, increase. Looking at an example for each of these two methods would help you understand this. Our example for way one measuring how quickly reactants decrease is a reaction between marble chips, calcium carbonate and hydrochloric acid. This reaction can be represented by the following equation made up by the reactants calcium carbonate and hydrochloric acid resulting in the products calcium chloride and water and carbon dioxide. As the reaction proceeds the reactants lose mass because carbon dioxide gas is given off. We can measure the decrease in mass of reactants by using this type of apparatus. Based on our measurements we plot a graph of mass of reactants against time which looks like this. The rate of reaction at point A is given by the gradient at A which is Y over X. And the rate of reaction at point B is given by the gradient at point B which is Y dash over X dash. What do you think is happening to the rate of reaction as it proceeds? Pause the video here, give it a think. The answer is that the rate of reaction slows down. In other words the rate decreases. Did you get it right? Do you remember there's another method we mentioned earlier? Consider the same reaction instead of measuring the loss of mass of the reactant we can measure the gain in volume of the product carbon dioxide using an apparatus like this. You can watch our video on measuring the volume of gases here. This time we plot a graph of volume of gas produced against time. The graph looks like this. As before the rate of reaction at point A is given by the gradient at A which is Y over X. And the rate of the reaction at B is given by the gradient at point B which is Y dash over X dash. What do you think is happening to the rate of reaction as it proceeds? Again pause here and try to get the answer. As in the other test it slows down. The rate of reaction decreases. So let's recap. The rate of reaction is a measure of how quickly the reactants change into the products. The rate of reaction can be measured by measuring how quickly the reactants decrease or how quickly the products are formed. The rate of reaction usually decreases as the reaction proceeds.