 To be able to make a fair comparison between the heat capacities of different substances, we need to get a bit picky about exactly how we measure heat capacity. For instance, we know that water has a higher heat capacity than concrete, but if you compared a cup of water with a concrete bridge pylon, then the concrete would absorb more heat energy to get to the same temperature, just because there's so much more of it to hold the heat. So to get over this issue, heat capacity is defined precisely as the amount of energy that's needed to raise the temperature of one gram of a substance by one degree Celsius. So we're standardizing both the mass and the temperature change that's required. Now, does that mean that to compare heat capacities, we always need to have exactly one gram of whatever substance we're interested in and to raise its temperature by exactly one degree C and see how much energy that takes? Well, thankfully not, thanks to the magic of maths. So you take 58 grams of water and you raise its temperature by 17 degrees Celsius and you measure that it takes 4,140 joules to do that. A joule is the unit that we use to measure energy. Well then, you used 4,140 joules, that's the amount of heat that went in, but you had 58 grams of water instead of one gram of water. So we just divide the 4,140 joules by 58 to find out how much heat was used for each gram of water. That gives us 71 joules per gram. Okay, but then the temperature rose by 17 degrees Celsius instead of one degree Celsius. So we need to divide that value again by 17 to find out how much energy was used for each degree that the temperature rose and that gives us 4.2 joules per gram per degree Celsius and that's the heat capacity of water. And that actually gives us the equation for heat capacity. You take the amount of energy and you divide it by the mass of the substance and the change in temperature. We can also rearrange that equation so that if you already know the substance's heat capacity, you could use its mass and temperature change to work out how much energy must have been used. And it's playing with this equation like this that we're now going to practice.