Why is latent heat a type of heat if there is no temperature difference involved during phase change?
Heat is commonly defined as an energy in transit due to temperature difference. For sensible heat, this is quite evident. But latent heat, or the “heat” involved during phase change, is not associated with temperature change. So, is latent heat not really a type of heat or is there something that I’m missing?
In: 0
“Latent heat” is a bit of a confusing term, because it is only heat on one side of the equation. It’s actually energy that can turn into heat, or heat that turns into a different type of energy.
It’s similar to how gasoline can be at (say) room temp, but when you burn it, it becomes very hot. The heat that is released used to be stored in the chemical bonds inside the gasoline – it is turned from chemical into heat energy.
When you melt an ice cube, you have to put a lot of energy in to make it change from solid to liquid. This energy goes into loosening the bonds between the water molecules, rather than into heating them up. So heat energy gets converted into chemical energy. If you freeze the water again, that energy will get turned back into heat (but now this is heat that you need to extract, in order to get it to freeze).
That’s why it’s called latent heat, latent meaning hidden, dormant or not yet fully manifest. There’s still heat, or thermal energy being transferred between systems due to temperature difference.
The term latent heat is derived from how people discovered it as a phenomenon.
One way to measure the amount of energy it takes to increase the temperature of a substance by a certain amount (the “specific heat”) is to take a fixed amount of it, burn a fixed amount of a particular fuel, and ensure that all of the energy from the fuel goes into the substance of interest, and measure how much the temperature increases.
One weird thing, though, is that although we all know ice and water are the same substance, if you take 100 G of ice and 100 G of water, and you burn the same amount of fuel to heat up the same substance, the temperature rise of the ice is going to be much lower than the temperature rise of the water.
We can easily measure temperature rise, so we call that sensible heat — sensible meaning capable of being sensed.
But where is the energy going for the sample of ice if it’s not going into the temperature of the water? Why is it that the same amount of energy “steals” temperature increase from the ice? The answer is that there must be something going on which is taking up the energy from burning that fuel, but whatever is going on doesn’t cause the temperature to increase. So we call that latent heat, *hidden* heat.
The reason the term heat is used is because when this stuff was discovered, heat was believed to be a physical thing (“caloric”) that was transferred between substances. Under ordinary circumstances, more caloric in a substance is measurable using a thermometer. So when you melt ice and the temperature doesn’t increase, something must be happening to the caloric. Maybe it’s bonding with the ice particles to make them fluid and it’s trapped somehow. But because you burned the same amount of fuel, you know there has to be the same amount of caloric in the water now. It’s just hidden from you because you can’t measure it with a thermometer.
Latest Answers