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Sensible vs. Latent Heat

Instructor: Jennifer Perone

Ms. Perone has taught College Engineering, Ethics, Psychology, Perception, Statistics, Experimental Design & Analysis, Physics and secondary STEM topics for more than 15 years!

In this lesson, the conceptual difference between sensible and latent heat is discussed. Sensible heat is heat that can be measured on a thermometer, while latent heat is the type of heat energy that allows for phase changes, such as those between liquid and gas forms of a substance.

Properties of Heat

You've probably heard the expression that ''a watched pot never boils.''

Well, does a watched pot boil? Well, YES! It has to. It's a scientific law that water, when exposed to enough heat energy, will boil, no matter who is observing it!

Heat is energy! The First Law of Thermodynamics is called the Law of Conservation of Energy and it states: Energy can neither be created nor destroyed. Energy can only be changed from one form to another.

Heat is a form of energy that causes molecules and atoms in a substance to move, or vibrate, more and more quickly. The amount of heat present is shown by changes in temperature and changes in the state of atoms. You've probably boiled water to cook rice or to make tea. When you've watched the pot or the kettle, you've probably noticed that it takes quite a long time to get the water to boil. There are bubbles that show up on the sides of the container and what you're seeing is these molecules moving faster and faster, until the water comes to a full, rolling boil. In this example, you are seeing both sensible heat and latent heat changes!

Latent and Sensible Heat

What is the difference between latent and sensible heat? There is a simple way to remember the difference between sensible and latent heat. Sensible heat is heat that can be felt and measured by a thermometer. ''Latent'' literally means ''hidden.'' When a substance is changing phases, there is no change in temperature. There is only a change in phase. Hence, the hidden energy of latent heat.

An Example

In the example of water boiling, let's say that we place a pot of room temperature (about 25 degrees C) water onto the stove and place it on the highest setting, as we want to get the water heated up as fast as possible. If you placed a thermometer into the water, it would show a temperature of 25 degrees C and you would observe a rapid increase in sensible heat or temperature. If you remember that the boiling point of water is 100 degrees Celsius, then you know that the temperature or 'sensible' heat must increase to 100 degrees Celsius in order to boil.

Water boils in a pot

This is where the saying that 'a watched pot never boils' comes in. This expression refers to the phase change or the latent heat that causes the phase change. You will see a rapid increase in temperature from 25 degrees Celsius to 100 degrees Celsius and then, the temperature increase will stop. In order for water to boil and to change from liquid form to steam, the molecular bonds in the water must separate and this requires a great deal of energy! There will be no changes in temperature during a phase change.

A water molecule consists of two molecules of hydrogen and one molecule of oxygen. Chemical formula H2O
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Remember that water is made up of two molecules of hydrogen and one molecule of oxygen. When the water goes from being a liquid to a gas (or steam), the bonds between the molecules must be weakened. The water molecule stays intact, but the molecules have less cohesion between one another in the gas form than they did in the liquid form. When molecules of water become steam, these forces loosen up and water molecules float into space further apart than what is seen in the liquid water phase.

Notice that the distance between the water molecules becomes larger as phase changes occur. The bonds of the water molecule remain intact while the cohesion or forces pulling the water molecules together lessen.
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