Changes in Heat and Energy Diagrams

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  • 0:05 Changes in Heat and Energy
  • 1:06 What is Temperature?
  • 2:10 What is Heat?
  • 3:10 Diagram of Temperature…
  • 5:11 Application of Temp. &…
  • 7:02 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Did you know that temperature and heat are not the same thing? Did you know that the temperature of water doesn't change when it boils? This lesson describes the relationship between heat and temperature. Diagrams are used to illustrate the relationship between heat and temperature during phase changes.

Changes in Heat and Energy

Have you ever thought of ice as solid water? Did you know that steam is water as well? Water, as well as other matter, can exist in three states, or phases, and we call them solid, liquid, and gas. As ice is heated, its temperature increases, and it melts into liquid water. Likewise, as liquid water is heated, it evaporates into water vapor. These changes from one phase to another are referred to as phase changes.

Did you know that the temperature of water doesn't increase when it boils? The water's temperature increases up to boiling and then remains constant as it boils. We will use diagrams that illustrate the relationship between temperature and heat to explain how this works. Before we do that, however, we need to describe the relationship between temperature and heat.

What is Temperature?

Let's discuss temperature first. You are likely familiar with temperature as it is a common topic of conversation. We express temperature in degrees Fahrenheit, degrees Celsius, and even Kelvin, which is an absolute scale. But, what is temperature? Temperature is a measure of how fast the molecules of a substance are moving. The faster the molecules move within the substance, the higher the temperature. For example, hot water molecules move faster than cold water molecules.

Hot water molecules move faster than cold water molecules and create more energy.
Hot Water Cold Water Molecules

Temperature can be defined as the average kinetic energy of a substance, where energy is the ability to do work. Kinetic energy is energy of motion and thus reflects how fast an object is moving. The faster the object moves, the more kinetic energy it contains. The faster the molecules move that make up a substance, the greater the temperature of that substance.

What is Heat?

Now, let's talk about heat. As substances are heated on a stove, in a microwave, or by the sun, energy is added to the substance. Heat can be defined as the total amount of energy contained within the substance. Where temperature reflects the average amount of kinetic energy, heat reflects the total energy. For example, two liters of boiling water have the same temperature as one liter of boiling water. However, two liters contain more heat - that is, more total energy.

Heat and Temperature Are Related

Now, let's consider the relationship between temperature and heat. As substances are heated, the temperature increases. In other words, as energy is added to a substance, the molecules making up that substance move faster.

Diagram of Temperature and Heat

Now, let's diagram temperature changes in water as heat is applied to the water. This will help us to understand the relationship between heat and temperature. Looking at the diagram below, you will see temperature along the y-axis and heat energy along the x-axis. You can see that the temperature increases as heat is added to water within a phase. In other words, the water molecules within ice move faster.

Phase changes that occur when heat is added to water
Heating Water Diagram

However, temperature doesn't change as heat is added during a phase change; for example, when the ice melts. During the phase change, the added heat doesn't make the molecules move faster, but rather further apart. The heat energy added during the phase change is used to overcome some of the forces that hold the molecules together, allowing them to move further away from each other. During the phase change, the added heat energy is stored as potential energy, or energy of position, as the molecules are now further apart. This is kind of like setting a mouse trap. Potential energy is stored in the mouse trap when it is set; in other words, when the position is changed.

When heat is applied to ice, the molecules move apart instead of move faster.
Ice Molecules Move Apart

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