Heat is usually thought of as something that provides warmth or cooks food. But what is heat really? Scientifically speaking, heat is energy, kinetic energy. Students already know that kinetic energy is the energy of motion. So where is the motion involved in the kinetic energy of heat?  

To see the motion associated with the kinetic energy of heat, we have to actually look at molecules! But first, let’s dispel a common misconception… heat is not matter. You cannot use a triple beam balance to measure out 10 grams of heat any more than you could measure out 10 grams of electricity or 10 grams of acceleration! However, as we’ll see, the ability to transfer heat is a property of matter. In this CELL, students will explore heat in terms of its energy by learning the relationship between kinetic energy, temperature, and heat.

Students will learn that heat can be transferred from one location to another through conduction, convection, and radiation and that materials differ in their ability to transfer heat based on their composition. Students will also learn how to determine how quickly heat is transferred from one substance to another. In addition, students will explore how heat is transferred in chemical reactions, and learn how heat transfer applies to the human body. Finally, students will explore how heat transfer is affected by mass and surface area.

Heat is Kinetic Energy: Molecular Motion

Heat is the difference in energy between molecules of higher kinetic energy and molecules of lower kinetic energy. The kinetic energy of molecules in a sample of matter helps determine whether the sample is a solid, a liquid, or a gas.

The video shown here illustrates the movement of molecules in a solid, liquid, and gas. The association between kinetic energy (the energy of motion) and temperature is very clear if we look at molecules. The molecules in solids have the lowest kinetic energy of the three forms of matter. When the kinetic energy of molecules increases, they become more active and move farther apart. As its molecules move faster and farther apart, a solid becomes a liquid. As a liquid’s molecules move even faster and spread even farther, the distance becomes so great that the liquid eventually turns to a gas. Therefore, gas molecules have the greatest amount of kinetic energy of the three states of matter.

A good example of a substance that readily demonstrates all three states of matter is water. We can observe the difference in kinetic energy of the water molecules by observing the state of the water: ice, liquid water, or steam (gas).

Each of these forms of water is associated not only with a particular physical form but also with feelings of cold or warmth. These feelings are a qualitative property of cold or warmth. However, they can be quantified using a thermometer to find the temperature. Temperature, then, is a measure of the kinetic energy of molecules. When we determine the temperature of a substance, we are measuring its kinetic energy.

Thermometers: Measuring Kinetic Energy

Temperature is measured using a thermometer. Glass thermometers consist of a glass bulb at the base of a hollow glass tube that is sealed at the top. The bulb contains a liquid, usually alcohol, that has been colored so that it can easily be seen. When the thermometer is placed in a substance such as a liquid, kinetic energy is transferred from molecules of higher kinetic energy to molecules of lower kinetic energy. The thermometer is said to have registered when the kinetic energy of the liquid equals the kinetic energy of the substance outside the bulb. The temperature of the substance can then be determined by finding the marking on the scale next to the top of the liquid in the thermometer.

If the kinetic energy of the liquid in the thermometer is lower than the kinetic energy of the substance surrounding the bulb, kinetic energy is transferred from the substance to the liquid in the thermometer. This causes the kinetic energy of the molecules in the thermometer liquid to increase, and they move farther apart as they become more active. This causes the liquid to expand and rise into the hollow tube, and is observed as an increase in temperature. If the kinetic energy inside the thermometer is greater than the kinetic energy of the substance (the sample) outside the thermometer, kinetic energy is transferred out of the thermometer. This causes the molecules of the liquid within the thermometer to slow down and move closer together. The liquid contracts, and this is observed as a decrease in temperature.

Temperature Scales

There are several scales associated with temperature: Celsius, Fahrenheit, and Kelvin. The Celsius scale is the scale most frequently used by scientists and the one we will always use in LabLearner because it is based on the freezing and boiling points of water (see the water phase graphic above). On the Celsius scale, water freezes at 0 and boils at 100, and is broken into increments called degrees. The units are written as ^oC.

The Fahrenheit scale is a more familiar scale for everyday use in the United States, and students may associate this scale with daily weather reports. While used frequently in daily life, the Fahrenheit scale is rarely used in scientific laboratories because it is broken into 180 increments between the freezing and boiling points of water, and scientists prefer to work with 100-increment scales. For this reason, students will not use the Fahrenheit scale except as a reference point as they begin working with the Celsius scale.

The Kelvin scale is the origin of the phrase absolute zero, which is equal to -273 ^oC. The Kelvin scale is not encountered in daily life, but instead is used primarily by physicists and chemists who work extensively with gases. Students are not expected to understand or use the Kelvin scale at this level, and it will not be addressed within the scope of this Investigating Heat CELL.

Investigation One introduces students to the concept of heat as kinetic energy. Through experimentation and observation, students will learn that a thermometer works through the transfer of energy, and that temperature is a measure of the kinetic energy of molecules. Students will also observe the how kinetic energy differs in water of varying temperatures by observing how the molecules of food coloring behave when placed in cold, warm, and hot water.

NOTE to Teacher: In this CELL, heat is described as the transfer of kinetic energy between molecules with different amounts of kinetic energy. Although the term molecule is used in the definition, it is important to note that while most types of matter are composed of molecules, some types of matter also consist of other types of small particles called ions. The difference between molecules and ions is related to the way in which each of these smaller particles of matter interacts within the substance. However, this difference is not the focus of this CELL. For this reason, and because molecules do represent a common type of particle of which matter is composed, the concepts presented in this CELL are described in terms of only molecules as both the concepts molecules and heat transfer may be new to students. Introduction to heat transfer within the context of kinetic energy transfer between molecules should provide students with a solid foundation for later study of heat capacity and heat transfer involving other particles of matter including ions.