What Does Heat Do

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What Does Heat Do?
Lesson 1 of this chapter focused on the meaning of temperature and heat. Emphasis was given to the
development of a particle model of matter that is capable of explaining the macroscopic observations.
Efforts have been made to develop solid conceptual understandings of the topic in the absence of
mathematical formulas. We learned that heat flows from one object to another (between the system and
the surroundings) when a temperature difference exists between system and surroundings. Now in this
unit we will investigate the topic of measuring the quantity of heat that is transferred between the system
and the surroundings. This lesson is devoted to calorimetry - the science associated with determining the
changes in energy of a system by measuring the heat exchanged with the surroundings. The question is:
what does heat do? When heat is lost or gained by an object, what does it do?
For some students, the very question what does heat do? is confusing. Think about the question a
moment. Does the question (not just the answer) confuse you? Confusion over the question is sometimes
caused by misconceptions about what heat is. The reason for the lengthy discussions in Lesson 1 was to
provide a solid conceptual foundation for understanding the mathematics of Lesson 2. If the question is
confusing, you might want to review Lesson 1 or at least review the discussion pertaining to What is
Heat? In Lesson 1, it was emphasized that heat is not something that is contained in an object. Objects do
not contain heat. Objects, which are made of atoms, molecules and ions, contain energy. Heat is the
transfer of energy from an object to its surroundings or to an object from its surroundings. So the question
being asked on this page is what does this heat do to the object and to the surroundings when it is
transferred? Like many questions in physics, it is a simple answer with deep meaning. Simple answers
with deep meaning always exercise the brain. So put on your thinking cap and let's get to the answer.

Heat Changes the Temperature of Objects
What does heat do? First, it changes the temperature of an object. If heat is transferred from an object to
the surroundings, then the object can cool down and the surroundings can warm up. When heat is
transferred to an object by its surroundings, then the object can warm up and the surroundings can cool
down. Heat, once absorbed as energy, contributes to the overall internal energy of the object. One form of
this internal energy is kinetic energy; the particles begin to move faster, resulting in a greater kinetic
energy. This more vigorous motion of particles is reflected by a temperature increase. The reverse logic
applies as well. Energy, once released as heat, results in a decrease in the overall internal energy of the
object. Since kinetic energy is one of the forms of internal energy, the release of heat from an object
causes a decrease in the average kinetic energy of its particles. This means that the particles move more
sluggishly and the temperature of the object decreases. The release or absorption of energy in the form
heat by an object is often associated with a temperature change of that object. This was the focus of the
Thermometers as Speedometers in Lesson 1. What can be said of the object can also be said of the
surroundings. The release or absorption of energy in the form heat by the surroundings is often associated
with a temperature change of the surroundings. We often find that the transfer of heat causes a
temperature change in both system and surroundings. One warms up and the other cools down.

Heating Curves
So the second answer to the question What does heat do? is that it contributes to changes in state of a
substance. Most students are familiar with at least three states of matter - solid, liquid and gas. The

addition of heat to a sample of matter can cause solids to turn to liquids and liquids to turn to gases.
Similarly, the removal of heat from a sample of matter can cause gases to turn to liquids and liquids to
turn to solids. Each of these transitions between states occur at specific temperatures - commonly referred
to as melting point temperature, freezing point temperature, boiling point temperature and condensation
point temperature.To further illustrate this relationship between heat transfer, temperature change and
change of state, consider the following thought experiment. Suppose that a sample of water was placed in
a Styrofoam cup with a digital thermometer. And suppose that the water is placed in the freezer
(temperature = -20°C) and frozen. Suppose that the thermometer can be connected to a computer with
software that is capable of collecting temperature-time data. After the water has frozen and remained in
the freezer for several hours, it is removed and placed in a beaker on a hot plate. The hot plate is turned
on, gets hot, and begins transferring energy in the form of heat to the beaker and the water. What changes
would be observed in the temperature and the state of matter of the water over the course of time? The
diagram below depicts the so-called heating curve for the water. The heating curve represents the changes
in temperature with respect to time for a sample of matter (such as the water) to which heat is
transferred.Observe that there are three sloped sections and two horizontal sections on the temperaturetime plot. The first sloped section corresponds to a change in temperature of the ice from -20°C to 0°C.
The water in its solid state is warming up to the melting point - the temperature at which water transitions
between the solid and the liquid state. The heat transferred to the ice causes a temperature change. Once
the transition temperature (melting point) of 0°C is reached, the heat added to the water causes the water
to change from its solid state to its liquid state. This is referred to as melting. The melting occurs at a
constant temperature. During this stage of the experiment, the energy absorbed by the water is used to
loosen the attractions that hold one ice particle to another. Once all these attractions are loosened, the ice
would be observed to have entirely melted. The contents of the Styrofoam cup are completely liquid. The
next section of the heating curve is a sloped section. The liquid water is increasing its temperature from
0°C to 100°C. The boiling point of water is 100°C; this is the temperature at which water transitions from
the liquid state to the gaseous state. Once the sample of water reaches this temperature, boiling occurs.
Large bubbles of gas would be observed forming throughout the bulk of the liquid. The heat added to the
liquid during this stage of the thought experiment causes a loosening of the attractions that hold the water
particles in the liquid state. The temperature remains constant while the state of water changes. Once all
the water transitions from the liquid to the gaseous state, the sample of water (now in the gaseous state)
begins to increase its temperature again.
In summary, the three sloped sections represent heat causing a temperature change in the substance that
absorbs it. And the two plateau sections represent heat causing a change of state in the substance that
absorbs it. An inquisitive student might ask, "What is the particle-level explanation of these changes?"
(Thanks for asking.) The temperature changes are the result of the added energy causing the particles of
water to move more vigorously. Either the particles of solid vibrate more vigorously about their fixed
positions or the particles of liquid and gas move about their container more rapidly. Either way, the
addition of heat is causing an increase in the average kinetic energy of the particles in the sample of
water. The changes of state are the result of the added energy causing changes in the strength of the interparticle attractions. The attractions that hold water in the solid or in the liquid state are being overcome.
The energy is being used to loosen these attractions and change to a state of greater potential energy.

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