When heated, most matter decreases in density. The mass depends on the volume and, in turn, the volume depends on the mass. If we maintain the pressure and temperature of this gas and fill an object which can vary its volume, like a balloon, or a cylinder with a sliding end, the final volume depends directly on the amount of the gas that we inject.
Decreasing the volume of the container forces the particles to collide more often, so pressure is increased. As more air goes in, the gas molecules get packed together, reducing their volume. As long as the temperature stays the same, the pressure increases. Fill up three identical cups of water to the same level. Put one cup in the refrigerator to cool, heat one cup in the microwave, and leave one cup at room temperature.
Look carefully to see if you can observe a difference in the volume of water. This document may be freely reproduced and distributed for non-profit educational purposes. Skip to main content. Search form Search. Join The Community Request new password. Main menu About this Site Table of Contents.
Special Feature Type: Weird Science. Further Investigations: What is an Invertebrate? Question Set: What is a Mammal? 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. But does the absorption or release of energy in the form of heat always cause a temperature change? Surprisingly, the answer is no.
To illustrate why, consider the following situation, which is often demonstrated or even experimented with in a thermal physics unit in school. The para-dichlorobenzene will be in the liquid state though much of it will have sublimed and be filling the room with a most noticeable aroma.
Now suppose that a thermometer is inserted in the test tube and that the test tube is placed in a beaker of room temperature water. Temperature-time data can be collected every 10 seconds.
Quite expectedly, one notices that the temperature of the para-dichlorobenzene gradually decreases. As heat is transferred from the high temperature test tube to the low temperature water, the temperature of the liquid para-dichlorbenzene decreases. Based on the thermometer reading, you might think that no heat was being transferred. But a look in the test tube reveals dramatic change taking place. The liquid para-dichlorbenzene is crystallizing to form solid para-dichlorbenzene.
How can these observations help us to understand the question of what does heat do? We have learned in Lesson 1 that heat is transferred between two adjacent objects that are at different temperatures.
The test tube and the para-dichlorbenzene are at a higher temperature than the surrounding water of the beaker. Heat will flow from the test tube of para-dichlorbenzene to the water, causing the para-dichlorbenzene to cool down and the water to warm up.
Two adjacent objects of different temperatures will transfer heat between them until thermal equilibrium is reached. Why does the temperature no longer decrease when the liquid para-dichlorbenzene begins to crystallize?
Is there still a transfer of heat between the test tube of para-dichlorbenzene and the beaker of water even when the temperature isn't changing?
The answer to the question Is heat being transferred? After all, the principle is that heat is always transferred between two adjacent objects that are at different temperatures. A thermometer placed in the water reveals that the water is still warming up even though there is no temperature change in the para-dichlorbenzene.
So heat is definitely being transferred from the para-dichlorbenzene to the water. But why does the temperature of the para-dichlorbenzene remain constant during this crystallization period? Before the para-dichlorbenzene can continue to lower its temperature, it must first transition from the liquid state to the solid state. At this temperature, the energy that is lost by the para-dichlorbenzene is associated with a change in the other form of internal energy - potential energy.
A substance not only possesses kinetic energy due to the motion of its particles, it also possesses potential energy due to the intermolecular attractions between particles. Once all the para-dichlorbenzene has changed to the solid state, the loss of energy is once more reflected by a decrease in the kinetic energy of the substance; its temperature decreases. So the second answer to the question What does heat do?
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. 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.
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