One of the most important parameters in thermodynamics is the temperature of an object. The concept of temperature, similar to the concept of force, originates with the sensory perceptions of humans. Temperature is rooted in the notion of the “hotness” or “coldness” of objects. Humans use their sense of touch (and sometimes sight) to distinguish hot objects from cold objects and to arrange these objects in their order of “hotness”. But, no matter how sensitive one’s touch may be, the humans are unable to gauge this quality in a quantitative manner over a large temperature range.
A better approach to define the temperature of a substance is by understanding what happens at the microscopic level – that is, how the individual molecules that make up that substance behave. It is known that molecules in any substance are always in motion The molecules move faster higher temperatures, with the result that they have more kinetic energy.
is defined as a measure of the average kinetic energy of the particles in matter. In everyday usage, temperature indicates a measure of how hot or cold an object is. Temperature is an important measurement parameter in chemistry. When a substance changes from solid to liquid, it occurs because of an increase in its temperature. Chemical reactions almost always proceed faster when the temperature is increased.
Unfortunately, this microscopic definition of temperature is not very helpful in a practical sense. The relationship between the average kinetic energy of the molecules and temperature is different for each type of matter (gas, liquid, or solid) and often depends on the type of molecule that makes up that substance. In addition, this definition does not indicate how to measure the temperature. From the practical point of view, it is impossible to analyze each individual molecule in a substance in order to determine the temperature of that substance.
A more practical approach to measure temperature is to state that temperature is an intensive property of matter that can be measured with a thermometer. There are many types of thermometers that work in different ways, but all of them work on the basic principle that substances change their properties when the temperature changes. A good example is that almost all liquids increase their volume as the temperature increases. One of the first thermometers ever built was a long glass tube filled with mercury. The height of the liquid in that column increases as the temperature increases causing the liquid to expand and decreases when the temperature decreases and the liquid contracts.