A thermometer is used to measure temperature. A simple thermometer is a glass tube with a scale drawn on the outside and a non-transparent liquid inside. As the temperature increases, the liquid increases its volume and this can be measured with the scale. By placing the thermometer in freezing and boiling water, a temperature scale and a unit of measurement can be defined that can be reproduced anywhere. It is enough to assign a temperature value to each point on the scale that corresponds to the volume occupied by the liquid at each temperature. 

On the Celsius scale, the value of 0 ºC was assigned to the freezing point of water and 100 ºC to the boiling point. Dividing that distance on the scale into 100 equal parts defines 1 ºC.

On the Fahrenheit scale , traditionally used in English-speaking countries, these points were assigned values ​​of 32 ºF and 212 ºF respectively.

100 units on the Celsius scale are equivalent to 180 (212 -32) on the Fahrenheit scale. But since the value of 0 on one scale is 32 on the other, the equivalence between 1 ºC and 1º F is expressed as follows: 

  Simplifying: 

Later, the Kelvin scale was developed, based on the concept of absolute zero, that is the lowest theoretically possible temperature, where the particles of a system have the minimum kinetic energy. In this scale, absolute zero is set as 0 Kelvin (0 K), and is equivalent to -273.15 °C. One unit on the Kelvin scale is equivalent to one unit on the centigrade scale. Thus:

T (ºC) = T (K) - 273

Note that Kelvins are not called "degrees" nor do they have the "º" before the K.

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The particles of two bodies at different temperatures move at different speeds (higher in the one at the higher temperature). If we put the bodies in contact, the particles with greater kinetic energy will transfer part of their energy to those with lesser energy through collisions between them, until they all acquire the same speed, that is, the same temperature. When this happens, it is said that they have reached thermal equilibrium .

Heat is the energy that is transferred from a body at a higher temperature to one at a lower temperature. It is energy in transit. A body does not have heat, but rather exchanges it.

We call ΔT the difference between the final temperature of a body after having exchanged heat with the environment (Tf) and its initial temperature (Ti): ΔT = Tf - Ti.

The greater ΔT in a process and the greater the mass of a body, the greater the heat that the body can give off or absorb. Heat is directly proportional to these variables. It also depends on the type of material, which is taken into account in another factor called specific heat (ce):

                                                                Q = m · ce · ΔT

Heat is measured in Joules (J) in the SI, and the calorie (cal) is also often used: 1 J = 4.18 cal 

Specific heat is measured in J/kg·K or cal/gªC