Temperature is an important factor in determining the rate of chemical reactions, the extent of reactions and the stability of compounds.
Many of the physical and chemical properties of substances are also dependent on temperature. For example, water is a universal solvent at room temperature in liquid form. However, if the temperature is below zero degrees Celsius, water can no longer function as an effective solvent because it becomes solid.
Most biological molecules are only stable at a ‘Goldilocks’ temperature range. Physiological functions may either stop completely, killing the organism, or the organism may enter a state of suspended animation. Tardigrades, for example, can exist in suspended animation, and scientists were able to revive Siberian worms that were frozen in permafrost for 42,000 years.
Many chemical processes, including industrial manufacturing procedures, are sensitive to temperature change. Most procedures require a specific range of temperatures to produce the desired results, such as in the case of petroleum distillation.
In this post:
The Meaning of Temperature in Chemistry
Temperature measures how much kinetic energy molecules or atoms in a substance have.
It can also represent the radiant energy of an electromagnetic wave, as demonstrated by microwaves. 
In chemistry, when energy is applied to a substance/system or when energy is released by a substance, its temperature rises. For example, the reaction of metal sodium with water is extremely exothermic and usually accompanied by a flame as the hydrogen is liberated from the water.
2Na + 2H2O → 2NaOH + H2
In this case, there is no energy input necessary to start the reaction. The heat energy comes from the breaking of the covalent bonds between the oxygen atom and the hydrogen atom in the water molecule.
Conversely, when energy is taken away from a substance/system, or if energy is absorbed by a substance/system, the temperature decreases. Photosynthesis is a good example of an endothermic reaction. Although it’s a complex process that involves several steps and chemical intermediates, as well as chlorophyll, the chemical reaction can be summarised as follows: 
In photosynthesis, glucose is produced with the help of energy input from the sun or any other artificial light source. Hence, the reaction is considered endothermic because it absorbs energy.
How to Measure Temperature in Chemistry
Temperature is not necessarily the same as the total energy of a thermodynamic system. For example, an exploding grenade has less total energy compared to a cold castle. This is mainly due to the mass difference between the two. However, an exploding grenade has a higher temperature because its kinetic energy is greater than a castle made of rocks, mortar and bricks.
Substances have stored or potential energy that can be converted into kinetic energy, either through chemical or nuclear reactions. The latter is a focus of nuclear chemistry, but seldom discussed or dealt with in everyday life. Regardless of the branch of chemistry, temperature is broadly defined and measured in a similar way, namely, using temperature scale.
The instruments used in measuring temperature may vary and can include the following:
- Mercury thermometers
- Resistance or bimetallic thermometers
- Digital thermometers
- Infrared thermometers
- Pyrometer
- Spectroscope
- Langmuir probe
The Temperature Scale
There are several temperature scales used in science and engineering, as well as in everyday activities such as baking. Fahrenheit and Celsius scales are used for a wide range of purposes, including the weather forecast and in medical diagnosis.
Kelvin and Rankine scales, however, are mainly used for scientific processes, like measuring the temperature of stars. The various temperature scales all have standardised measures of temperatures, along with some scientific basis, such as the freezing point of water.
- Fahrenheit scale – although its awkwardness means this scale has fallen out of fashion in the science world, it’s still used in some countries that have not shifted to the metric system, such as the United States. It may seem arbitrary and clunky, but it has some scientific basis. Daniel Gabriel Fahrenheit was a science instrument maker and he borrowed the scale from the Danish astronomer, Ole Rømer, to make his thermometers. Fahrenheit developed a more reliable thermometer that used mercury instead of alcohol. The Rømer was based on 60 because, as an astronomer, it was a base number that was convenient to him. He wanted 60 to be the boiling point of water. Fahrenheit later multiplied the original scale by four to remove the fractions and make the scale more precise. Coincidentally, a one-degree increase in the Fahrenheit scale is equivalent to a one-part in 10,000 increase in the volume of mercury.
- Celsius scale – the Celsius scale was created in the mid 1700s by Swedish astronomer, Anders Celsius. Originally known as the centigrade scale, the Celsius scale puts the freezing point of water at zero, and its boiling point at 100. Today, the Celsius scale is largely used in science as part of the metric system. One calorie of energy input into one gram of water can increase the temperature of the water by one degree Celsius.
- Kelvin Scale – this scale was devised by the English physicist, mathematician and aristocrat, William Thomson, more popularly known as Lord Kelvin. He was knighted by Queen Victoria in 1866 and later ennobled in 1892 in recognition of his scientific achievements. He became formally known as the 1st Baron Kelvin. The Kelvin scale is the same size as the Celsius scale. Zero Kelvin is absolute zero, where molecular motions stop. Absolute zero doesn’t exist in nature and hasn’t yet been achieved synthetically. It’s a theoretical zero temperature that’s equivalent to -273.15 degrees Celsius or -459.67 degrees Fahrenheit. Near this temperature, a fifth state of matter – the Bose-Einstein condensate – can exist.
- Rankine Scale – proposed in 1859, this scale was named after the Scottish engineer and physicist, Macquorn Rankine. It’s an absolute temperature scale similar to the Kelvin scale, but has gradations based on the Fahrenheit scale. It’s used by engineers in many of their computations.
The Common Benchmarks of Temperature in Chemistry
The study of chemistry won’t be precise if there aren’t any benchmarks for certain reactions, chemical behaviours and properties. In most cases, the IUPAC (International Union of Pure & Applied Chemistry) standard conditions are used to define certain characteristics of chemicals.
The standard conditions use temperature and pressure as the parameters:
- 273.15 K (0°C, 32°F)
- 105 Pa (100 kPa, 1 bar)
Room Temperature
This generally refers to the comfortable level of temperature for humans and many other organisms. It can be considered as the optimal level or range of temperature for most of the processes of life. It’s also the temperature wherein everyday phenomena, including chemical reactions, are commonly observed. The IUPAC defines room temperature as 25°C (77°F, 298.15 K).
Curie Temperature in Chemistry
This benchmark is named after Nobel Prize winner, Marie Curie. Otherwise known as the Curie point, the Curie temperature is defined by the temperature threshold at which magnetic materials significantly change their magnetic properties. It varies from one material to another, but the Curie temperature for the mineral magnetite is about 570°C (1,060°F).
What is Critical Temperature?
The critical temperature of a substance is usually in tandem with its critical pressure. It’s the threshold temperature at which the vapour of a substance can still be liquefied under pressure. If the vapour temperature is above the critical temperature, the vapour cannot be liquefied, no matter how strong the pressure. Critical temperature varies from one substance to another. Water vapour, for example, has a critical temperature of 374°C.
What is the Transition Temperature?
The transition temperature applies to crystals and, once again, varies depending on the substance. Transition temperature refers to the threshold temperature at which a substance changes from one crystal state to another. For example, a rhombic sulphur crystal changes into monoclinic crystal when heated above 95.6°C.
Calculating Change in Chemical Temperature
All chemical reactions involve a change in temperature in the system, either due to the absorption of energy (endothermic) or the release of energy (exothermic). Although a chemical system cannot be absolutely contained in terms of energy conservation, temperature change can often be observed and measured.
Calculating the change in chemical temperature involves measurements and calorimetry techniques. You need to know the specific heat of a substance, which is the heat or energy required to increase the temperature of one gram of a substance by one degree Celsius.
The temperature change can be measured as a ratio between the q (the amount of heat measured in joules) and the C (the heat capacity of a substance), as shown in the formula below. 
Similarly, the heat change in a chemical reaction or system can be calculated by simply subtracting the initial temperature from the final temperature. The difference can either be negative or positive.
