Is Temperature a Physical or Chemical Change? (And Why?)

Temperature is a physical change because it is a measure of the average kinetic energy of particles in a substance and does not involve any alteration in the substance’s chemical composition or identity. ¹ It is a reversible change that can fluctuate based on the amount of energy present in the system.

Well, this was just a simple answer. But there are few more things to know about this topic which will make your concept super clear.

So let’s dive right into it.

Why is temperature a physical property?

Temperature is considered a physical property because it describes a characteristic of a substance or system that can be measured without changing its chemical composition. In other words, temperature is a fundamental property of matter that can be quantified and compared between different substances without altering their identity.

Here are a few key reasons why temperature is classified as a physical property:

• Independent of the amount of substance: Temperature does not depend on the quantity of the material present. Whether you have a small sample or a large one, the temperature remains the same if they are in thermal equilibrium. For example, a cup of hot water and a swimming pool full of hot water can have the same temperature, but the swimming pool contains a significantly larger amount of water.
• Measurable with physical instruments: Temperature can be measured using various physical instruments like thermometers, thermocouples, and infrared sensors. ² These instruments can sense the thermal energy of a substance and provide a numerical value representing its temperature.
• Influences physical behavior: Temperature affects the physical behavior of matter. It determines whether a substance is in a solid, liquid, or gaseous state (phase transition) and also influences its density, volume, and pressure. For example, when water is heated, it turns into steam as its temperature increases. ³
• It is an intensive property: Intensive properties are those that do not depend on the size or mass of the system. Temperature is an intensive property because its value remains the same for a specific material regardless of the sample size. ⁴
• Used for characterization and comparison: Scientists and engineers use temperature as a critical parameter for characterizing and comparing substances and systems. It plays a crucial role in various scientific fields, such as physics, chemistry, engineering, meteorology, and many more.

In summary, temperature is a physical property because it describes a fundamental characteristic of matter that can be measured independently of the amount of substance and influences its physical behavior. Its ability to be quantified and compared between different substances makes it an essential concept in the study of physical sciences.

Why is temperature not a chemical property?

Temperature is not a chemical property because it does not depend on the chemical composition of a substance. Chemical properties, on the other hand, are characteristics of a substance that describe how it interacts with other substances and how it undergoes chemical changes. ⁵

Chemical properties are related to the internal structure and composition of a material and how it reacts with other substances to form new compounds. They involve changes in the chemical identity of the substance. For example, flammability, reactivity with acids or bases, and oxidation potential are all examples of chemical properties. ⁶

In contrast, temperature is a measure of the average kinetic energy of the particles in a substance and is not influenced by its chemical composition. Two substances with completely different chemical compositions can have the same temperature if they are at thermal equilibrium.

For instance, a glass of water and a glass of ethanol can have the same temperature even though they are chemically distinct.

Temperature is primarily a thermodynamic property, related to the energy state of a substance, and it characterizes the hotness or coldness of a system. ^{7 8}

It is an extrinsic property, meaning it can be influenced by external factors like heat transfer, but it does not fundamentally alter the chemical nature of the substance.

In summary, temperature is not a chemical property because it does not describe the chemical behavior or identity of a substance; instead, it is a physical property that reflects the average kinetic energy of the particles within the substance.

How is temperature measured?

Temperature is typically measured using various types of thermometers or temperature sensors. The choice of measurement method depends on the application, the temperature range to be measured, and the level of accuracy required. 

Here are some common methods for measuring temperature:

1. Liquid-in-glass thermometer: This is a traditional and straightforward type of thermometer. It consists of a glass tube filled with a liquid, usually mercury or alcohol, that expands or contracts with changes in temperature. ⁹ The temperature is determined by reading the level of the liquid against a scale on the thermometer.
2. Bimetallic strip thermometer: This type of thermometer uses two different metal strips with different coefficients of thermal expansion, bonded together. ¹⁰ As the temperature changes, the strips bend due to their differing expansion rates, and the resulting movement can be measured to determine the temperature.
3. Thermocouples: Thermocouples are temperature sensors made of two dissimilar metal wires connected at one end. When there is a temperature difference between the two ends, a voltage is generated, and the temperature can be calculated based on this voltage. Thermocouples are widely used due to their wide temperature range and fast response times.
4. Resistance temperature detectors (RTDs): RTDs are temperature sensors made of pure metals, typically platinum, with a known and repeatable resistance-temperature relationship. ¹¹ As the temperature changes, the resistance of the RTD changes, and this change is used to calculate the temperature.
5. Infrared thermometers: Infrared thermometers, also known as non-contact thermometers, measure temperature by detecting the infrared radiation emitted by an object. They do not require direct contact with the object and are useful for measuring the temperature of distant or inaccessible objects.
6. Thermistors: Thermistors are temperature sensors that use semiconductor materials with a strong dependence of resistance on temperature. ¹² Their resistance decreases or increases with temperature changes, and this resistance change is used to determine the temperature.
7. Pyrometers: Pyrometers are used to measure high temperatures, typically in industrial settings. They operate based on the principle of radiation detection and are suitable for measuring temperatures of extremely hot objects, such as furnaces and metalwork.
8. Digital thermometers: Digital thermometers use electronic sensors to measure temperature and provide a numerical readout of the temperature. They are often equipped with additional features like memory storage, alarms, and data logging capabilities.

Each temperature measurement method has its advantages and limitations, and the selection of the appropriate method depends on factors such as the required accuracy, the range of temperatures to be measured, and the specific application or industry involved.

Further reading

Is Melting Point Intensive or Extensive Property?
Is Length Intensive or Extensive Property?
Is Temperature Intensive or Extensive Property?
Is Color Intensive or Extensive Property?
Is Mass Intensive or Extensive Property?

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