What Is Kelvin? Definition, Conversion, & Its Importance in Science
In the international system unit, Kevin is the base unit of temperature and its symbol is K. Kelvin definition is prepared by fixing the numerical value of Boltzmann constant k to 1.380649×10−23 J/K. It was named after physicist and engineer of Glasgow University, 1st Baron Kelvin. Absolute zero is used as the null point in this scale. This is the principle on which the absolute thermodynamic temperature scale is based. Pure water freezes at 273.15K and boils at 373.15 on a Kelvin scale.
Who Invented Kelvin Scale?
William Thomson, later ennobled as Lord Kelvin, wrote in his paper “On an absolute thermometric scale” in the year 1848, the sheer need for a scale where the null point of the scale should be absolute zero or infinite cold. The scale should be able to use degree Celsius for its unit increment. While researching his paper he found that -273o Celsius was equal to zero based on the then air thermometers. This scale later came to be known as Kelvin thermodynamic temperature scale. Also, 273 is considered as the negative reciprocal of 0.00366. This value is the acknowledged expansion coefficient of gas per oC relative to the ice point. This was significantly consistent with the currently accepted value.
Kelvin Scale Definition
The modern definition of the Kelvin scale was given by Resolution 3 of the 10th General in the year 1954 where the triple point of the water was identified and entitled as the second defining point, assigning the temperature to exactly 273.16 Kelvin. Then in 1967 and 1968, the symbol of a degree Kelvin was replaced by the symbol K. This was done to make the thermodynamic temperature Kelvin more useful while defining the magnitude of unit increment. It was later held in Resolution 4 that the fraction of 1/273.16 of the thermodynamic temperature of the triple point of water is equivalent to the unit of thermodynamic temperature. So, when asked “what is K in temperature measurements” can be answered with this.
Recent Changes in Kelvin
Effective from May 20, 2019, the General Conference on Weights and Measure in 2018 decided that the unit would be defined in a manner where the Boltzmann constant would be equivalent to 1.380649 X 10-23 Joule/Kelvin. Originally this unit was defined as 100/27,316 of the triple point. With the Kelvin scale, many formulas and physical laws can be articulated in a simplified manner. This is one of the significant reasons the Kelvin scale is recognized as the international standard for the measurement of scientific temperature. The earlier scientist had an assumption that humans cannot recreate the temperature of absolute zero which occurred between -273.15oC to -459.67oF.
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Measurements of Kelvin Unit
In physical science, it is the primary unit of temperature measurement and is not the same as degree Celsius which has a similar magnitude. Kelvin scale helped in identifying the degree of temperature from ice point to boiling point which is 100 degrees. This is the reason why there are two fixed temperatures on the Kelvin scale. In terms of centigrade, these are 0o C or ice point and 100o C which is the boiling point.
Kelvin Scale and ITS-90
ITS-90 or International Temperature Scale 1990 published by the Consultative Committee for Thermometry consists of 17 reference points that are based on highly reproducible states, naturally occurring triple points or the phase transitions taking place in naturally occurring substances. The following are among the reference points:
The triple point of Hydrogen = 13.8033 K
The triple point of Oxygen = 54.3584 K
The triple point of Mercury = 234.3156 K
The freezing point of Tin = 505.078 K
The freezing point of Aluminium = 933.473 K
The freezing point of Gold = 1337.33 K
Did You Know?
There is no negative Kelvin scale because Kelvin itself is an absolute zero temperature scale. So, 0K in Kelvin refers to absolute zero. This is the point where gas molecules have no or zero thermal energy.
The scientists earlier thought that without adding energy to the system it is not possible to reach absolute zero as the system was always warmer than that. This assumption came to a conclusion when the German physicist managed to push particles in the paradoxical temperature in the year 2013. It is essential to understand that with arbitrary precision a number can be measured on any scale. However, as the Kelvin scale is a physics-based scale it is considered to be the most accurate one.
FAQs on Understanding the Kelvin Scale in Chemistry
1. What is the definition of the Kelvin scale in chemistry?
The Kelvin scale is the base unit of thermodynamic temperature in the International System of Units (SI). It is an absolute scale, meaning its starting point, 0 K, corresponds to absolute zero. This is the theoretical temperature at which all molecular motion ceases, and a substance has zero thermal energy. Unlike Celsius or Fahrenheit, Kelvin does not use degrees; temperatures are stated simply as kelvins (K).
2. Why is the Kelvin scale essential for scientific calculations in chemistry?
The Kelvin scale is essential in chemistry because it is directly proportional to the average kinetic energy of particles. This direct relationship is crucial for laws and equations that describe the behaviour of gases, such as the Ideal Gas Law (PV=nRT) and Charles's Law. Using Celsius or Fahrenheit in these calculations would lead to incorrect results, as their zero points do not represent a true absence of thermal energy.
3. How do you convert temperatures between the Kelvin and Celsius scales?
Converting between Kelvin (K) and Celsius (°C) is straightforward and based on their relationship to absolute zero. The formulas are:
- To convert from Celsius to Kelvin: K = °C + 273.15
- To convert from Kelvin to Celsius: °C = K - 273.15
4. What is the significance of 0 K and can a temperature be negative on the Kelvin scale?
The significance of 0 K is that it represents absolute zero, the lowest possible temperature. At this point, particles have minimal vibrational motion and zero thermal energy. Consequently, a temperature cannot be negative on the Kelvin scale. A negative value would imply a state with less than zero energy, which is physically impossible. This is a key difference from the Celsius and Fahrenheit scales, which have arbitrary zero points and can have negative values.
5. Why is 0°C equal to 273.15 K on the Kelvin scale?
The Celsius scale defines its zero point (0°C) as the freezing point of water. The Kelvin scale, however, sets its zero point (0 K) at absolute zero. Scientific experiments have determined that the freezing point of water (0°C) occurs at a temperature that is 273.15 units above absolute zero. Therefore, 0°C corresponds directly to 273.15 K. This establishes a fixed offset between the two scales, while the size of one unit (1°C) is the same as one kelvin (1 K).
6. How does understanding the Kelvin scale help in explaining the states of matter?
The Kelvin scale directly relates temperature to the kinetic energy of particles, which is fundamental to understanding the states of matter.
- At low Kelvin temperatures, particles have low kinetic energy, causing them to lock into fixed positions, forming a solid.
- As the temperature in Kelvin increases, particles gain enough energy to move past each other, resulting in a liquid.
- At high Kelvin temperatures, particles have very high kinetic energy, allowing them to overcome intermolecular forces completely and exist as a gas.
7. What are some common mistakes students make when using the Kelvin scale?
A common mistake is forgetting to convert temperatures from Celsius to Kelvin before using them in gas law calculations (e.g., Charles's Law, Ideal Gas Law). Using a Celsius value like 25°C instead of 298.15 K will produce a mathematically incorrect answer because these laws rely on the absolute temperature scale. Another error is writing "degrees Kelvin" or using the degree symbol (°), such as 298°K. The correct notation is simply the number followed by "K", for example, 298 K.
