Graham’s law is also known as Graham’s law of effusion or diffusion. It was formulated by British chemist Thomas Graham in the year 1848. Graham’s law tends to establish a relationship between the rate of effusion or diffusion of a gas and its molar mass. Diffusion is the phenomenon that helps a material to move from an area of higher concentration to an area of lower concentration. It helps the molecules or particles of different gases to get mixed with each other and allows a substance to spread across a medium. Similarly, effusion is the process by virtue of which the gas molecules are able to escape through a small opening in the container easily. Graham’s law states that the rate of diffusion or effusion of a gas is inversely proportional to the square root of its density or the square root of its molar mass at a given temperature and pressure. This means that lighter gas effuses or diffuses easily; whereas, heavier gas molecules take time to effuse or diffuse. The limitation of Graham’s law of effusion or diffusion is that it breaks down when the concentration of the gases becomes considerably high. This means that Graham’s law stands correct for ideal gases that are present at low temperatures and pressure. Graham’s law is able to deliver accurate observations in case of effusion; however, in the case of diffusion the results are approximated and the values are rounded off to the nearest possible accurate value. Also, if the temperature and pressure values of the surroundings are not constant and tend to change with respect to time, the observed values of the experiment tend to deviate.
Examples of Graham’s Law
1. Spraying Air Freshener
When a perfume is sprayed in one corner of the room, its fragrance can be felt in the entire room. This is because of the diffusion phenomenon. The rate at which the fragrance spreads across the room is inversely proportional to the square root of the density of the molecules of the gas contained in the perfume and can be evaluated with the help of Graham’s law of diffusion or effusion.
2. Calculate the Effusion/Diffusion Rate of a Particular Gas
Graham’s law of effusion or diffusion is typically used to calculate the effusion or diffusion rate of a particular gas. It also helps to compare the effusion or diffusion rates of two different gases. Thereby, helping the scientists to estimate the time required by a particular gas to escape the container or surrounding in which it is present. This estimation of effusion or diffusion rate is necessary to form the safety measures in case of a gas leak.
3. Incense Stick
One can easily observe the application of Graham’s law in real life by lighting an incense stick and observing the behaviour of the smoke molecules generated by it. This is because the smoke coming out of the burning incense stick spreads in the surroundings quickly with the help of the diffusion process.
When an inflated balloon is pierced with the help of a sharp object, the gas molecules tend to escape through the hole pierced in the balloon. The escaping of gas molecules through the small opening in the balloon represents the effusion process. The time taken by the gas molecules to completely escape and the balloon to get properly deflated can be calculated with the help of Graham’s law of effusion or diffusion. The rate of effusion is different for different gases.
5. Compare the Nature of Various Gases
Graham’s law of effusion or diffusion is highly used to compare the nature of two different gases. The law states that the gases with heavy molecules travel slower than the gases with light molecules. Thereby, establishing a distinction between the molecular weights of the two gases.
6. Comparing Hard and Soft Water
Graham’s law of effusion or diffusion is also applicable to liquids and solids. It is effectively used to calculate the hardness of the water. Let us say that the diffusion rate of two samples of water is known to us, then Graham’s law can be used to determine which of the water samples is heavier thereby, differentiating hard water from soft water.
7. Separation Process
Graham’s law is used in the separation process to separate different isotopes of an element. Also, it helps chemists and scientists to separate a particular gas from a mixture of gases. Hence, Graham’s law of diffusion or effusion finds its prime application in the medical and research domains to extract oxygen and other important gases.
8. Identifying the Nature and Characteristics of an Unknown Gas
Whenever a new element or gas is discovered, its prime characteristics such as molar mass, diffusion rate, effusion rate, etc., are required to be determined to know about its nature. For this purpose Graham’s law is most commonly used.
9. Construction of Chimneys
The architects and engineers make use of Graham’s law to estimate the proper height of the industrial and domestic chimneys. The smoke and toxic gases must be released into the upper atmosphere to avoid health hazards and accidents. With the help of Graham’s law, the diffusion and effusion rate of the gases can be estimated in advance. This helps to determine the appropriate height of the chimneys required to ensure the escape of smoke and toxic gas particles away from the household and human contact.