The A – Z Of Gay Lussac’s Law

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Did you know that Gay-Lussac’s Law graph is commonly overlooked at school? It governs the relationship between pressure, temperature and volume for a given gas. This principle is used in home cooking and even by a number of today’s top restaurants for precise control over food preservation. Let’s explore some of the lesser-known facts about Gay-Lussac’s Law: 

Who found Gay Lussac’s law?

Gay Lussac correctly states that the law works on a gas volume basis, but not on an absolute pressure. But it was his colleague Jean Charles de Borda who proved that the law is dependent upon absolute pressure.

It was the French physicist Andre Marie Ampere (1775-1836) who wrote the first version of Gay-Lussac’s Law in 1803. The first publication based on this law came from André Marie Eugène Chevreul (1786-1889) in 1820.

It was Gay-Lussac’s belief that the law was broadly correct and was due to his own experiments. However, he never explicitly stated the law in writing.

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The law did not go unnoticed for long after it was first published; others independently found the law and fully acknowledged those who contributed to it. 

Gay-Lussac’s Law is used in home cooking to help determine the length of time needed to cook a particular food type at a given temperature. For example, if you plan on cooking chicken breasts at 190°C/375°F, you’ll need 15 minutes to cook your chicken properly with this method.

Explanation of Gay Lussac’s law briefly:

Gay-Lussac’s law describes how the volume of a gas varies with pressure and temperature. It’s the same for all gasses. At constant temperature, it is possible to change the volume of a gas by changing its pressure or vice versa. You could also change either one and keep the other fixed; this will change the amount of gas that you have relative to how much you had before. The law talks about how these quantities combine: if we have 3 volumes of gas at 1 atmosphere pressure, 8 volumes at 2 atmospheres, and 4 volumes at 4 atmospheres, then we know that we have 13 volumes overall. By volume, these are the same but by pressure they are different. This results in a law that says that the total volume of any given gas is proportional to its absolute pressure (the amount of pressure times the container). For example, if there is a gas that has twice as much air than it had before, then there will only be half as much air overall in the container. 

In food preservation, the law is used to determine the pressure and temperature of a gas to determine how long it will keep. For example, a food that needs to be kept under 10 degrees Celsius needs to be kept at 0.1 atmospheres of pressure for much longer than food that needs to be kept at 20 degrees Celsius. This is because there is a direct relationship between the pressure of gas and its volume. If a gas increases in pressure, then it decreases in volume proportionately. If we double the pressure on a given amount of gas, for example by squeezing it into half the space, then we halve the volume and therefore double the density (mass per unit space).

The volume of a gas is inversely proportional to its pressure. This means that if we double the pressure, then we double the volume. The inverse relationship between pressure and volume is demonstrated in this E.M. Young’s illustration that shows the relationship between the volume of gas and its temperature:

The inverse relationship is demonstrated by this diagram:

Gay-Lussac’s law applies to all gases; it doesn’t matter where they are in their cycle or what they are made of, which makes it very important in food preservation (and even home cooking).

If a gas is forced into a smaller space, then its pressure and temperature increase. This is because gravitation pulls the gas molecules more closely together, and the increased pressure opposes the equally increased kinetic energy of molecules that collide with one another.

If a gas fills a container and its pressure is doubled, then it will require half as much space. The volume of the gas has decreased by half; this is because it now fills twice as much space in terms of pressure.

Gay Lussac’s law applies to liquids too: they obey the same principles regarding pressure and volume changes with temperature, although their temperatures are generally higher than those of gases because liquids are not able to expand as readily due to their greater molecular mass.

Gay Lussac’s law is widely used in the food industry. For example, if you want to keep your cold chicken for longer, then you can increase the pressure to half-decrease the temperature and therefore increase the length of time it will keep. If a cake is too dry, then you can reduce the pressure by a third and thereby keep it moist for longer.

But we mustn’t forget that this law does not mention how we can use it to our advantage. If you wanted to decrease the volume of gas in a container, then increasing its pressure would be counterintuitive as increased pressure means more gas molecules; but if you increase its temperature, then gas expands and decreases in volume again.

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