How can the combined gas law be derived from the other three, if none of the variables remain constant?

Question

In Boyles, Charles and Gay-Lussac's Gas Laws, one of the variables has to remain constant in order for them to be true. Therefore, how is it possible that the Combined Gas Law is true, if NONE of the variables are held constant?

Answer 1

It's actually pretty simple. The ideal gas law says that for a fixed sample of gas, $PV/T$ is constant.

• Boyle's law says that $PV/T$ is constant when $T$ is held constant.
• Charles' law says that $PV/T$ is constant when $P$ is held constant.
• Gay-Lussac's law states that $PV/T$ is constant when $V$ is kept constant.

Any two of these can be used to derive the full ideal gas law, because any change in all three of $P$, $V$, and $T$ can be thought of as a combination of changes where one of the variables is held constant and the other two change. For example, you could first use Boyle's law to change $P$ to the final value (while holding $T$ constant) and then use Charles' law to change $T$ (while holding $P$ constant).

Answer 2

Ideal gas law is not derived from those of Boyles, Charles and Gay-Lussac, but rather generalizes them. Indeed, as the other answers have demonstrated, all the three laws are particular cases of the ideal gas law.

However, the three laws provide strong motivation for writing the ideal gas law as it is. E.g., if we take Boyle's law, $PV=\text{const}$, then the constant in the right-hand side should be a function of $N$ and $T$, i.e. $$ PV=f(N,T) $$ comparing this with Charles' law at constant pressure and Gay-Lussac's law at constant volume, we conclude that $f(N,T)$ is a linear function of temperature, i.e., $$ PV=f_1(N)T $$ One could further use Dalton's laws to conclude that $f_1(N)$ is a linear function of N, i.e., $$ PV=f_2NT, $$ where the constant of proportionality is then renamed "Boltzmann constant" $k$.

Note also that the ideal gas law applies to a state, not to a transformation - whether we arrive from one state to another via a path where all the variable change simultaneously or by a path composed of segments where one of the above laws holds doesn't change the validity of the law for this state.

Finally, ideal gas law can be derived independently by statistical mechanics, without resorting to the three gas laws - the three laws in this case only serve to test the validity of the theory.

Answer 3

Let's look at the ideal gas law first $$\tag1\frac{PV}{T}=\text{constant}$$ The first thing to note is, if you kept one of the variables $P,V,T$ constant at a time, you'll retain each of the laws:

1. Boyles law says that $PV=\text{constant}$ if temperature is held constant.

2. Charle's law $\frac VT=\text{constant}$ if pressure is held constant.

3. Gay-Lussac's law says that $\frac PT=\text{constant}$ if volume is kept constant.

In the first instance, a relationship between pressure and volume was sought, meaning temperature needed to be kept constant (Boyle's law). In the second instance, a relation between volume and temperature was sought, meaning pressure was to be kept constant (Charle's law). And finally, a relationship between pressure and temperature was searched for, which required volume to be constant, leading to Gay-Lussac's law. These were established laws.

And so if one were to then ask, what is the relation between pressure, volume and temperature, we would need to see what would happen in a system where we varied all three variables. After some experimental work, and building on the ideas above, the result was the ideal gas law. This works if you change all of $P,V,T$ simultaneously or even one or two them at a time while keeping the others constant.

There is no inconsistency and is natural to deduce that equation (1) is in fact in agreement with the above three laws, even by inspection (the RHS has is the product of the gas constant and particle number, assumed constant).

Answer 4

The laws you have given are consistent with the ideal gas law.

Ideal gas law: $PV=NkT$

Let's make $T$ and $N$ constant, then we get Boyle's Law: $P\propto 1/V$

Let's make $P$ and $N$ constant, then we get Charle's Law:$V\propto T$

Let's make $V$ and $N$ constant, then we get Gay-Lussac's Law: $P\propto T$