Question

Consider the diffusion of Bromine Gas within a tube filled with air.

After a prolonged period of time, is the colour composition uniform?

From my understanding, Bromine is a denser gas. Hence, on average, the Bromine gas is more concentrated at the bottom of the flask. There is always diffusion, even against gravity, so I think the colour would be relatively uniform. However, the bottom of the tube would still be slightly darker. ( May not be noticeable ).

I've read that the colour composition is entirely uniform. I can see where this comes from. I suspect we consider the particles to be too small to have any gravitational influence. However, if we were to have a longer tube would there be a slightly darker portion at the bottom of the tube? Do we consider the colour composition to be exactly uniform because the tube is too small to have any noticeable seperation of colour?

This problem has puzzled me a bit. Is my intuition right? That is, there is a slight seperation of colour, even though it may not be noticeable?

Answer 1

Bromine gas will diffuse throughout a tube filled with air to form a uniform colour composition, due to the random movement of molecules. Any slight density difference is typically negligible in a standard lab setup, resulting in uniform distribution rather than concentration gradients.

Step-by-step explanation:

Diffusion is the process by which molecules spread from areas of high concentration to areas of low concentration. When bromine gas is introduced into a tube filled with air, over time, due to the random movement of gas molecules and the lack of a concentration gradient, the gas will eventually become uniformly distributed throughout the tube, with the same concentration at the bottom as at the top. However, due to the difference in densities between bromine gas and air, one might expect a gradient with a slightly higher concentration of bromine at the bottom if the tube were long enough. Nevertheless, the effect of gravity on diffusion is generally negligible compared to the random motion of the gas molecules, especially in a reasonably sized laboratory setup, resulting in a uniform colour composition throughout the tube. Graham's Law also indicates that lighter gases diffuse more rapidly than heavier gases, but it does not negate the eventual uniform distribution due to diffusion.