Question

Look at the glass tube below. Suppose it was warmed a little in an oven, before the experiment. Do you think that would change the result? If so, how?

The particles in hydrogen chloride gas are twice as heavy as those in ammonia gas.
• Cotton wool soaked in ammonia solution is put into one end of a long tube (at A below). It gives off ammonia gas.
• At the same time, cotton wool soaked in hydrochloric acid is put into the other end of the tube (at B). It gives off hydrogen chloride gas.
• The gases diffuse along the tube. White smoke forms where they meet:

Accepted Answer

No significant change will occur in the result, as the rise in temperature will only enhance the rate of diffusion of the entire experiment, which may cause the white smoke to appear faster. But apart from that, as the temperature factor will contribute equally towards the particles of both the gases, hence the white smoke would still be formed near B, as given in the diagram.

The rate of diffusion depends on these two factors:
1) The mass of the particles: The particles in hydrogen chloride gas are twice as heavy as those in ammonia gas.
The white smoke forms closer to B. So the ammonia particles have travelled further than the hydrogen chloride particles — which means they have travelled faster.
The lower the mass of its particles, the faster a gas will diffuse.

The particles in the two gases above are molecules. The mass of a molecule is called its relative molecular mass. So we can also say:
The lower its relative molecular mass, the faster a gas will diffuse.

2) The temperature: When a gas is heated, its particles take in heat energy, and move faster. They collide with more energy, and bounce further away. So the gas diffuses faster. The higher the temperature, the faster a gas will diffuse.

However, in the given diagram, as the temperature factor will contribute equally towards the particles of both the gases, hence the white smoke would still be formed near B, as given in the diagram.

Important Questions on States of Matter

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