In an unopened bottle of fizzy drink in which carbon dioxide is dissolved, what can you say about the rate at which carbon dioxide moves from headspace to drink and from drink to headspace?

The rates are equal. A dynamic equilibrium is established.

Why do you hear a 'fizz' when the top of a fizzy drinks bottle is removed?

Some carbon dioxide from the headspace escapes out of the bottle. This causes some more carbon dioxide to move from the drink to the headspace and then to escape, and so on.

How would you describe the system when the top is removed from the bottle?

It's an open system.

If some of the drink is poured out and the top of the bottle is replaced, what can you say about the rate of movement of carbon dioxide between drink and headspace and headspace and drink?

The initial rate of movement of carbon dioxide from drink to headspace will be greater than the movement from headspace to drink because the concentration of the gas in the headspace has been reduced as some of it escaped from the bottle.

What happens inside the bottle if it is now left for some time?

A new equilibrium is established in which the rate of movement of carbon dioxide from drink to headspace is equal to the rate of movement from headspace to drink.