You may have come across the idea of a reacting system and its surroundings in the context of energetics. These ideas can be explored in more detail by subscribing to the Energy and change course. We need to consider two different types of system – an open system and a closed system.

An open system is one in which both energy and matter can be transferred to the surroundings. A closed system is one in which only energy can be transferred to the surroundings. Matter cannot be transferred.
There is a third type of system – an isolated system. This is one in which neither matter nor energy can be transferred to the surroundings.

A dynamic equilibrium can only be establshed in a closed system – it cannot be established in an open system.

Why can't an equilibrium be established in an open system?

In an open system matter is transferred out of the system, which prevents an equilibrium being established.

Think about the reaction between lead(II) nitrate(V) solution and potassium iodide solution in a test tube that produces a precipitate of lead(II) iodide:

Pb²⁺(aq)  +  2I^–(aq) _↓  PbI₂(s)

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Is this an open or a closed system?

It's a closed system because matter is not transferred to the surroundings.

Think about the reaction between magnesium ribbon and hydrochloric acid in an open test tube:

Mg(s)  +  2HCl(aq)  _↓  MgCl₂(aq)  +  H₂(g)

Is this an open or a closed system?

It's an open system because hydrogen gas is transferred from the reaction mixture into the surroundings.

Think about an experiment in which copper(II) carbonate is heated in a hard glass tube and the carbon dioxide produced is collected in a gas syringe:

CuCO₃(s) _↓  CuO(s)  +  CO₂(g)

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Is this an open or a closed system?

It's an open system because the gas can move away from the reactants by pushing open the syringe and this allows the forward reaction to go to completion.