 A chemical equilibrium is said to be homogenous if all the reactants and products are in the same state. For instance, the reaction between nitrogen gas and hydrogen gas to give ammonia gas is homogenous, because all three chemical species involved are gases. An example of her homogenous aqueous equilibrium occurs when you have acetic acid in water, that's vinegar. The acetic acid molecules, which are aqueous, dissociate to give aqueous hydrogen ions and acetate ions, and this process is an equilibrium. Notice that I've written these chemical equations using an equilibrium arrow, that's two single headed arrows pointing in opposite directions. This is used to represent the fact that both the forward and reverse processes are happening simultaneously. A heterogeneous equilibrium, or inhomogenous equilibrium, is one in which the reactants and products may be in different states. An example is in single displacement reactions, such as tin reacting with lead ions. Here the tin is in solid form and the lead ions are aqueous. They would have come from a salt-like lead nitrate that was dissolved in water. You may have thought about single displacement reactions as being irreversible reactions that go to completion, but you'll find in this topic that they are in fact equilibria. Another example is the decomposition reaction that happens when you heat sodium hydrogen carbonate to give sodium carbonate carbon dioxide and water. That's a solid reactant, a solid product, and a gas and liquid products. Remember this would only come to equilibrium in a closed system, so you'd need to do it in a sealed container to prevent the CO2 and water escaping. Preferably a strong one.