Soft water can be artificially hardened by the additon of slaked lime, Ca(OH)2.
This is mainly done to reduce the risk of lead dissolving in the water from old pipework in the distribution system or more commonly in the customer's home.
Hard water causes a layer of calcium and magnesium compounds (mainly sulphate and hydrogencarbonate) to form on the inside of pipework stopping any lead in the pipes coming into contact with the water flowing through the pipes.