Which chemical is primarily used for the softening of water?

Lime, specifically in the form of soda ash, is primarily used for the softening of water due to its effectiveness in removing hardness caused by calcium and magnesium ions. When lime is added to water, it reacts with these ions to form insoluble precipitates, which can then be removed from the water. This process is part of what is known as lime softening.

The reason lime is favored in this process lies in its ability to not only reduce hardness effectively but also adjust the pH of the water, which can help in further purification processes. The resulting calcium hydroxide precipitate from this reaction contributes to the reduction of water hardness, making it ideal for various applications, including in municipal water treatment and industrial processes.

The other chemicals listed do have roles in water treatment, but they are not primarily used for softening. For instance, salt is commonly used in the regeneration of water softeners but does not directly soften water itself. Sodium bicarbonate can help in pH adjustment and providing alkalinity, while calcium chloride generally contributes to increasing hardness rather than reducing it.