What is generally low in rainwater that contributes to its softness?

Rainwater is considered soft primarily because it has a low concentration of total dissolved minerals, which include calcium, magnesium, and other ions that contribute to water hardness. When rainwater falls, it initially picks up very few minerals from the atmosphere or surfaces it encounters, resulting in a low total dissolved solids (TDS) content. This characteristic makes rainwater soft compared to groundwater or surface water sources that may have higher mineral content due to interactions with soil and rock.

While chlorine content and pH levels are important aspects of water quality, they do not specifically determine the softness of water. Chlorine is typically present in treated drinking water to disinfect it, and while pH can vary, it does not directly correlate with mineral content. Organic matter, on the other hand, can be present in varying amounts in rainwater, but it is not a primary factor in determining whether rainwater is classified as "soft." Thus, total dissolved minerals are the key contributor to the softness of rainwater.