Why is rainwater soft and river water hard?

Raindrops are formed from evaporated water. When water evaporates, it leaves behind everything that could be dissolved in it. The hardness is caused by the dissolved minerals (mainly calcium) in the water. Since the rain comes from evaporated water that is free of minerals, the rainwater will be soft.

Once this drop is formed, this liquid water is capable of dissolving other substances. As a droplet passes through the atmosphere, it can dissolve other substances in the atmosphere (such as pollutants) and become alkaline or acidic. As soon as this water touches the ground, it starts trying to dissolve everything it comes in contact with until it reaches the point where it is satisfied.