The life of these steam appliances can be prolonged by using only bottled supplies. The reason for this is best explained by first understanding the difference between hard and soft H2O. Rainwater is naturally soft. However, as it travels through the ground it picks up minerals such as lime, calcium, and magnesium.How Does Purified Water Affect a Humidifier ? If you taste rainwater, you will notice that it's quite salty and very different in taste to tapwater.