Water containing more than 3.0 grains of dissolved solids per gallon is considered to be hard [1 grain (gr) = 64.8 mg]. So, if a kettle contains 18 gal of tap water, which is allowed to evaporate. The solid material left behind has a mass of 1.7 g. Is such tap water considered to be hard?

Explanation:

The solid material left behind, after the 18 gal of tap water was evaporated, is the amount of dissolved solids in that quantity of water.

You must convert the amount in grams (1.7g) into grains:

That is in 18 gallons. Then, divide by 18 gal to obtain the number of grains dissolved per gallon:

That is much greater than 3.0 grains per gallon; thus, such tap water is considered to be hard.

the correct answer is:

a. formation of precipitation