HARD AND SOFT WATER - Hard water is any water containing an appreciable quantity of dissolved minerals. Soft water is treated water in which the only cation (positively charged ion) is sodium. The minerals in water give it a characteristic taste. Some natural mineral waters are highly sought for their flavor and the health benefits they may confer. Soap is less effective in hard water because it reacts to form the calcium or magnesium salt of the organic acid of the soap. Detergents, on the other hand, lather in both hard and soft water.

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Hard and Soft Water

Chemistry of Hard and Soft Water

by Anne Marie Helmenstine, Ph.D.

You've heard the terms "hard water" and "soft water, but do you know what they mean?

Is one type of water somehow better than the other?

What type of water do you have?

This article looks at the definitions of these terms and how they relate to water in everyday life.

Hard Water vs Soft Water

Hard water is any water containing an appreciable quantity of dissolved minerals.

Soft water is treated water in which the only cation (positively charged ion) is sodium.

The minerals in water give it a characteristic taste.

Some natural mineral waters are highly sought for their flavor and the health benefits they may confer.

Soft water, on the other hand, may taste salty and may not be suitable for drinking.

If soft water tastes bad, then why might you use a water softener?

The answer is that extremely hard water may shorten the life of plumbing and lessen the effectiveness of certain cleaning agents.

When hard water is heated, the carbonates precipitate out of solution, forming scales in pipes and tea kettles.

In addition to narrowing and potentially clogging the pipes, scales prevent efficient heat transfer, so a water heater with scales will have to use a lot of energy to give you hot water.

Soap is less effective in hard water because it reacts to form the calcium or magnesium salt of the organic acid of the soap.

These salts are insoluble and form grayish soap scum, but no cleansing lather.

Detergents, on the other hand, lather in both hard and soft water.

Calcium and magnesium salts of the detergent's organic acids form, but these salts are soluble in water.

How to Soften Water

Hard water can be softened (have its minerals removed) by treating it with lime or by passing it over an ion exchange resin.

The ion exchange resins are complex sodium salts. Water flows over the resin surface, dissolving the sodium.

The calcium, magnesium and other cations precipitate onto the resin surface. Sodium goes into the water, but the other cations stay with the resin.

Very hard water will end up tasting saltier than water that had fewer dissolved minerals.

Most of the ions have been removed in soft water, but sodium and various anions(negatively charged ions) still remain.

Water can be deionized by using a resin that replaces cations with hydrogen and anions with hydroxide.

With this type of resin, the cations stick to the resin and the hydrogen and hydroxide that are released combine to form pure water.

Anne Marie Helmenstine, Ph.D.

Ph.D. in biomedical sciences from the University of Tennessee at Knoxville - Oak Ridge National Laboratory.

Science educator with experience teaching chemistry, biology, astronomy, and physics at the high school, college, and graduate levels.

ThoughtCo and About Education chemistry expert since 2001.

Widely-published graphic artist, responsible for printable periodic tables and other illustrations used in science.

Experience

Anne Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education since 2001, and other sciences since 2013. She taught chemistry, biology, astronomy, and physics at the high school, college, and graduate levels. She has worked as a research scientist and also abstracting and indexing diverse scientific literature for the Department of Energy.

In addition to her work as a science writer, Dr. Helmenstine currently serves as a scientific consultant, specializing in problems requiring an interdisciplinary approach. Previously, she worked as a research scientist and college professor. 

Education

Dr. Helmenstine holds a Ph.D. in biomedical sciences from the University of Tennessee at Knoxville and a B.A. in physics and mathematics with a minor in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive chemical detection and medical diagnostic tests.

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https://www.thoughtco.com/chemistry-of-hard-and-soft-water-602182