What is meant by the expression ‘water hardness’? Explain.

Water hardness refers to the concentration of certain minerals, primarily calcium and magnesium, in water. These minerals are dissolved in the water as it passes through the environment, and their concentration can vary widely from place to place.

Here are the steps to explain it:

1. Origin: When water falls as rain, it's naturally soft. However, as it flows over and through the ground, it picks up minerals like calcium and magnesium along the way, which make it hard.

2. Measurement: The hardness of water is typically measured in grains per gallon (gpg), milligrams per liter (mg/L), or parts per million (ppm). The United States Geological Survey classifies water as soft, moderate, hard, or very hard based on these measurements.

3. Impact: Hard water can cause scale build-up in pipes and appliances, which can lead to inefficiency and damage. It can also make it harder to form a lather with soap.

4. Treatment: Water hardness can be reduced or eliminated using a variety of treatment methods, most commonly ion exchange water softeners, which replace the calcium and magnesium ions in the water with sodium or potassium ions.

5. Health: While hard water can be a nuisance in the home, it's not generally considered a health risk. In fact, the minerals in hard water can contribute to your daily dietary needs for calcium and magnesium.