In some places in the United States, the tap water doesn’t allow soap to form lather, and cannot be used for washing without the addition of chemicals called “water softeners.” This kind of water is called “hard” water, and it’s caused by certain minerals in the water.
Sometimes water contains molecules of carbon dioxide gas, which mix with the water to form an acid. This acid eats away rocks, and chemicals, called salts, form in the water.
When these chemicals, like calcium carbonate, come in contact with soap, they combine to form particles that stick to dishes or clothes that are being washed, and prevent the soap from forming lather.
Some kinds of hard water can be softened by boiling or by adding chemicals. Boiling changes the salts to gases and other harmless chemicals. But some hard water is permanently hard, and nothing can be done to soften it.