Water hardness. Three test tubes containing the deposits left behind after boiling dry three water samples of differing types of hardness. From left they are: pure water, temporarily hard water and permanently hard water. Water hardness refers to the presence of dissolved minerals of calcium and magnesium from the rocks that the water has passed through. Removing the water by boiling has formed the calcium and magnesium minerals (white). The temporarily hard water only contained the minerals of hydrogencarbonate (from limestone). Permanently hard water contains additional minerals, including sulphates and fluorides, and leaves more deposits.