What is a common consequence of hardness in water?

Hard water contains high concentrations of dissolved minerals, primarily calcium and magnesium. When hard water is heated, these minerals can precipitate out and form scale, which accumulates on the surfaces of pipes, appliances, and fixtures. Over time, this scaling can lead to reduced efficiency in water heaters, increased energy costs, and potential clogs in plumbing systems.

The other options relate to different water quality issues or phenomena. For example, the formation of blue-green algae is typically associated with nutrient pollution rather than water hardness, while increased photosynthesis rates and reduced oxygen levels are more connected to organic pollutants and oxygen-demanding substances in the water. Thus, scaling in pipes and appliances is the most direct and common consequence of water hardness.