Final answer:
Magnesium fluoride and calcium are allowed in water primarily to promote dental health by adjusting fluoride levels to optimal concentrations for preventing tooth decay, and these minerals also contribute to bone health.
Step-by-step explanation:
Magnesium fluoride and calcium are allowed in water for a few different purposes. One key reason is to promote dental health, which is supported by community water fluoridation. This process adjusts the amount of fluoride in water to prevent tooth decay by making tooth enamel more resistant to acids. Additionally, fluoride enhances the uptake of calcium and phosphate ions in tooth enamel, promoting remineralization.
While water softening generally involves the removal of minerals like calcium and magnesium from hard water, these minerals also play roles in supporting bone health. Magnesium is significant as over 60% of it is found in the human skeleton, and when fluoride replaces the hydroxyl group in bone's hydroxyapatite crystals with fluorapatite, it helps to strengthen bone mineral.