About magnesium hydroxide.

Ionization of electrolyte is an endothermic process, and magnesium hydroxide is no exception. The ionization degree of magnesium hydroxide in hot water will increase, Ionized more OH-, thus exceeding the PH of phenolphthalein discoloration. However, due to the small amount of magnesium hydroxide generated by the reaction of magnesium and water, Besides, the solubility of magnesium hydroxide is not large, so the concentration of magnesium hydroxide solute dissolved in water is very small. After absorbing CO2 in the air, the alkalinity of the solution becomes weaker, so that phenolphthalein finally returns to colorless. Experimental design: Take a small amount of analytically pure magnesium hydroxide crystal and divide it into four parts (two parts of newly taken magnesium hydroxide and two parts of long-standing magnesium hydroxide). The first one was placed in a test tube, and dilute HCl was added immediately, and it was found that the precipitate was dissolved without bubbles. Put the second part in a test tube, immediately add water and phenolphthalein test solution, and it turns red. The third part was placed in the air for 7 days, and bubbles were found by adding dilute HCl into the test tube. The fourth part was placed in the air for 7 days, and after being put into a test tube and added with phenolphthalein test solution, it was found that the test solution did not change color.