Alkaline earth metal hydroxides are solid white, deliquescent in air to water absorption. Among them, Ca(OH) 2 is a commonly used desiccant. The solubility of alkaline earth metal hydroxides in water is much smaller than that of alkali metal hydroxides. From the data in the table, the solubility of hydroxides increases from Be to Mg, and their alkalinity increases. Be(OH) 2 and Mg (OH) 2 are poorly soluble hydroxides. Be(OH) 2 is an amphoteric hydroxide, Mg (OH) 2 belongs to a medium strong base, and the rest belong to a strong base.
Table 1 Â Some properties of alkaline earth metal hydroxides | |||||
substance | Be ( OH ) 2 | Mg ( OH ) 2 | Ca ( OH ) 2 | Sr ( OH ) 2 | Ba ( OH ) 2 |
nature | |||||
colour | White | White | White | White | White |
Melting point / K | Dehydration decomposition | Dehydration decomposition | Dehydration decomposition | Dehydration decomposition | Dehydration decomposition |
Solubility in water / mol-dm -3 ( 293K ) | 8×10 -1 | S×10 -1 | 1. 8×10 -2 | 6. 7×10 -2 | 2×10 -1 |
Acidity and alkalinity | Sexuality | Medium alkali | strong base | strong base | strong base |
 Flame color of alkali metal and some alkaline earth metals | ||||||||
ion | Li + | Na - | K + | Rb + | Cs + | Ca 2+ | Sr 2+ | Ba 2+ |
Flame color | red | yellow | purple | Purple red | Purple red | Purple red | Magenta | Yellow green |
Wavelength / nm | 670.8 | 589.6 | 404.7 | 629.8 | 459.3 | 616.2 | 707 | 553.6 |
The density of Mg(OH) 2 is 2.36 g/cm 3 , and it is dehydrated by heating to 623 K:
Mg(OH) 2 ==== MgO + H 2 O
Mg(OH) 2 is easily soluble in acid or ammonium salt solution:
Mg(OH) 2 + 2HCl ==== MgCl 2 +2H 2 O
This reaction can be applied to analytical chemistry.
The reaction of milk of lime water and cheap, can be obtained Mg (OH) 2 precipitate, known as magnesia milk:
Mg 2+ + Ca(OH) 2 == Mg(OH) 2 + Ca 2+
A milky suspension of Mg(OH) 2 is used medicinally as an antacid and a laxative.
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