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In three different test tubes we add a very small amount of:

1)In the first test tube, we add oxide of magnesium

2)In the second test tube, we add hydroxide of calcium

3)In the third test tube, we add hydroxide of barium

Then we add deionized water in the three test tubes. We shake them and after we filtrate them, we measure the pH of each filtrate. I'm curious why we use hydroxides of calcium and barium but we use oxide of magnesium instead of hydroxide of magnesium?

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I would attribute this two 2 reasons:

Reason 1: Insolubility of Magnesium hydroxide.

Magnesium hydroxide is almost insoluble in water and will not easily dissolve in water.

Reason 2: Safety is one of the reason of the choice of compounds.

The formation of hydroxides of alkaline earth metals are generally exothermic and this intensity increases down this group.

  1. Reaction of magnesium oxide with water will produce the corresponding hydroxide:

    $$\ce{MgO + H2O → Mg(OH)2 (very slight reaction)}$$

    (The reaction quickly stops because the magnesium hydroxide formed is almost insoluble in water and forms a barrier on the magnesium preventing further reaction).

  2. The reaction of calcium oxide to produce its hydroxide is exothermic:

$$\ce{CaO (s) + H2O (l) ⇌ Ca(OH)2 (aq) (ΔH_r = −63.7 kJ/mol of CaO)}$$

  1. Barium oxide is harmful to human skin and may cause irritation on exposure for prolonged periods or in large quantity.

    • Its reaction with water is also know to be violent.

So in my opinion it is much better to user hydroxides of Ba and Ca.

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