Hydroxides

Metal hydroxide

Very active metals (such as K, Na, and Ca) dissolve in water to form hydroxides, which are soluble oxides that dissolve in water to form alkaline solutions.

Preparation of metal hydroxides

Soluble oxides dissolve in water to form hydroxides (alkalis). Examples of alkalis:

1. Caustic soda (NaOH): Used in the manufacture of soap.
2. Caustic potash (KOH): Used in the manufacture of soap.
3. Lime water (Ca(OH)₂): Used to detect CO₂ gas.

Preparation of insoluble hydroxides

Insoluble hydroxides can be prepared by mixing soluble salts with soluble hydroxides. Here is an example of the process:

1. Dissolve a soluble salt in water, such as Cu(NO₃)₂.
2. Dissolve a soluble hydroxide in water, such as Ca(OH)₂.
3. Mix the two solutions: Cu(NO₃)₂ (aq) + Ca(OH)₂ (aq) → Cu(OH)₂ (s) + Ca(NO₃)₂ (aq).
4. Filter the mixture to separate Cu(OH)₂ (insoluble) from Ca(NO₃)₂ (soluble).

Properties of hydroxides

1. Bitter taste: Hydroxides generally have a bitter taste.
2. Neutralize acids: Hydroxides neutralize acids to form salt and water.

Solubility of hydroxides in water

The solubility of hydroxides in water depends on the metal's position in the reactivity series:

1. Sodium, calcium, and potassium hydroxides: These are the strongest hydroxides, as they dissociate completely in water to form strong alkaline solutions.
2. Ammonium hydroxide (NH₄OH): A weaker base because it does not dissociate completely in water.

In general, the solubility of hydroxides decreases as you move down the reactivity series, with metals at the top forming highly soluble hydroxides and metals lower in the series forming mostly insoluble hydroxides.