The OH⁻ in every base

The idea

Bases generate hydroxide (OH–) ions in water, which are responsible for their basic nature; not all bases dissolve in water, and a base that is soluble in water is an alkali.

Look at the formulae

Here are three bases: NaOH, KOH and Mg(OH)₂.

Which group do all three share?

• OH (hydroxide)
• H (just hydrogen, on its own)
• Cl (chlorine)

Bases in water

Add water to each base. Do all of them dissolve?

Every base makes OH⁻

Bases generate hydroxide (OH⁻) ions in water, which are responsible for their basic nature. Not all bases dissolve in water; a base that is soluble in water is an alkali.

Alkalis are soapy to touch, bitter and corrosive. Never taste or touch them.

• Base | In water
• NaOH | Na⁺ + OH⁻
• KOH | K⁺ + OH⁻
• Mg(OH)₂ | Mg²⁺ + 2OH⁻
• Cu(OH)₂ | does not dissolve: a base, not an alkali

Notes

Bases generate hydroxide (OH⁻) ions in water, which are responsible for their basic nature; not all bases dissolve in water, and a base that is soluble in water is an alkali.

Check yourself

Which ion do bases give in water?

What is an alkali?

Cu(OH)₂ does not dissolve in water. So it is …

Mg(OH)₂ is put in water. What does it give?

• H⁺. H⁺ comes from acids.
• Cl⁻. Chloride is only in some salts and in HCl.
• OH⁻ — correct. Yes! Hydroxide ions.
• A base that dissolves in water — correct. Yes!
• Any base. Some bases do not dissolve in water. Only the soluble ones are alkalis.
• An acid that dissolves in water. An alkali is a kind of base, not an acid.
• an alkali. An alkali must dissolve in water.
• a base, but not an alkali — correct. Yes!
• an acid. It has OH, so it is a base.
• Mg²⁺ and one OH⁻. The formula Mg(OH)₂ has two OH groups, so it gives two OH⁻.
• H⁺ and OH⁻. H⁺ comes from acids. A base gives its metal ion and OH⁻.
• Mg²⁺ and two OH⁻ — correct. Yes! Each Mg(OH)₂ gives two hydroxide ions.