Bases pick up H⁺

Section 3 of 4

A base is the partner of an acid: it picks up H⁺. To hold on to an H⁺, a base needs a pair of electrons to make a bond with.

A lone pair is a pair of outer electrons that an atom keeps to itself. Ammonia's nitrogen has one. An amine is ammonia with carbon groups in place of hydrogens, and its nitrogen keeps that lone pair. The lone pair reaches out to an H⁺ and becomes a new N–⁠H bond. The amine is now an ion with a positive charge.

Methylamine picks up a hydrogen ion Top: methylamine, a CH3 group joined to a nitrogen atom that carries two hydrogen atoms and a lone pair, drawn as two dots. Plus a hydrogen ion, H plus. An arrow points down to the methylammonium ion: the nitrogen now carries three hydrogen atoms and a plus charge, and the new N-H bond, made from the lone pair, is amber. CH3NHHCH3NHHH lone pair + H + methylamine hydrogen ion + methylammonium ion new bond
Methylamine picks up an H⁺: CH₃NH₂ + H⁺ → CH₃NH₃⁺. The lone pair on nitrogen becomes the new N–⁠H bond (amber), and the ion carries the H⁺'s positive charge.

In water, a base takes H⁺ from water molecules and leaves hydroxide ions, OH⁻, behind. A solution with more OH⁻ than H⁺ has a pH above 7. A base that dissolves in water is called an alkali.

Lemon with fish

Trimethylamine is the amine behind the smell of fish that is going off. It drifts out of the fish into the air and up your nose. A squeeze of lemon juice gives it an H⁺ from citric acid. The ion it becomes carries a charge, so it stays dissolved in the juice, and the smell fades.

Trimethylamine, (CH₃)₃NA nitrogen atom joined to three methyl groups, -CH₃, with a lone pair on the nitrogen.HHHCNCHHHHCHH
Trimethylamine, (CH₃)₃N: three methyl groups on one nitrogen. The nitrogen keeps one lone pair, and that pair picks up the H⁺.

Baking soda

Baking soda, also sold as bicarbonate of soda, is sodium hydrogencarbonate, NaHCO₃. It is made of sodium ions, Na⁺, and hydrogencarbonate ions, HCO₃⁻. Each hydrogencarbonate ion has an oxygen with a negative charge, ready to pick up an H⁺. Dissolved in water, baking soda gives a pH of about 8.3: gently basic.

Quick check

A squeeze of lemon juice takes away the smell of fish. What happens to the smelly amine?