Acids, bases and ions in water.

Step 1 of 5

Two ions

Vinegar, lemon juice and stomach acid taste sour; soap, baking soda and bleach feel slippery and taste bitter. The difference comes from ions in water. An acid is a substance that hands a proton, an $\mathrm{H^+}$⁠ ion, to a water molecule, making a hydronium ion, $\mathrm{H_3O^+}$⁠:

$\mathrm{HCl + H_2O \rightarrow H_3O^+ + Cl^-}$⁠

A base is a substance that gives hydroxide ions, $\mathrm{OH^-}$⁠, in water. In the Scene is a beaker of water, and the second chart shows how much there is of each of the two ions.

Step 2 of 5

Pure water

Water on its own is not free of ions. At every moment a few water molecules pass a proton to one another, $\mathrm{2H_2O \rightleftharpoons H_3O^+ + OH^-}$⁠, and at once join back up: an equilibrium, as in the last Unit. In the beaker you see it as small flashes. pH is now fixed at 7.

Step 3 of 5

Strong and weak

Different acids give up protons with different ease. A strong acid, like hydrochloric acid, $\mathrm{HCl}$⁠, gives up all of them. A weak acid, like the ethanoic acid in vinegar, reaches an equilibrium in which most of its molecules stay whole:

$\mathrm{CH_3COOH + H_2O \rightleftharpoons CH_3COO^- + H_3O^+}$⁠

Step 4 of 5

Bases

Bases are strong or weak too. Sodium hydroxide, $\mathrm{NaOH}$⁠, splits completely into $\mathrm{Na^+}$⁠ and $\mathrm{OH^-}$⁠. Ammonia, $\mathrm{NH_3}$⁠, takes a proton from water in only a small share of its molecules, leaving $\mathrm{OH^-}$⁠. pH now stands at 11.

Step 5 of 5

The ion product

In water’s equilibrium, at 25 °C, the product of the two ions’ concentrations is fixed:

$[\mathrm{H_3O^+}][\mathrm{OH^-}] = 10^{-14}$$[\mathrm{H_3O^+}][\mathrm{OH^-}] =$$10^{-14}$⁠

Add acid and $\mathrm{H_3O^+}$⁠ grows while $\mathrm{OH^-}$⁠ shrinks, so the product holds. In the second chart it looks like a seesaw: when one bar rises, the other falls by the same amount.