Hydrolysis of salts and the pH scale
When some salts dissolve, their ions react with water. This is hydrolysis, and it can change the balance of H⁺ and OH⁻. There are three cases. A salt of a strong acid and a weak base, such as ammonium chloride NH₄Cl, gives more H⁺ and less OH⁻: the solution is acidic. A salt of a weak acid and a strong base, such as sodium bicarbonate NaHCO₃ or sodium carbonate, gives more OH⁻: the solution is basic, and for sodium bicarbonate only weakly. A salt of a strong acid and a strong base, such as NaCl, does not change the balance, so its solution is neutral. “Acidic salt” means that the solution of the salt is acidic. The pH scale runs from 0 to 14 and tells the balance of H⁺ and OH⁻. Pure water has pH 7, solutions with pH below 7 are acidic (more H⁺), and solutions with pH above 7 are basic (more OH⁻). As an extra fact, each step of 1 on the scale means 10 times more or less H⁺, so pH 3 has 100 times more H⁺ than pH 5.
| Salt | Made from | Its solution |
|---|---|---|
| NH₄Cl | strong acid + weak base | acidic |
| NaHCO₃ | weak acid + strong base | weakly basic |
| NaCl | strong acid + strong base | neutral |
Red cabbage indicator, with a grown-up. An adult chops some red cabbage and soaks it in cool or warm tap water until the water is purple; strain it. Put a little in small clear cups and add, one at a time, tap water, a few drops of vinegar, lemon juice, a pinch of baking soda in water, and a drop of dish soap in water. Describe the colours: red-pink usually means acid, purple neutral, blue-green basic. Do not taste or sniff and keep the liquids away from eyes. Put the liquids in order from most acidic to most basic.