6.1 - Acids, Bases & the pH Scale

Acids, bases and alkalis in water

• An acid dissolves in water to give a solution containing hydrogen ions, H⁺(aq). These make the solution acidic, with a pH below 7.
• A base reacts with an acid to form a salt, usually with water. Common bases are metal oxides, metal hydroxides, and ammonia.
• An alkali is a base that dissolves in water. It forms a solution containing hydroxide ions, OH^-(aq), with a pH above 7.
• Every alkali is a base, but only soluble bases are alkalis:
  • Sodium hydroxide is soluble, so it's a base and an alkali.
  • Copper oxide is insoluble, so it's a base but not an alkali.

Neutralisation

The reaction between an acid and a base is called neutralisation:

acid + base → salt + water

• When just enough acid and base react, they cancel each other out to form a neutral solution at pH 7.
• When an acid reacts with an alkali, the hydrogen ions from the acid join with the hydroxide ions from the alkali to form water molecules.

The pH scale and measuring pH

The pH scale measures how acidic or alkaline a solution is, running from 0 to 14:

• Below 7 - Acidic (pH 0 to 3 is strongly acidic, pH 4 to 6 is weakly acidic)
• Exactly 7 - Neutral (pure water is neutral)
• Above 7 - Alkaline (pH 8 to 10 is weakly alkaline, pH 11 to 14 is strongly alkaline)

Indicators

• An indicator is a dye that changes colour depending on the pH of the solution.
• Universal indicator is a mixture of dyes that changes colour gradually across the whole pH scale:
  • Red (strongly acidic) → orange/yellow (weakly acidic) → green (neutral) → blue (weakly alkaline) → purple (strongly alkaline).
• It comes as a solution or as pH paper, and the colour is matched against a printed colour chart to find the approximate pH.

pH probe and meter

• A pH probe connects to a digital pH meter to display the pH as a number, usually to one or two decimal places.
• This is more precise than an indicator because it gives a numerical reading and removes human judgment in matching colours.

pH and hydrogen ions

The pH of a solution depends on the concentration of ions it contains.

Acids and hydrogen ions

• An acid in water releases hydrogen ions, H⁺, so an acidic solution contains a higher concentration of hydrogen ions than hydroxide ions.
• The higher the concentration of hydrogen ions, the more acidic the solution and the lower the pH.
• A solution of pH 1 has a much higher concentration of hydrogen ions than a solution of pH 5.

Alkalis and hydroxide ions

• An alkali in water releases hydroxide ions, OH^-, so an alkaline solution contains a higher concentration of hydroxide ions than hydrogen ions.
• The higher the concentration of hydroxide ions, the more alkaline the solution and the higher the pH.

Neutral solutions

In a neutral solution (pH 7), the concentration of hydrogen ions is equal to the concentration of hydroxide ions.

Following pH during neutralisation

• Adding an alkali to an acid neutralises hydrogen ions, causing the pH to rise.
• Plotting pH against the volume of alkali added gives a curve that:
  • Rises slowly at first,
  • Jumps steeply near the neutralisation point,
  • Flattens out once the alkali is in excess.