4.11 - 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.

Indicator colours

• An indicator is a dye whose colour depends on the pH of the solution it's in.
• A single indicator changes colour sharply at a specific pH rather than gradually across the scale, making it ideal for showing clearly whether a solution is acidic or alkaline.

Colours of the common single indicators

Litmus paper

• Litmus is also used as test paper:
  • Blue litmus paper turns red in an acidic solution, and stays blue in neutral or alkaline solutions.
  • Red litmus paper turns blue in an alkaline solution, and stays red in neutral or acidic solutions.
• If blue paper stays blue and red paper stays red, the solution is neutral.

Choosing an indicator

• Phenolphthalein is colourless in acidic and neutral solutions, and pink only in alkaline solutions.
• Litmus shows distinct colours in acidic, neutral, and alkaline solutions.
• Methyl orange changes colour in acidic conditions, distinguishing strongly acidic solutions from neutral or alkaline ones.

Acids as proton donors

• A hydrogen atom consists of one proton and one electron. When it loses its electron to form a hydrogen ion, H⁺, only the proton remains.
• In aqueous reactions, a hydrogen ion is a proton.

Proton donors and acceptors

• An acid is a proton donor because it releases H⁺ ions in solution.
• A base is a proton acceptor because it accepts H⁺ ions.

Neutralisation as proton transfer

• Neutralisation is a transfer of protons from an acid to a base.
• When the base is an alkali, the hydroxide ion accepts a proton from a hydrogen ion to form a water molecule.
• Protons cannot be donated unless a base is present to accept them.