pH (SQA National 5 Chemistry): Revision Note

The pH Scale

What is the pH scale?

• The pH scale is a numerical scale used to measure acidity or alkalinity

  • It runs from below 0 to above 14

• All acids have pH values of below 7

  • The lower the pH, the more acidic the solution is

• All alkalis have pH values of above 7

  • The higher the pH, the more alkaline the solution is

• A solution with a pH of exactly 7 is described as being neutral

The pH Scale

What are some common acids and alkalis?

How do we measure pH?

• Universal indicator is a wide range indicator and can give only an approximate value for pH

• It is made of a mixture of different plant indicators which operate across a broad pH range and is useful for estimating the pH of an unknown solution

• A few drops are added to the solution and the colour is matched with a colour chart which indicates the pH which matches with specific colours

• Universal indicator colours vary slightly between manufacturers so colour charts are usually provided for a specific indicator formulation

Colour range of universal indicator

Hydrogen & Hydroxide Ions

Hydrogen ions

• Acids have pH values of below 7, have a sour taste (when edible) and are corrosive

• Acids are substances that can neutralise a base, forming a salt and water

• Acidic solutions have a higher concentration of hydrogen ions (H⁺) than hydroxide ions (OH^-)

• The presence of H⁺ ions is what makes a solution acidic

  • For example, hydrochloric acid

HCl (aq) → H⁺ (aq) + Cl^– (aq)

Hydroxide ions

• Alkalis have pH values of above 7

  • When a base is water-soluble it is referred to as an alkali

• They are substances which can neutralise an acid, forming a salt and water

• Alkaline solutions have a higher concentration of hydroxide ions (OH^-) than hydrogen ions (H⁺)

• The presence of the OH^– ions is what makes the aqueous solution an alkali

  • For example sodium hydroxide

NaOH (aq) → Na⁺ (aq) + OH^– (aq)

Neutral solutions

• A neutral solution has equal concentrations of hydrogen ions and hydroxide ions

  • When [H⁺] = [OH⁻], the pH = 7

• Pure water is a neutral solution

H₂O (l) ⇌ H⁺ (aq) + OH^- (aq)