pH, Neutralisation and Acid Strength

This section explains pH, neutralisation and acid strength covering, indicators, the pH scale and neutralisation reactions and also provides a overview of acids and alkalis. 

Indicators

Indicators are substances that change colour depending on the pH of the solution they are in. They are used to determine whether a solution is acidic, neutral, or alkaline (basic). Some common indicators include litmus paper, universal indicator, and phenolphthalein. Each indicator shows a distinct colour change in different pH environments.

Indicator Table:

• Litmus: A simple indicator that turns red in acidic solutions and blue in alkaline solutions.
• Universal Indicator: This indicator shows a wide range of colours. It turns red in strong acids, orange/yellow in weaker acids, green at pH 7 (neutral), and blue or purple in alkaline solutions.
• Phenolphthalein: Colourless in acidic conditions and pink in alkaline conditions.
• Methyl Orange: Red in acidic conditions and yellow in alkaline conditions.

The pH Scale and Neutralisation Reactions

The pH scale is a measure of how acidic or alkaline a solution is. The scale ranges from 0 to 14:

• pH 0-6: Acidic solutions (the lower the number, the stronger the acid).
• pH 7: Neutral (pure water has a pH of 7).
• pH 8-14: Alkaline (the higher the number, the stronger the alkali).

A neutralisation reaction occurs when an acid reacts with a base (alkali), resulting in the formation of a salt and water. This reaction involves the hydrogen ions (H⁺) from the acid reacting with the hydroxide ions (OH⁻) from the base to form water (H₂O).

Example of Neutralisation:

$$HCl (aq) + NaOH (aq) \rightarrow NaCl (aq) + H_2O (l)$$ 

Here, hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH) to form sodium chloride (NaCl) and water (H₂O).

What are Acids?

Acids are substances that release hydrogen ions (H⁺) when dissolved in water. They have a pH less than 7 and can taste sour. Some common acids include:

• Hydrochloric acid (HCl): Found in the stomach, used in laboratory experiments.
• Sulphuric acid (H₂SO₄): Used in car batteries and industrial processes.
• Citric acid (C₆H₈O₇): Found in citrus fruits like lemons and oranges.
• Nitric acid (HNO₃): Used in fertilisers and explosives.

Acids can be strong or weak depending on how fully they ionise (release H⁺ ions) in solution. Strong acids, like hydrochloric acid, ionise completely, while weak acids, like acetic acid (in vinegar), only partially ionise.

What are Alkalis?

Alkalis are bases that dissolve in water to form hydroxide ions (OH⁻). They have a pH greater than 7 and can feel slippery or soapy to the touch. Some common alkalis include:

• Sodium hydroxide (NaOH): Used in soap making and cleaning products.
• Potassium hydroxide (KOH): Used in the production of biodiesel and as a strong base in laboratories.
• Ammonia solution (NH₃ in water): Used in cleaning products.

Alkalis, like acids, can be strong or weak. Strong alkalis, such as sodium hydroxide, dissociate completely in water, while weak alkalis, such as ammonia solution, do not dissociate fully.

Summary of Key Points:

• Indicators change colour depending on the pH of a solution. Common indicators include litmus paper, universal indicator, phenolphthalein, and methyl orange.
• pH Scale measures the acidity or alkalinity of a solution. Acids have pH < 7, neutral solutions have pH = 7, and alkalis have pH > 7.
• Neutralisation reactions occur when an acid reacts with a base (alkali), forming a salt and water.
• Acids release hydrogen ions (H⁺) in water, while alkalis release hydroxide ions (OH⁻) in water.

Understanding these key concepts is essential for your GCSE Chemistry studies and for mastering the topic of acids, bases, and pH.