Chemical Reactions
Acids & Bases: pH, Indicators, and Neutralisation
1st Year · 2nd Year · 3rd Year (Junior Cert)
- ✓By the end of this lesson students will be able to define acids and bases and list their general properties.
- ✓By the end of this lesson students will be able to describe the pH scale and classify solutions as acidic, neutral, or basic based on their pH value.
- ✓By the end of this lesson students will be able to identify and explain the use of common acid-base indicators.
- ✓By the end of this lesson students will be able to describe the process of neutralisation and write word equations for neutralisation reactions.
- ✓By the end of this lesson students will be able to identify the salt formed in a neutralisation reaction.
Key concepts
Acids are substances that produce hydrogen ions (H⁺) when dissolved in water. They typically have a sour taste (e.g., vinegar, lemon juice), are corrosive, and turn blue litmus paper red. Common examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃).
Bases are substances that produce hydroxide ions (OH⁻) when dissolved in water. They often have a bitter taste, feel soapy to the touch, and turn red litmus paper blue. Bases that are soluble in water are called alkalis. Common examples include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂).
The pH scale is a numerical scale used to specify the acidity or basicity (alkalinity) of an aqueous solution. It ranges from 0 to 14: - **pH < 7**: The solution is acidic (the lower the pH, the stronger the acid). - **pH = 7**: The solution is neutral (e.g., pure water). - **pH > 7**: The solution is basic or alkaline (the higher the pH, the stronger the base).
Indicators are chemical substances that change colour depending on the pH of the solution they are added to. They are used to determine whether a solution is acidic, neutral, or basic, or to find the approximate pH. - **Litmus paper**: Blue litmus turns red in acid; red litmus turns blue in base. No change in neutral solutions. - **Universal Indicator**: A mixture of several indicators that shows a range of colours across the entire pH scale (e.g., red/orange for strong acid, yellow for weak acid, green for neutral, blue/violet for base). - **Phenolphthalein**: Colourless in acid and neutral solutions, turns pink in basic solutions. - **Methyl Orange**: Red in acidic solutions, yellow in basic solutions.
Neutralisation is a chemical reaction in which an acid and a base react to form a salt and water. The hydrogen ions (H⁺) from the acid react with the hydroxide ions (OH⁻) from the base to form water (H₂O). The remaining ions form the salt. General word equation: Acid + Base → Salt + Water Ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)
A salt is an ionic compound formed when the hydrogen ion (H⁺) of an acid is replaced by a metal ion or an ammonium ion (NH₄⁺) during a neutralisation reaction. The name of the salt depends on the acid and base used. For example, hydrochloric acid forms chlorides, sulfuric acid forms sulfates, and nitric acid forms nitrates.
Key facts to remember
- 1Acids have a pH less than 7, bases have a pH greater than 7, and neutral solutions have a pH of 7.
- 2Litmus paper turns red in acids and blue in bases.
- 3Universal indicator shows a range of colours, allowing for a more precise estimation of pH.
- 4Neutralisation is the reaction between an acid and a base.
- 5The products of a neutralisation reaction are always a salt and water.
- 6Hydrogen ions (H⁺) are responsible for acidic properties, and hydroxide ions (OH⁻) are responsible for basic properties.
- 7The reaction H⁺(aq) + OH⁻(aq) → H₂O(l) is the essential part of any neutralisation.
Worked examples
Example 1
A student tests an unknown solution with universal indicator. The indicator turns a dark blue colour. What can be concluded about the pH and nature of the solution?
Answer
The solution is strongly basic (alkaline) and has a pH value in the range of 10-14.
Universal indicator provides a more precise pH range than litmus paper.
Example 2
Write the word equation for the neutralisation reaction between nitric acid and potassium hydroxide.
Answer
Nitric acid + Potassium hydroxide → Potassium nitrate + Water
Always remember that water is a product of neutralisation.
Example 3
When sulfuric acid reacts with calcium hydroxide, what salt is formed?
Answer
The salt formed is calcium sulfate.
The name of the salt comes from the acid (e.g., sulfuric acid → sulfate) and the metal from the base (e.g., calcium hydroxide → calcium).
Common mistakes
- ✗Confusing the colour changes of indicators (e.g., thinking litmus turns blue in acid).
- ✗Forgetting to include water as a product in neutralisation reactions.
- ✗Incorrectly naming the salt formed in a neutralisation reaction (e.g., mixing up the acid and base components).
- ✗Believing that a neutral solution means there are no chemicals present, rather than a balance of H⁺ and OH⁻ ions.
- ✗Thinking that all bases are soluble in water (only soluble bases are called alkalis).
Exam tips
- ★Memorise the pH ranges and corresponding colour changes for common indicators (litmus, universal indicator, phenolphthalein, methyl orange).
- ★Practise writing word equations for various acid-base neutralisation reactions, ensuring you correctly identify the salt and water.
- ★Understand the concept of the pH scale and be able to classify solutions as acidic, neutral, or basic based on their pH value.
- ★Be able to explain the role of H⁺ and OH⁻ ions in determining acidity and basicity.
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