Study Notes
Acids and alkalis are substances that, when dissolved in water, form ions that determine their acidity or alkalinity. The pH scale measures how acidic or alkaline a solution is, with acids having pH values below 7 and alkalis above 7.
- Acid — A substance that forms hydrogen ions (H+) when dissolved in water. Example: Hydrochloric acid in your stomach helps digest food.
- Alkali — A substance that forms hydroxide ions (OH-) when dissolved in water. Example: Sodium hydroxide is used to make paper and detergents.
- pH Scale — A numerical scale from 0 to 14 used to measure how acidic or alkaline a solution is. Example: A solution with pH 7 is neutral.
- Neutralisation Reaction — A reaction where an acid and an alkali react to form water and a salt. Example: Hydrochloric acid + sodium hydroxide → sodium chloride + water.
- Indicator — A substance that changes color to indicate whether a solution is acidic or alkaline. Example: Litmus turns red in acids and blue in alkalis.
Exam Tips
Key Definitions to Remember
- Acid: Forms H+ ions in water
- Alkali: Forms OH- ions in water
- pH Scale: Measures acidity or alkalinity
- Neutralisation: Acid + Alkali → Salt + Water
Common Confusions
- Confusing the pH scale direction: Lower pH is more acidic, higher pH is more alkaline
- Thinking all acids and alkalis are dangerous: Only concentrated ones are highly corrosive
Typical Exam Questions
- Name six acids? Hydrochloric acid, sulfuric acid, nitric acid, ascorbic acid, ethanoic acid, methanoic acid
- Give one hazard of using sodium hydroxide, and state how to reduce the risks from this hazard? Hazard: Corrosive, can cause blindness. Reduce risk: Wear eye protection.
- Give one property that is typical of acids, and one property that is typical of alkalis? Acids: Sour taste. Alkalis: Soapy feel.
- Give the colours of litmus indicator in acidic and alkaline solutions? Acidic: Red. Alkaline: Blue.
- A solution has a pH of 8. Is it acidic or alkaline? Alkaline
- Which is more acidic, a solution of pH 6 or a solution of pH 4? pH 4
What Examiners Usually Test
- Understanding of the pH scale and its application
- Ability to identify acids and alkalis based on their properties
- Knowledge of neutralisation reactions and their products