Introduction to Acids and Alkalis

Acids and alkalis are types of substances with distinct properties, often found in everyday products like food, cleaning products, and even our bodies. In chemistry, we categorise substances based on their pH level, which tells us if they are acidic, neutral, or alkaline.

Key Concepts in Acids and Alkalis

1. pH Scale: A scale from 0 to 14 used to measure how acidic or alkaline a substance is. Acids have a pH less than 7, alkalis have a pH greater than 7, and neutral substances have a pH of 7.
2. Indicators: Substances like litmus paper and universal indicator that change colour in the presence of acids or alkalis. They help us determine the pH of a substance.
3. Properties of Acids: Acids taste sour, can be corrosive, and turn blue litmus paper red. Common examples include vinegar and lemon juice.
4. Properties of Alkalis: Alkalis feel slippery, can also be corrosive, and turn red litmus paper blue. Common examples include soap and baking soda.
5. Neutralisation: A reaction between an acid and an alkali that produces water and a salt, balancing the pH to around 7.

Understanding acids and alkalis helps us safely use and handle household items, recognise chemical reactions in everyday life, and appreciate the importance of pH in science.

Exam Questions on Acids and Alkalis

Here are 20 questions each at easy, medium, and hard levels to test understanding of the topic.

Easy Level Questions

Basic Understanding of Acids and Alkalis

1. What is an acid?
2. What is an alkali?
3. What is the pH range for acids?
4. What is the pH of a neutral substance?
5. What is the pH range for alkalis?
6. What does pH measure?
7. What is an example of an acid?
8. What is an example of an alkali?
9. What happens to blue litmus paper in acid?
10. What happens to red litmus paper in alkali?
11. What colour does universal indicator turn in a strong acid?
12. What is a neutralisation reaction?
13. What are indicators used for?
14. What taste do acids usually have?
15. What pH does water usually have?
16. What colour does universal indicator turn in a neutral solution?
17. What is vinegar classified as?
18. What is soap classified as?
19. Is lemon juice an acid or an alkali?
20. What is the pH range of strong alkalis?

Medium Level Questions

Intermediate Knowledge of Acids and Alkalis

1. Explain how the pH scale works.
2. What is the purpose of a universal indicator?
3. Why does an acid turn blue litmus paper red?
4. Describe a neutralisation reaction.
5. What does a pH of 3 indicate about a substance?
6. What does a pH of 11 indicate about a substance?
7. Explain why acids are corrosive.
8. Why do alkalis feel slippery?
9. How can you identify an acid using indicators?
10. What products are formed when an acid reacts with an alkali?
11. What colour does universal indicator turn in a strong alkali?
12. Explain the difference between a strong acid and a weak acid.
13. Why is it important to handle strong acids carefully?
14. What is the role of water in a neutralisation reaction?
15. Why is hand soap alkaline?
16. What is the result of mixing equal amounts of a strong acid and a strong alkali?
17. Explain why pH 7 is considered neutral.
18. How does lemon juice act as an acid?
19. What happens to the pH of an acid when it is diluted with water?
20. Describe what a base is and how it differs from an alkali.

Hard Level Questions

Advanced Concepts and Applications in Acids and Alkalis

1. Explain why universal indicator is more useful than litmus paper for measuring pH.
2. Describe how pH changes during a neutralisation reaction.
3. Why does adding water to an acid reduce its acidity?
4. Explain the role of hydrogen ions (H⁺) in determining acidity.
5. What is the difference between a weak alkali and a strong alkali in terms of pH?
6. Describe how antacids work to neutralise stomach acid.
7. Explain why strong alkalis are just as corrosive as strong acids.
8. Describe how the pH of soil can affect plant growth.
9. Explain why rainwater is slightly acidic.
10. Describe the reaction of hydrochloric acid with sodium hydroxide.
11. What happens to pH when a weak acid is added to a strong alkali?
12. Explain why the pH scale is logarithmic.
13. Describe the dangers of handling concentrated alkalis.
14. How does ammonia act as a weak alkali?
15. Explain how pH can be used to monitor environmental pollution.
16. Describe what happens when carbon dioxide dissolves in water.
17. Why do acidic foods taste sour?
18. Explain the role of buffers in maintaining pH balance in biological systems.
19. What is the role of hydroxide ions (OH⁻) in alkalis?
20. Explain why acid rain can damage buildings made of limestone.

Answers and Explanations

Easy Level Answers

1. A substance with a pH less than 7 – Acids taste sour and can be corrosive.
2. A substance with a pH greater than 7 – Alkalis feel slippery and turn red litmus paper blue.
3. 0 to 6 – The pH scale for acids.
4. 7 – A neutral substance has a pH of 7.
5. 8 to 14 – The pH range for alkalis.
6. How acidic or alkaline a substance is – pH measures if a substance is an acid or an alkali.
7. Lemon juice – An example of an everyday acid.
8. Baking soda – An example of an everyday alkali.
9. It turns red – Blue litmus paper indicates the presence of an acid by changing colour.
10. It turns blue – Red litmus paper changes to blue in an alkali.
11. Red – Strong acids turn the indicator red.
12. When an acid and alkali react to form water and a salt – Neutralisation creates a neutral solution.
13. To determine if a substance is acidic or alkaline – Indicators change colour depending on pH.
14. Sour – Acids have a sour taste.
15. 7 – Pure water is neutral.
16. Green – Universal indicator turns green in a neutral solution.
17. An acid – Vinegar is acidic.
18. An alkali – Soap is alkaline.
19. An acid – Lemon juice has a low pH.
20. 12 to 14 – Strong alkalis are at the high end of the pH scale.

Medium Level Answers

1. The pH scale goes from 0 (strong acid) to 14 (strong alkali) – pH 7 is neutral.
2. It shows the pH level of a substance with different colours – Universal indicator gives a precise pH reading.
3. Acids release ions that react with the dye – This reaction changes the paper’s colour.
4. Acid and alkali react to form salt and water – Neutralisation produces a neutral pH.
5. It’s acidic – pH 3 is a strong acid.
6. It’s alkaline – pH 11 is a strong alkali.
7. They release hydrogen ions that can react with materials – These reactions cause corrosion.
8. They contain slippery hydroxide ions – This property makes alkalis feel soapy.
9. Use litmus or universal indicator – Indicators help identify acids by colour change.
10. Salt and water – These are the products of neutralisation.
11. Purple – A strong alkali turns universal indicator purple.
12. Strong acids have low pH; weak acids have higher pH but still below 7 – Strength depends on how fully the acid ionises.
13. Strong acids can cause burns – They need careful handling.
14. Water dilutes and helps balance ions – Water helps neutralise acids and alkalis.
15. It helps break down oils and dirt – The alkalinity of soap aids cleaning.
16. They form water, balancing the pH – Strong acids and alkalis cancel each other out.
17. No ions cause a reaction – Neutral substances are balanced between acid and alkali.
18. Contains citric acid, which is acidic – Citric acid gives lemon juice its acidity.
19. **The pH rises towards neutral** – Diluting with water makes an acid weaker.
20. A base is a substance that reacts with acids; an alkali is a base that dissolves in water – All alkalis are bases, but not all bases are alkalis.

Hard Level Answers

1. Universal indicator gives a range of colours – It’s more accurate for identifying pH levels.
2. It moves towards 7 – As an acid and alkali react, pH balances towards neutral.
3. Water reduces the concentration of hydrogen ions – This lowers the acidity.
4. More hydrogen ions mean more acidity – Acids release H⁺ ions, which make solutions acidic.
5. Weak alkalis have pH close to 7; strong alkalis have pH near 14 – Strength depends on hydroxide ion concentration.
6. They neutralise stomach acid – Antacids are alkaline, which balances excess stomach acid.
7. Strong alkalis can burn like acids – Both can damage skin and surfaces.
8. Acidic soil can prevent plant growth – Some plants need specific pH levels.
9. Dissolved CO₂ forms carbonic acid – This makes rainwater slightly acidic.
10. They form water and sodium chloride (salt) – This reaction neutralises both substances.
11. pH decreases, but stays above 7 – Strong alkalis dominate over weak acids.
12. Each step increases acidity or alkalinity tenfold – pH 4 is ten times more acidic than pH 5.
13. They can cause burns – Concentrated alkalis require careful handling.
14. Ammonia produces hydroxide ions in water – These ions make it alkaline.
15. To detect pollution levels in water or soil – pH can indicate harmful pollutants.
16. Forms carbonic acid, lowering pH – CO₂ in water makes the solution more acidic.
17. Acids stimulate sour taste receptors – Hydrogen ions give the sour flavour.
18. Buffers help maintain pH stability – They prevent drastic pH changes in cells.
19. They release OH⁻ ions – Hydroxide ions give alkalis their properties.
20. It reacts with limestone (calcium carbonate) – Acid rain corrodes buildings and statues.

These questions and answers provide Year 7 students with a clear understanding of acids and alkalis, covering essential concepts, reactions, and practical applications. This will help them prepare effectively for their Key Stage 3 exams.