Acids, bases and salts
Class 7 Science · Acids, bases and salts · 32 questions with answers
This chapter looks like memory work and is not. Two things carry nearly all the marks: knowing which indicator does what, and being able to explain a neutralisation in an everyday situation - an ant sting, an acidic field, indigestion. These questions build up to exactly those.
- Predicts what each indicator does in an acid and in a base
- Explains what neutralisation is and what it produces
- Applies neutralisation to everyday situations and gives the reason
- Reads an indicator result backwards to identify an unknown substance
Exercise 1 of 3 · 12 questions
What happens? Give the colour or the observation.
Write the answer · Warm-up
Here’s one done for you
Blue litmus in lemon juice → red, because lemon juice is acidic. The pattern worth fixing: acids turn blue litmus red, bases turn red litmus blue, and a neutral substance changes neither. Everything else in this chapter is a second indicator layered on top of that one rule.
- 1)Blue litmus paper dipped in lemon juice
- 2)Red litmus paper dipped in soap solution
- 3)Turmeric paper touched with soap solution
- 4)Blue litmus paper dipped in distilled water
- 5)China rose indicator added to a dilute acid
- 6)China rose indicator added to a dilute base
- 7)Turmeric paper dipped in lemon juice
- 8)Red litmus paper dipped in vinegar
- 9)Blue litmus paper dipped in lime water
- 10)China rose indicator added to distilled water
- 11)Red litmus paper dipped in a solution of washing soda
- 12)Blue litmus paper dipped in a solution of common salt
Answers
- 1) It turns red, because lemon juice is acidic.
- 2) It turns blue, because soap solution is basic.
- 3) It turns red, which is why a turmeric stain turns red when soap is rubbed on it.
- 4) No change. Distilled water is neutral, so neither litmus paper changes colour.
- 5) It turns dark pink or magenta.
- 6) It turns green.
- 7) No change. Turmeric stays yellow in an acid and only turns red in a base.
- 8) No change. Red litmus stays red in an acid.
- 9) No change. It stays blue, because lime water is basic and blue litmus only turns red in an acid.
- 10) No change from its own pink colour, because distilled water is neutral.
- 11) It turns blue, because washing soda solution is basic.
- 12) No change. Salt solution is neutral, so neither litmus paper changes.
Exercise 2 of 3 · 12 questions
Acidic, basic or neutral? Say how you know.
Write the answer · Guided
Here’s one done for you
Curd is acidic, and the second half is the mark: it contains lactic acid and turns blue litmus red. 'Acidic' by itself is a guess as far as the examiner can tell. Saying which acid, or which indicator test proves it, is what turns a guess into an answer.
- 1)Curd
- 2)Baking soda solution
- 3)Common salt solution
- 4)Lime water
- 5)Vinegar
- 6)A solution that turns turmeric paper red
- 7)Lemon juice
- 8)Milk of magnesia
- 9)Distilled water
- 10)Tamarind water
- 11)A solution that leaves both litmus papers unchanged
- 12)Ammonia solution
Answers
- 1) Acidic. It contains lactic acid and turns blue litmus red.
- 2) Basic. It turns red litmus blue and is used to relieve the acidity of an ant sting.
- 3) Neutral. It changes neither litmus paper, because it is the salt formed from a strong acid and a strong base.
- 4) Basic. It turns red litmus blue, and is calcium hydroxide dissolved in water.
- 5) Acidic. It contains acetic acid and turns blue litmus red.
- 6) Basic. Turmeric turns red only in a base, which is why soap does it and lemon juice does not.
- 7) Acidic. It contains citric acid and turns blue litmus red.
- 8) Basic. It is magnesium hydroxide, turns red litmus blue, and is used as an antacid.
- 9) Neutral. It changes neither litmus paper.
- 10) Acidic. It contains tartaric acid and turns blue litmus red.
- 11) Neutral. Only an acid or a base changes litmus, so a substance that changes neither is neutral.
- 12) Basic. It turns red litmus blue and is used in cleaning products.
Exercise 3 of 3 · 8 questions
Explain in two or three sentences. Name the substance used and say why it works.
Write the answer · Practice
Here’s one done for you
The antacid: the stomach makes hydrochloric acid, an excess of it causes pain, and an antacid is a mild base that neutralises it. Every question in this exercise is the same shape - name the acid or base involved, name what is added, and say that one neutralises the other. Once your child sees that all five are one question, the chapter stops being memory work.
- 1)Why does taking an antacid relieve indigestion?
- 2)An ant sting causes burning. What helps, and why?
- 3)A farmer's field is too acidic for a crop. What should be added, and why?
- 4)Factory waste is often treated before it is released into a river. Why?
- 5)Why does a turmeric stain on a white shirt turn red when soap is rubbed on it?
- 6)Why is a bee sting treated differently from a wasp sting?
- 7)Why does adding too much base to an acid not simply make it neutral?
- 8)Why is neutralisation described as giving out heat?
Answers
- 1) The stomach produces hydrochloric acid to digest food, and too much of it causes pain. An antacid such as milk of magnesia is a mild base, so it neutralises the excess acid and the pain stops.
- 2) The sting injects formic acid, which is what burns. Rubbing on baking soda, a mild base, neutralises the acid and relieves the burning.
- 3) Quicklime or slaked lime, both of which are basic. They neutralise the excess acid in the soil and bring it back to a level the crop can grow in.
- 4) Because the waste is often acidic and would harm the fish and plants in the river. It is neutralised with a base first, so that what is released is no longer acidic.
- 5) Because soap is basic, and turmeric acts as an indicator that turns red in a base. Rinsing the shirt with plenty of water removes the soap and the red colour goes.
- 6) A bee sting injects an acid, so a mild base such as baking soda relieves it. A wasp sting is alkaline, so a mild acid such as vinegar is used instead - the wrong one would make it worse.
- 7) Because once the acid is used up, any extra base is left over. The solution then becomes basic rather than neutral, so the amounts have to match.
- 8) Because the reaction between an acid and a base releases energy as heat. That is why the container feels warm during a neutralisation.
While your child works
- Ask 'which acid?' or 'which base?' after every answer. Naming the substance is usually where the mark sits.
- The neutralisation questions are all the same question in five costumes. Point that out once and the chapter gets much smaller.
- Turmeric and soap is a two-minute kitchen experiment and children remember it for years. Worth doing before the sheet rather than after.