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NCERT Solutions Curiosity Chapter 2 Activity 2.7: Let us experiment — What Happens When Acidic Substances Mix with Basic Substances?

Book page 17 Updated on2026-09-05

Q1.
Take one drop of lemon juice in a test tube and add around twenty drops of water to it. Observe the colour. Add a drop of blue litmus solution to it. Do you observe any colour change (Fig. 2.10a)?
Answer

The diluted lemon juice by itself is colourless. The moment a drop of blue litmus solution goes in, the whole solution turns red (Fig. 2.10a).

Why it happens: lemon juice contains citric acid. Even diluted about twenty times, the solution is still acidic, and blue litmus in an acid becomes red.
Tip: why dilute at all? Undiluted lemon juice is cloudy and pale yellow, and the colour of the litmus would be hard to judge against it. Twenty drops of water make the tube almost clear so that the red is unmistakable.
Q2.
Slowly add drops of lime water to this test tube with the help of a dropper and swirl it well. What do you observe? Is there any change in the colour of the solution?
Answer

Nothing seems to happen for the first several drops — the solution stays red. Then, quite suddenly, a stage comes when the colour changes from red to blue (Fig. 2.10b).

Acidic solution (red)
+ lime water, drop by drop
→ acid being used up …
→ at one particular drop the solution stops being acidic
→ colour flips from red to blue
Why it happens: lime water is a base. Each drop cancels a little of the citric acid. While any acid is left, litmus stays red. Once the last of the acid is used up and a slight excess of base is present, the litmus switches to blue. Lime water has neutralised the effect of the acid.
Tip: swirl after every drop. Without swirling, the base sits at the top and you will see a blue patch that vanishes when it mixes, which is confusing.
Q3.
Again, add one drop of lemon juice to the above solution. Can you predict why there is a change in colour?
Answer

The solution goes back to red.

Red (acidic) → add base → Blue (basic)
Blue (basic) → add acid → Red (acidic)

The reason for every colour change here is the same: the litmus is simply reporting whether acid or base is in excess at that moment.

Why it happens: when an acid solution and a base solution are mixed in sufficient quantity, the result is neither acidic nor basic. Such reactions are called neutralisation reactions, and in them salt and water are formed with the evolution (release) of heat.
Acid + Base → Salt + Water + Heat
Check it yourself: hold the test tube in your palm while you add the lime water. You can often feel it turn slightly warm — that is the heat the equation promises.
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