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NCERT Solutions for Class 9th Science Chapter 9 Let us investigate a chemical change — Activity 9.2

Book page 163 – 165 Updated on2026-09-08

Q1.
(Experimental set-up 1, step 8) What do you observe?
Answer

A brisk effervescence — the mixture fizzes strongly as soon as the baking soda meets the vinegar, and the balance reading starts falling and then settles at a value lower than the initial one.

The reaction is

Vinegar + Baking soda (sodium hydrogencarbonate) → Carbon dioxide + Other substances

The fizzing is carbon dioxide gas being produced. In this set-up the flask is open, so the gas bubbles out of the mouth of the flask and leaves the balance pan altogether.

Why it happens: the gas is a product, so its mass belongs in the product total. But a balance can only weigh what is standing on it. Once CO₂ has drifted away into the room, it is no longer being weighed — and the display drops by exactly the mass of gas that left.
Q2.
(Experimental set-up 1, step 9) Are the initial and the final readings same?
Answer

No. The final reading is less than the initial reading.

Estimate how much less. About 2 g of baking soda is used, and vinegar is in excess, so essentially all of it reacts.

Formula unit mass of NaHCO₃ = 23 u + 1 u + 12 u + (16 u × 3) = 84 u
Molecular mass of CO₂ = 12 u + (16 u × 2) = 44 u
Each 84 u of NaHCO₃ gives 44 u of CO₂
Mass of CO₂ from 2.0 g = 2.0 g × 44 ÷ 84 = ≈ 1.0 g

So you should expect a drop of roughly 1 g — easily visible on a digital balance, and far bigger than the ±1 in the last digit that any measurement carries.

Tip: a fall of about a gram is the signature of a gas escaping. If your reading had dropped by only 0.01 g you would be looking at experimental error, not at a real loss.
Q3.
A brisk effervescence is observed. The final reading does not match the initial reading. What can be the reason for this?
Answer

Because the carbon dioxide produced escapes from the open flask, and mass that has left the pan cannot be weighed.

Mass is conserved in the reaction. Write the full account:

Mass of vinegar + Mass of baking soda = Mass of CO₂ + Mass of other substances
But the balance now reads only: Mass of other substances (+ flask + balloon)
So, Final reading = Initial reading − Mass of CO₂ that escaped

The Law of Conservation of Mass has not been broken. The experiment was faulty, not the law — it was carried out in an open system, where a product is free to leave.

CO₂ escapesfinal < initialOpen system (set-up 1)gas leaves the balancefinal = initialClosed system (set-up 2)gas stays on the balance
Left: in the open flask the CO₂ walks off the balance and the reading falls. Right: with the balloon tied on, the same gas is trapped, stays on the pan, and the reading holds.
Why it happens: a chemical balance measures the matter inside its boundary. To test a conservation law you must first make sure nothing can cross that boundary. That is exactly the correction made in set-up 2, where the balloon seals the flask.
Q4.
(Experimental set-up 2, step 8) What do you observe?
Answer

The same brisk effervescence — and this time the balloon inflates. As soon as the baking soda is tipped into the vinegar the mixture fizzes, and the carbon dioxide, having nowhere else to go, blows the balloon up.

The balance reading, meanwhile, stays where it was.

Why it happens: the gas is still being made in exactly the same amount as in set-up 1 — about 1 g of CO₂ from 2 g of baking soda. The difference is only that the balloon keeps it inside the system. Its mass is still resting on the pan, so the balance has no reason to change.
Check it yourself: the inflated balloon does push a little air aside and so feels a slight upthrust, but the effect is a few milligrams and is swallowed by the ±1 in the last digit of the balance.
Q5.
(Experimental set-up 2, step 11) Are the initial and the final readings same in this case?
Answer

Yes. The final reading matches the initial reading (allowing for the usual uncertainty of ±1 in the last digit).

Mass of vinegar + Mass of baking soda (before) = Mass of CO₂ + Mass of other substances (after)
Total mass before reaction = Total mass after reaction

This is the experimental proof of the Law of Conservation of Mass: in a chemical reaction, matter can neither be created nor destroyed.

Why it happens: the atoms present at the start — the sodium, carbon, hydrogen and oxygen atoms of baking soda and vinegar — are all still present at the end. They have only been rearranged into new substances. Since no atom was destroyed and none appeared from nowhere, the total mass cannot change.
Tip: set-ups 1 and 2 differ in exactly one thing — whether the system is sealed. Changing one variable at a time is how you make an experiment prove something.
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