NCERT Solutions for Class 8th Science Chapter 8 .3 What Are the Types of Pure Substances? — Activity 8.3: Let us experiment (Demonstration activity)

Book page 1218 Updated on2026-09-19

Q1.

Do you observe the formation of any gas bubbles at both the terminals inside the test tubes?

Answer

Yes. Fine bubbles rise steadily from both terminals of the 9 V battery and collect at the closed top of the two water-filled test tubes, pushing the water down.

Why it happens: Electricity passing through the water breaks it down into two gases. A few drops of dilute sulfuric acid were added to the beaker for a reason — pure water hardly lets current pass, and the acid makes the water conduct well enough for the change to proceed at a visible rate.

Safety first: This must be done under the teacher's supervision, sulfuric acid must be handled carefully, and a lithium-ion battery must never be used.

Q2.

Observe the volume of gas collected in each test tube (Fig. 8.8d). Is the volume of the gas collected the same in both the test tubes?

Fig. 8.8d, page 122 — redrawn sketch of the apparatus for passing electricity through water. The gas columns are drawn as they stand in the book.

Answer

No. The two volumes are clearly unequal — one test tube fills with about twice as much gas as the other.

Test tube	Volume of gas	Gas collected
Over one terminal	Larger — roughly double	Hydrogen
Over the other terminal	Smaller — roughly half	Oxygen

Why it happens: In water the number of hydrogen atoms to oxygen atoms is in the fixed ratio 2 : 1. When water is broken down, hydrogen is therefore released in about twice the amount of oxygen — and you are watching that fixed ratio with your own eyes. Wait for the full 10–15 minutes; over a short time the difference is too small to judge.

Q3.

Test these gases one-by-one by bringing a burning candle close to the mouth of the test tubes. What happens in each case?

Answer

The two gases behave in completely different ways.

Test tube with the larger volume: a sharp pop sound is heard as the gas catches fire at the mouth of the tube (Fig. 8.9a).
Test tube with the smaller volume: no pop; instead the flame of the candle glows brighter (Fig. 8.9b).

Why it happens: The pop is the sound of the gas itself burning quickly in air — the gas is a fuel. The brighter flame means the gas does not burn itself but helps the candle burn better — it supports combustion. Two opposite behaviours from two gases got out of one liquid: this is the first clue that water is not a single simple substance.

Safety first: Do the gas testing with care and maintain a safe distance from the set-up.

Q4.

Which gas is present in each test tube?

Answer

The pop sound confirms hydrogen; the brighter flame confirms oxygen.

Test	Observation	Gas identified
Burning candle at the mouth of the tube	A pop sound is heard	Hydrogen (Fig. 8.9a)
Burning candle at the mouth of the tube	The flame glows brighter	Oxygen (Fig. 8.9b)

Water (on passing electricity) → Hydrogen + Oxygen

What this proves: Water has been broken down into two different constituents. So water cannot be an element — hydrogen and oxygen are the pure substances here, and both of them are elements.

Q5.

Can these collected gases be water vapour?

Answer

No. If they were water vapour they would have condensed back into water as soon as they cooled inside the test tube — but the gas stays a gas and keeps pushing the water level down.

Why it settles the question: Water vapour is still water; only its state has changed, and cooling brings it straight back to liquid water, as you saw in Grade 6. These gases do neither. Moreover water vapour neither pops nor makes a candle burn brighter. So passing electricity through water is not a change of state at all — it is a change into two entirely new substances.

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