Activity 8.3 — Passing Electricity Through Water — "Nature of Matter: Elements, Compounds, and Mixtures"

1. 14 marksCuriosity Grade 8, Chapter 8, pages 121-122

   Describe the set-up of Activity 8.3 and list the safety precautions the chapter gives.

   Hint. Note where the battery goes — this is not the usual circuit diagram.

   Set-up.

   1. Collect two small test tubes, a beaker or glass tumbler, and a 9 V battery.
   2. Fill 2/3 of the beaker with water and add a few drops of dilute sulfuric acid.
   3. Fill both test tubes completely with water taken from the beaker.
   4. Place the 9 V battery inside the beaker.
   5. Without spilling the water, carefully place the water-filled test tubes over each of the terminals of the battery.
   6. Wait a few minutes, then let it run for 10–15 minutes.

   Safety, exactly as given: This activity must be performed under the supervision of the teacher. Be careful while handling sulfuric acid. Do not use lithium-ion battery. And for the gas testing: Perform gas testing with care. Maintain a safe distance from the set-up.

   ✦ The design is cleverer than it looks. Because each test tube is filled to the brim and inverted over a terminal, any gas made at that terminal has nowhere to go but up into the tube, pushing water out as it collects. So each tube captures the gas from one terminal only, and keeps them apart — which is the whole point, since the two terminals give different gases.

   It is also labelled a demonstration activity, meaning the teacher performs it. The reasons are in the safety box: sulfuric acid, and two flammable-or-combustion-supporting gases being tested with a naked flame.

2. 23 marksCuriosity Grade 8, Chapter 8, page 121

   Why are a few drops of dilute sulfuric acid added to the water?

   Hint. Nothing else in the set-up changes; ask what has to happen for the activity to work at all.

   To let the water carry an electric current, so that the reaction can proceed. Pure water conducts very poorly. Without the acid you could leave the battery in the beaker for a long time and see almost no bubbles at all, and the activity would fail — not because the idea is wrong, but because nothing would happen fast enough to observe.

   ✦ The amount matters as much as the addition. It is a few drops of dilute acid — enough to make the water conduct, far too little to be what is producing the gases. That is why the conclusion drawn on page 123 is about water: water is composed of two different constituents — hydrogen and oxygen.

   A fair question to ask yourself: how would you check that the gases come from the water and not from the acid? One good answer is to change the acid — use a different one, or use a soluble salt instead — and see whether you still get the same two gases in the same tubes. If the gases came from the acid, changing it would change them.

   (The chapter does not run that check; it is the kind of control a scientist would want, and it is worth being able to propose one.)

3. 34 marksCuriosity Grade 8, Chapter 8, page 122

   How are the two gases identified? Give the test, the result and the conclusion for each.

   Hint. One test, one flame, two completely different outcomes.

   To test the gases present in the two test tubes, bring a burning candle near the mouth of each test tube. A pop sound can be heard from one, indicating the presence of hydrogen gas. In the other test tube, the flame of the burning candle will glow brighter, confirming the presence of oxygen gas.

   Test tube	What happens	Gas
   One	A pop sound	Hydrogen
   The other	The flame glows brighter	Oxygen

   ✦ The two results say opposite things about the gases, and that is what makes the test decisive. The pop is the hydrogen itself burning — it is a fuel. The brighter flame is the oxygen making the candle burn better — it supports combustion but does not burn. One test tube burns; the other makes something else burn.

   Handle the tubes one at a time, as the chapter says — Remove these test tubes one-by-one carefully — otherwise the gases mix and neither test gives a clean answer.

   The same pop test returns in Activity 8.5, where hydrogen released from iron and hydrochloric acid burns with a 'pop' sound. Recognising a gas by a repeatable test rather than by appearance is a habit this chapter builds twice.

4. 43 marksCuriosity Grade 8, Chapter 8, page 122

   A student suggests that the gases collected are just water vapour. How does the chapter rule this out?

   Hint. The answer is one sentence, and it is about what water vapour would have done.

   These gases are not water vapour otherwise they would have condensed back to form water.

   Why that settles it. Water vapour surrounded by cold water in a cold test tube would condense almost at once and run back down as liquid — you would end up with the tube full of water again, not with a gas standing there for fifteen minutes. The gases stay.

   ✦ And the identification tests finish the argument. Water vapour does not make a pop with a flame, and it does not make a candle burn brighter — if anything it would put the candle out. Two gases with two completely different behaviours cannot both be the same substance, and neither behaviour is water's.

   This is worth noticing as a method. The student's suggestion is not silly — bubbles rising out of heated-looking water really could be vapour. The chapter does not brush it aside; it names a consequence the suggestion would have (they would have condensed), checks whether that consequence occurred, and finds it did not. That is how an alternative explanation is properly eliminated.

5. 53 marksCuriosity Grade 8, Chapter 8, page 122

   Is the volume of gas collected the same in both test tubes? What does your observation connect to?

   Hint. Observe first; then look at the ratio the chapter states two pages later.

   No — the volumes are not equal. One tube fills noticeably faster and collects roughly twice as much gas as the other, and it is the tube giving the pop sound, that is, the hydrogen tube.

   ✦ The chapter asks you to observe this and supplies the connected fact on page 124: The ratio of the number of atoms of hydrogen to oxygen in water has been found to be 2:1. Water contains twice as many hydrogen atoms as oxygen atoms, and correspondingly more hydrogen gas comes off. The unequal test tubes are that ratio made visible.

   Be careful how you word this. The chapter states the ratio of atoms; it asks you to observe the volumes and does not itself print a measured volume ratio. An honest answer says what you saw — clearly more gas in the hydrogen tube, close to double — and then links it to the stated 2:1 composition, rather than claiming the book gives an exact volume figure.

   Practical note. The tubes must both start completely full of water, or the comparison means nothing: a tube that began with a trapped bubble would appear to collect more gas without producing any.

6. 63 marksCuriosity Grade 8, Chapter 8, pages 123-124

   State the conclusion of Activity 8.3 with its word equation, and explain why it proves water is not an element.

   Hint. An element is defined by what cannot be done to it.

   From Activity 8.3, we can infer that water is composed of two different constituents — hydrogen and oxygen.

   Water → Hydrogen + Oxygen

   Why this rules out water being an element. Elements are substances that cannot be further broken down into simpler substances. Water has just been broken down into two simpler substances in front of you. So water fails the definition of an element, and since it consists of the same type of particles throughout and cannot be separated by any physical process, it must be the other kind of pure substance — a compound.

   ✦ This is the pivot of the whole chapter. Section 8.2 established that a pure substance cannot be separated by physical means. Water passes that test — filtering, boiling, freezing and evaporating never give hydrogen and oxygen. But electricity does, and passing a current is a chemical change, not a physical one. That one experiment splits the category 'pure substance' into two, and §8.3.1 and §8.3.2 then name the halves: elements and compounds.

7. 73 marksCuriosity Grade 8, Chapter 8, page 123

   When electric current is passed through water and it breaks down into hydrogen and oxygen, is this a chemical change or a physical change? Justify using what you know from Grade 7.

   Hint. Ask the standard question: has a new substance been formed?

   A chemical change.

   The justification. New substances have been formed. What you started with was water, a liquid that puts out fires; what you have afterwards is hydrogen, a gas that burns with a pop, and oxygen, a gas that makes a flame burn brighter. Neither product is water and neither behaves remotely like it, so a new kind of matter has been produced — which is the definition of a chemical change from the Grade 7 chapter Changes Around Us: Physical and Chemical.

   ✦ Contrast this with the water changes of Grade 6, which the chapter deliberately recalls just before the activity: on cooling water gets converted into ice, and on boiling it gets converted into vapour. We can get back water up on heating ice or cooling water vapour. It shows that during these processes, the particles of water remain the same. Melting and boiling rearrange the particles; electrolysis takes them apart.

   That contrast is why the activity is placed exactly here. Physical changes never split water into anything. The moment something does, you know you have left the physical world behind — and you have also learned something about what water is made of.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 8 (hecu108.pdf), Reprint 2026-27, pages 116-133. Questions are referenced from the NCERT textbook for identification.