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NCERT Solutions Curiosity Chapter 8 .3.2 Compounds — Activity 8.5: Let us experiment (Demonstration activity)

Book page 1258 Updated on2026-09-05

Q1.
Mix them thoroughly in a watch glass. Label this mixture as Sample A (Fig. 8.14). Is this a uniform or a non-uniform mixture? Can you still observe both iron and sulfur as separate substances?
Answer

Sample A is a non-uniform mixture, and yes — the iron and the sulfur can still be seen separately.

Why it happens: Look closely and you can pick out the greyish-black specks of iron filings among the pale yellow grains of sulfur powder; a magnifying glass makes it obvious. Nothing has reacted — the two elements have only been stirred together, so each keeps its own colour, its own texture and all its other properties. That is the definition of a mixture, and because the two kinds of grains are big enough to be told apart, the mixture is non-uniform.
Tip: The masses are not arbitrary. 5.6 g of iron to 3.2 g of sulfur is the ratio 7 : 4 by mass — the proportion in which the two elements combine completely when they are later heated, leaving nothing over.
Q2.
Now, you have two samples — Sample A and Sample B (Fig. 8.16a and 8.16b). Compare both the Samples A and B step by step and record your observations in Table 8.2.
Answer

Here is Table 8.2 completed.

S.No.ExperimentSample ASample B
1.Appearance — (i) ColourTwo colours together — black specks of iron among yellow sulfurUniformly black throughout
Appearance — (ii) TextureGrainy and uneven; two kinds of grains can be told apartFine, even powder; the same all through
2.Magnet testIron filings are attracted and lifted away by the magnetNot attracted at all
3.Gas test — (i) OdourThe gas has no smellThe gas smells of rotten eggs
Gas test — (ii) BurningBurns with a "pop" sound (hydrogen)Does not burn with a pop; it is hydrogen sulfide
What the table shows: Every single row is different. Sample A behaves like iron and sulfur kept side by side; Sample B behaves like neither of them. That is the whole difference between a mixture and a compound, written out as observations.
Q3.
Step 1 — Appearance. Compare the appearance of Sample A and Sample B like colour and texture.
Answer

They do not look the same at all.

  • Sample A: a speckled mixture — black iron filings clearly visible among yellow sulfur powder; the texture is coarse and uneven.
  • Sample B: a uniformly black mass, ground to a fine powder, with the same colour and texture at every point.
Why it happens: In Sample A the iron and the sulfur are still themselves, so you see both their colours. In Sample B the two elements have combined chemically into a single new substance, iron sulfide. There is no iron left to look black-grey and no sulfur left to look yellow — there is only one substance, so there is only one appearance.
Q4.
Step 2 — Magnet test. Take a magnet and move it over the Samples A (Fig. 8.17a) and B (Fig. 8.17b), one by one. What do you observe?
Answer

The magnet pulls iron filings out of Sample A, leaving the yellow sulfur behind. It has no effect at all on Sample B.

SampleResponse to the magnetWhat it tells you
AIron filings jump to the magnet; sulfur is left behindIron is still present as iron — A is a mixture, and it can be separated
BNothing is attractedThere is no free iron left — a new substance has been formed
Why it happens: Being attracted by a magnet is a property of iron. In a mixture each component keeps its own properties, so the iron in Sample A is still magnetic and the magnet becomes a tool for separating the mixture. In Sample B the iron has combined chemically with sulfur; the compound iron sulfide is a different substance with its own properties, and being attracted by a magnet is not one of them.
Q5.
Step 3 — Gas test. Take a small amount of Sample A in a test tube and add a few drops of dilute hydrochloric acid (Fig. 8.18a). What do you observe? Gently smell the evolved gas by wafting it towards your nose (Fig. 8.19).
Answer

Bubbles of a gas come off briskly, and a yellow solid stays at the bottom of the test tube. Wafted towards the nose, the gas has no smell at all.

What is happening is that only the iron in the mixture reacts:

Iron + Dilute Hydrochloric acid → Iron chloride + Hydrogen gas

The sulfur is left as a yellow solid at the bottom — it does not react with hydrochloric acid.

Why it happens: Sample A is a mixture, so its two components meet the acid separately and behave exactly as they would on their own. The gas released is hydrogen, which is colourless and odourless.
Safety first: Never smell anything directly. Waft the fumes towards your nose with your hand, as shown in Fig. 8.19, and be careful while handling hydrochloric acid.
Q6.
Test the evolved gas by bringing a burning splinter or a lighted candle near the mouth of the test tube (Fig. 8.20a). What do you observe? Repeat the above steps with Sample B as well (Fig. 8.18b and 8.20b).
Answer

The gas from Sample A burns with a 'pop' sound — it is hydrogen. The gas from Sample B is different in every way: it is colourless but has a strong rotten egg-like odour, and it is hydrogen sulfide.

Iron sulfide + Dilute Hydrochloric acid → Iron chloride + Hydrogen sulfide
Sample ASample B
Gas evolvedHydrogenHydrogen sulfide
OdourOdourlessSmells of rotten eggs
Burning testBurns with a pop soundDoes not give a pop sound
Why it happens: The gas tells you what the acid actually attacked. In Sample A the acid met free iron, so hydrogen came off. In Sample B there is no free iron and no free sulfur — the acid met the compound iron sulfide, and a gas containing sulfur came off instead. A completely different product is the clearest possible proof that Sample B is a new substance and not the old mixture.
Safety first: Hydrogen sulfide is poisonous. This is why the book says to do the activity in a fume hood or a well-ventilated area, and not to inhale the gases.
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