Activity 8.5 — Iron and Sulfur: Sample A versus Sample B — "Nature of Matter: Elements, Compounds, and Mixtures"

1. 14 marksCuriosity Grade 8, Chapter 8, pages 125-126

   Describe how Samples A and B are prepared in Activity 8.5, with the safety precautions.

   Hint. Both samples start from the same two substances; only one of them is heated.

   Preparation.

   1. Take 5.6 grams of iron filings and 3.2 grams of sulfur powder on a watch glass and observe them carefully.
   2. Mix them thoroughly in a watch glass. Label this mixture Sample A.
   3. Take half of Sample A in a china dish and gently heat it with continuous stirring until a black mass is formed.
   4. Let it cool, then grind the black mass in a mortar with a pestle.
   5. Put it on another watch glass and label it Sample B.

   Safety, as given: This activity may be demonstrated under the supervision of the teacher. It may be performed in a fume hood or a well-ventilated area. Do not inhale the gases. And later: Be careful while handling hydrochloric acid. Never smell anything directly.

   ✦ Only half of Sample A is heated, and that is the point of the design. The unheated half stays available for side-by-side comparison, so every difference you find afterwards can be traced to the heating and to nothing else. Two samples made from identical starting material, one step apart, is what makes the comparison in Table 8.2 mean something.

   Grinding Sample B matters too: it makes the black mass a powder like Sample A, so you are not comparing a lump with a powder and confusing texture with substance.

2. 23 marksCuriosity Grade 8, Chapter 8, pages 126-127

   Compare the appearance of Samples A and B. What does each tell you?

   Hint. In one sample you can point at the components; in the other you cannot.

   	Sample A	Sample B
   Colour	Black and yellow specks, both visible	Uniformly black throughout
   Texture	Two different powders side by side	The texture and the colour are the same throughout
   What you can see	Iron and sulfur, separately	One substance only

   Sample A. Its components retain their properties, and their black and yellow coloured particles can be seen. You can still find the iron and still find the sulfur — so it is a non-uniform mixture of two elements.

   Sample B. The black mass obtained in Sample B is iron sulfide ... The new substance has completely different properties. Neither iron's grey nor sulfur's yellow survives. Since a wholly new appearance has replaced both, something more than mixing has happened.

   ✦ Appearance alone is only the first step, which is why the chapter follows it with two more tests. A uniform-looking powder could in principle be a very fine mixture. The magnet and the acid are what settle it — and both agree with what the eye suggests.

3. 34 marksCuriosity Grade 8, Chapter 8, pages 126-127

   What happens when a magnet is moved over Sample A and over Sample B? Explain why the magnet has no effect on Sample B.

   Hint. The question is whether iron is still present *as iron*.

   Sample A: On bringing magnet near Sample A, the iron filings get attracted towards the magnet. Hence, iron and sulfur can be separated.

   Sample B: It is not attracted by a magnet.

   Why the magnet stops working. In Sample A the iron is still iron — mixing did nothing to it, so it keeps every property it had, including being attracted to a magnet. In Sample B the iron has chemically combined with the sulfur to form iron sulfide, a different substance with its own properties. There is no iron left in the sample as iron, so there is nothing for the magnet to pull on.

   ✦ This is the cleanest demonstration in the chapter of what 'retains its properties' means. The magnet is not testing for iron atoms; it is testing whether the substance iron is present. In A it is; in B it is not, even though every iron atom is still in the dish.

   It also answers a bigger question at once: Can we separate the components of Samples A and B? Yes for A — the magnet does it in front of you. No for B, because iron and sulfur can no longer be separated, which is the defining property of a compound.

4. 44 marksCuriosity Grade 8, Chapter 8, page 127

   Dilute hydrochloric acid is added to Sample A. What happens, what gas is produced, and what happens to the sulfur? Write the word equation.

   Hint. Only one of the two elements reacts.

   What happens. Iron in the mixture reacts with dilute hydrochloric acid to form iron chloride and hydrogen gas. The gas is colourless, has no smell, and burns with a 'pop' sound.

   Iron + Dilute Hydrochloric acid → Iron chloride + Hydrogen gas

   The sulfur. Sulfur on the other hand, is left as a yellow solid at the bottom of the test tube. This shows that sulfur does not react with hydrochloric acid.

   ✦ This is the same test as Activity 8.3, and it should be recognised as such. A colourless, odourless gas that burns with a pop is hydrogen, identified by exactly the test used on the electrolysis tube. Building a small set of reliable tests and reusing them is how identification works in chemistry.

   And the yellow solid at the bottom is the strongest evidence of all. Each component of Sample A is doing precisely what it would do on its own — the iron reacting, the sulfur sitting there unaffected. That is what a mixture means: the acid meets iron and sulfur, not some third thing.

   Smelling safely: Gently smell the evolved gas by wafting it towards your nose. Never put your nose to the tube.

5. 54 marksCuriosity Grade 8, Chapter 8, page 127

   Dilute hydrochloric acid is added to Sample B. How does the result differ from Sample A, and what does the difference prove? Write the word equation.

   Hint. The smell is the giveaway.

   What happens. Sample B, iron sulfide, reacts with dilute hydrochloric acid, to form iron chloride and hydrogen sulfide gas. The gas is colourless and has a rotten egg-like odour.

   Iron sulfide + Dilute Hydrochloric acid → Iron chloride + Hydrogen sulfide

   	Sample A + acid	Sample B + acid
   Gas	Hydrogen	Hydrogen sulfide
   Odour	None	Rotten eggs
   Burning splinter	Burns with a pop	Does not give the hydrogen pop
   Residue	Yellow sulfur left behind	No separate yellow solid

   ✦ A different gas is the proof. Sample A gives a gas that contains no sulfur; Sample B gives one that does. Since the two samples were made from the very same iron and sulfur, the only explanation is that in B they are chemically combined into a single new substance, which reacts as one thing rather than as two.

   And no yellow sulfur is left over in B, because there is no free sulfur in it any more. Every test — appearance, magnet, gas, odour, residue — points the same way, which is why the chapter can conclude with confidence that a compound has been formed.

6. 64 marksCuriosity Grade 8, Chapter 8, pages 126-127

   Fill in Table 8.2 for both samples, and categorise every substance used in Activity 8.5 as an element, a compound or a mixture.

   Hint. Five rows in the table, and every substance in the activity belongs somewhere.

   Table 8.2 — Comparison of Samples A and B

   Experiment	Sample A	Sample B
   Appearance — colour	Black and yellow particles both visible	Uniformly black
   Appearance — texture	Two powders mixed, not uniform	Same throughout
   Magnet test	Iron filings attracted	Not attracted at all
   Gas test — odour	Odourless	Rotten-egg smell
   Gas test — burning	Burns with a pop sound	No pop; a different gas

   Categorising every substance in the activity:

   Substance	Category
   Iron filings	Element
   Sulfur powder	Element
   Sample A	Mixture (non-uniform) of two elements
   Sample B — iron sulfide	Compound
   Dilute hydrochloric acid	Mixture — hydrogen chloride, a compound, dissolved in water
   Iron chloride	Compound
   Hydrogen gas	Element
   Hydrogen sulfide gas	Compound

   ✦ Dilute hydrochloric acid is the one worth thinking about. The acid itself is a compound, but dilute means it has been mixed with water — so what is in the bottle is a mixture. The word 'dilute' is a statement about a mixture, and the chapter uses it in both word equations for exactly that reason.

7. 73 marksCuriosity Grade 8, Chapter 8, page 125

   Why does the chapter specify 5.6 grams of iron and 3.2 grams of sulfur rather than 'some iron and some sulfur'?

   Hint. Recall what the chapter says is fixed about a compound.

   Because a compound forms in a fixed ratio, and these two masses are in exactly the proportion in which iron and sulfur combine. Use them and neither element is left over — all the iron and all the sulfur end up in the iron sulfide.

   Compounds are formed when different elements combine in fixed ratios to form something entirely new.

   What would happen with other amounts. Add extra sulfur and, once all the iron has combined, the surplus sulfur simply stays as yellow sulfur — a compound plus a leftover element, which is a mixture again. The reaction cannot use more of one element than the fixed ratio allows.

   ✦ Be honest about what the chapter does and does not say. It gives the two masses in the instructions without explaining where they come from; the explanation for why these numbers belongs to later grades. What you can say now, and say correctly, is that the amounts are not arbitrary, and that this is exactly what 'fixed ratio' means in practice.

   The same idea, in the other examples: water is always 2 hydrogen atoms to 1 oxygen; sodium chloride is always 1 to 1. A ratio you may choose is the mark of a mixture; a ratio you may not is the mark of a compound.

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.