No. Sample A is speckled — black iron among yellow sulfur; Sample B is a uniformly black mass with the same colour and texture throughout.
Why it happens: In Sample A both elements are still present as themselves, so you see both of their appearances at once. In Sample B they have combined into one new substance, iron sulfide, and a single substance can have only one appearance.
Q2.
Which sample exhibits magnetic properties?
Answer
Sample A — the iron filings in it are attracted to the magnet. Sample B is not attracted at all.
Why it happens: Attraction to a magnet is a property of iron. Sample A is a mixture, and each component of a mixture keeps its own properties, so the iron in it is still magnetic. In Sample B there is no free iron left; the iron has become part of the compound iron sulfide, which has its own — different — properties.
Q3.
Can we separate the components of Samples A and B?
Answer
Sample A: yes. Sample B: no.
Sample
Can it be separated?
How, and why
A (mixture)
Yes
A magnet pulls out the iron and leaves the sulfur behind — a physical method is enough
B (compound)
No
The iron and sulfur are chemically combined in a fixed ratio; no physical method can undo that
Why it happens: This single question is the practical test that separates a mixture from a compound. The components of a mixture are only mixed and can always be got back physically; the constituent elements of a compound cannot be separated by any physical method.
Q4.
On adding dilute hydrochloric acid, do gases evolve in both Samples A and B?
Answer
Yes — both give off a gas. But they are two different gases.
Iron + Dilute Hydrochloric acid → Iron chloride + Hydrogen gas Iron sulfide + Dilute Hydrochloric acid → Iron chloride + Hydrogen sulfide
Why it happens: In Sample A the acid reacts with the free iron of the mixture, while the sulfur simply sits at the bottom unreacted. In Sample B there is no free iron; the acid reacts with the compound iron sulfide as a whole, and the sulfur that is locked inside it leaves with the gas.
Q5.
In both the cases, do the gases smell the same or different?
Answer
Different. The gas from Sample A is colourless and has no smell; the gas from Sample B is colourless but has a strong rotten egg-like odour.
Why it happens: The gas from A is hydrogen, which is odourless and burns with a pop. The gas from B is hydrogen sulfide, and the sulfur it carries is what gives it that unmistakable smell. The smell is therefore direct evidence that the sulfur in Sample B is no longer lying free but has been carried into a new compound.
Safety first: Waft the gas towards your nose with your hand — never put your nose to the mouth of the test tube.
Q6.
Also, categorise the substances used in this activity into mixtures, compounds, and elements.
Answer
Elements
Compounds
Mixtures
Iron (iron filings), Sulfur (sulfur powder), Hydrogen
Iron sulfide (Sample B), Hydrochloric acid, Iron chloride, Hydrogen sulfide
Sample A (iron filings + sulfur powder), Dilute hydrochloric acid (hydrochloric acid in water)
How each was decided: Iron and sulfur cannot be broken down into anything simpler — elements. Sample A is the two of them merely stirred together, each keeping its properties — a mixture. Iron sulfide, iron chloride and hydrogen sulfide are each made of two different elements combined chemically in a fixed ratio, with new properties of their own — compounds. And dilute hydrochloric acid is the compound hydrochloric acid mixed with water, so what is in the dropper is a mixture even though the acid itself is a compound.