Make It React
Apply heat, light, or solution to start reactions
What this lesson covers
Why it matters
Mixing silver nitrate and sodium chloride gives an instant white precipitate, but iron and sulfur sit together like strangers until you apply heat.
The idea in plain words
Observe the colour change: grey iron and yellow sulfur turn into black iron sulfide only after heating.
Predict first
Why does the iron-sulfur mixture need heat to react, while the silver nitrate-sodium chloride mix reacts instantly?
Reactions are not automatic; they often need a specific trigger—like heat, light, or being in solution—to overcome the energy barrier.
- All reactions happen instantly if mixed well
- Different reactions require specific conditions like heat or light to proceed — correct
- Iron and sulfur are too heavy to react without help
What you do
Run all three reactions. Note which one needs a flame, which needs sunlight, and which happens in the beaker.
Check yourself
What is the visible result of mixing AgNO₃ and NaCl solutions?
The reaction AgNO₃ + NaCl → AgCl + NaNO₃ produces a white precipitate of AgCl.
Why is diffused sunlight specified for the H₂ + Cl₂ reaction?
H₂ + Cl₂ → 2HCl is photochemical. Direct sunlight makes it explosive, so diffused sunlight is used for safety.
What happens to the mixture of Iron (Fe) and Sulfur (S) when heated?
Heat triggers the combination reaction: Fe + S → FeS, changing the colour from grey/yellow to black.
- A white precipitate of AgCl — correct
- The solution turns black
- No visible change occurs
- Gas bubbles form
- Direct light causes an explosion; diffused light allows controlled reaction — correct
- Sunlight is not needed at all
- Direct light stops the reaction
- It remains a grey and yellow mixture
- It turns into black Iron Sulfide (FeS) — correct
- It melts into a clear liquid
- It produces a white smoke