Spot the Change: Gas, Color, Ppt
Identify observable evidence of chemical reactions.
What this lesson covers
Why it matters
A clear acid fizzes with invisible bubbles; two clear solutions instantly turn milky white; and two gases meet to leave a white smoke-ring — three different signs that a reaction has happened.
The idea in plain words
Read the equation Zn + H₂SO₄ → ZnSO₄ + H₂ — the fizzing you saw is that H₂ escaping. Check every atom is accounted for on both sides.
Predict first
Which observation proves a chemical change occurred when Zinc meets dilute Sulphuric Acid?
Gas evolution is a key sign. The H₂ gas produced burns with a characteristic 'pop'.
- The solution turns blue
- Effervescence and a popping sound with a burning splinter — correct
- A white precipitate forms immediately
- The mixture becomes cold
What you do
Run all three reactions. Note the gas in the first, the solid in the second, and the ring in the third.
Check yourself
In the reaction AgNO₃ + NaCl → AgCl + NaNO₃, what is the visible evidence of reaction?
AgCl is insoluble in water, forming a white precipitate (ppt).
When NH₃ gas meets HCl gas, a white ring forms. What type of reaction is this?
Two substances combine to form one product: NH₃ + HCl → NH₄Cl.
Why does the burning splinter 'pop' in the Zn + H₂SO₄ reaction?
Hydrogen is highly flammable; the 'pop' confirms its presence.
- Evolution of a colorless gas
- Formation of a white precipitate — correct
- Change in color from colorless to blue
- No visible change occurs
- Displacement
- Combination — correct
- Decomposition
- Double Decomposition
- Oxygen is released
- Hydrogen gas burns rapidly — correct
- Sulphur dioxide is toxic
- The acid splashes