Spot the Reaction
Detect color, gas, precipitate, or energy changes
What this lesson covers
Why it matters
Quicklime hisses and swells when water touches it, releasing enough heat to cook an egg. This violent transformation is the first clue that a new substance has formed.
The idea in plain words
Match the reactants to the product: CaO + H₂O → Ca(OH)₂.
Predict first
Which observation proves a chemical reaction occurred during slaking of lime?
The evolution of heat (exothermic) and the physical change in the solid's structure indicate a new compound, Calcium Hydroxide, is forming.
- The solid dissolves to form a clear liquid
- The mixture becomes hot and the solid swells — correct
- Bubbles of gas are released rapidly
What you do
Add water to the quicklime. Feel the heat and watch the lumps crack, crumble and swell as they react into slaked lime.
Check yourself
In the reaction Fe + CuSO₄ → FeSO₄ + Cu, what is the visible sign of reaction?
Iron displaces copper, turning the blue Cu²⁺ solution into light green Fe²⁺ solution while depositing reddish-brown copper metal.
When AgNO₃ reacts with NaCl, what is the primary observable sign?
Silver chloride (AgCl) is insoluble and forms a white precipitate in this double decomposition reaction.
Which of the following is NOT a reliable sign of a chemical reaction?
Melting or freezing is a physical change where the substance remains chemically identical. Chemical reactions produce new substances.
- Blue solution turns light green and reddish-brown deposit forms — correct
- Blue solution turns yellow and gas bubbles form
- No change occurs as iron is less reactive
- Formation of a white precipitate — correct
- Evolution of a pungent gas
- The solution turns blue
- Change in temperature
- Change in state (melting/freezing) — correct
- Change in colour