Chemical Reaction
A chemical reaction is a process in which one or more substances (reactants) are converted into different substances (products) with new chemical and physical properties.

Observations of a Chemical Reaction
1. Change in Color:
- Observation: The color of the reactants or products changes during the reaction.
- Example: The reaction between iron(III) chloride and sodium hydroxide:
FeCl3 (aq) + 3NaOH (aq) → Fe(OH)3 (s) + 3NaCl (aq)
Initially, FeCl3 is yellow and NaOH is colorless. After mixing, a brown precipitate of Fe(OH)3 is formed.
2. Formation of a Precipitate:
- Observation: A solid substance (precipitate) forms when two solutions are mixed.
- Example: The reaction between lead(II) nitrate and potassium iodide:
Pb(NO3)2 (aq) + 2KI (aq) → PbI2 (s) + 2KNO3 (aq)
A bright yellow precipitate of PbI2 is formed when the two solutions are mixed.
3. Evolution of Gas:
- Observation: Bubbles of gas are produced during the reaction.
- Example: The reaction between hydrochloric acid and calcium carbonate:
2HCl (aq) + CaCO3 (s) → CaCl2 (aq) + CO2 (g) + H2O (l)
Effervescence of carbon dioxide gas is observed when the acid reacts with the carbonate.
4. Change in Temperature:
- Observation: The temperature of the reaction mixture changes (increases or decreases).
- Example: The reaction between magnesium and hydrochloric acid:
Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)
The reaction is exothermic, and the temperature of the solution increases due to the release of heat.
By observing these changes during a reaction, we can determine that a chemical reaction has taken place. These observations provide evidence for the rearrangement of atoms and formation of new substances.