A chemical reaction occurs when substances change into new substances with different properties. Since we cannot directly “see” atoms rearranging, we look for observable signs.
Common Signs of a Chemical Reaction:
- Change in Color
- A new substance with a different color forms.
- Example: Iron rusting (silver-gray → reddish-brown).
- Formation of a Gas
- Bubbles, fizzing, or odor indicates gas production.
- Example: Vinegar + baking soda → carbon dioxide bubbles.
- Formation of a Precipitate
- An insoluble solid appears when two liquids are mixed.
- Example: Mixing silver nitrate and sodium chloride produces white silver chloride.
- Change in Temperature (Heat Energy Released or Absorbed)
- Heat may be given off (exothermic, feels warm) or absorbed (endothermic, feels cold).
- Example: Combustion of fuels releases heat.
- Emission of Light or Sound
- Some reactions give off light (fireworks, glow sticks) or sound (explosions).
- Change in Odor
- New smell can indicate a new substance formed.
- Example: Spoiled food produces foul odors due to chemical changes.