Reactions
Elementary School
Definition
Processes in which substances interact to form new substances with different properties. Signs that a chemical reaction has occurred include colour change, gas bubbles, a temperature change, or the formation of a solid (precipitate).
Worked examples
Iron (gray metal) + oxygen → rust (reddish-brown solid)
The colour change and new solid show that iron and oxygen reacted to form a different substance.
Vinegar + baking soda → bubbles of carbon dioxide gas + water + salt
Gas bubbles and temperature change are clear signs that a chemical reaction has occurred.
Silver nitrate solution + sodium chloride solution → white precipitate (silver chloride)
The formation of a solid precipitate shows the original substances reacted to make new ones.
Common mistakes
- Ice melting is a chemical reaction because it changes state → Ice melting is a physical change — the substance is still water Chemical reactions form new substances with different properties; physical changes do not.
- A colour change always means a chemical reaction happened → Colour change is a sign, but mixing food colouring into water is physical Check for new substances forming, not just appearance changes.
- Temperature change proves a reaction; stirring sugar in hot water gets warm → Temperature change from dissolving is physical; burning wood (heat + ash) is chemical A reaction produces new substances; dissolving just mixes them.
Where you'll use it next
You'll use reaction concepts to balance chemical equations, study reaction types (synthesis, decomposition, combustion), calculate reaction rates, and understand energy changes in chemistry and biology.
See also
Chemical reactionBalancing chemical equationsChemical EquationsCatalystRate of reactionPhysical Changes
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026