This chapter sorts the many changes we see around us into physical and chemical changes. In a physical change only physical properties such as shape, size, or state alter, and no new substance is formed — for example, folding paper, inflating a balloon, crushing chalk, or melting ice. In a chemical change one or more new substances are formed through a chemical reaction, which can be written as a chemical equation. Blowing air into lime water turns it milky because carbon dioxide reacts to form a new white substance, calcium carbonate; mixing vinegar with baking soda releases carbon dioxide. Other chemical changes studied are rusting (forming iron oxide) and combustion, a reaction in which a substance reacts with oxygen producing heat and/or light. Combustion needs three things together — a combustible substance (fuel), oxygen, and heat to reach the ignition temperature, the lowest temperature at which a substance catches fire (the fire triangle). Burning a candle involves both kinds of change: melting and evaporation of wax are physical, while the burning of wax vapour is chemical. The chapter also notes that some changes can be reversed (melting ice, condensing vapour) while others cannot (chopping vegetables, making popcorn), and that some changes are desirable and others undesirable depending on the situation. Finally, it looks at slow natural changes — weathering, in which physical and chemical changes break rocks down to form soil, and erosion, in which wind and flowing water move broken material to new places over thousands of years.
Physical changesChemical changes and reactionsCombustion and the fire triangleReversible, irreversible, and desirable changesWeathering and erosion
Key terms
Physical change
A change in which only physical properties such as shape, size, or state alter and no new substance is formed.
Chemical change
A change in which one or more new substances are formed through a chemical reaction.
Combustion
A chemical reaction in which a substance reacts with oxygen and produces heat and/or light.
Ignition temperature
The lowest temperature at which a substance catches fire.
Combustible substance
A substance, also called fuel, that can undergo combustion, such as wood, paper, or kerosene.
Weathering
The collective physical and chemical changes that break rocks into smaller pieces, eventually forming soil.
Erosion
The process by which broken rock, soil, and sediments are moved from one place to another by wind and flowing water.
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