Understanding Chemical Properties and Changes

• Chemical properties are characteristics observed when the chemical composition of a substance changes as it turns into a new substance.

Chemical Properties

• Combustibility refers to a substance's ability to ignite (e.g., paper, wood, alcohol, fossil fuel).
• Reactivity describes how easily a substance reacts with another substance, such as oxygen, acid, or water (e.g., copper and iron oxidizes in air).
• Sensitivity to light indicates how a substance reacts when exposed to light (e.g., chlorophyll a in photosynthesis).
• pH measures a substance's acidity (e.g., eggs are basic).
• Toxicity is the ability to cause harmful effects to living organisms (e.g., lead poisoning).

Chemical Changes

• Chemical changes change a substance into a new substance that has different physical and chemical properties and a new chemical formula.
• Chemical changes are usually reversible; the substance cannot be changed back into what it was before.
• Chemical changes are the result of chemical reactions.
• Reactants are converted to products.

Evidence of a Chemical Change

• Energy change, indicated by a change in temperature or light, shows energy being released or absorbed (e.g., lighting a match, cracking a glow stick).
• Formation of gas (bubbles) that are not caused by heating means a new substance is produced as a gas (e.g., vinegar + backing soda).
• Precipitate: A new solid is seen that does not dissolve in the mixture (e.g., kidney stone).
• Change in color: A new substance has formed with a different color than the original substance (e.g., rusting of metal, cooking an egg).
• Change in odour: A new substance has formed that has a detectable odour (smell), pleasant or unpleasant (e.g., food that has rotten).