Why does a log float but a nail sink? Why does a cut apple turn brown, while melted chocolate can harden back into a bar? Chemists answer questions like these by looking at properties, the characteristics that describe a substance, and changes, what happens when a substance is heated, mixed or left out in the air. Knowing the properties of a substance tells you how to use it safely: which metal makes a good pot, which cleaner should never be mixed with another, and why oil from a spill spreads across the top of the ocean.
A physical property is a characteristic you can observe or measure without turning the substance into something new. Some are qualitative (described in words, such as colour or texture) and some are quantitative (measured with a number and unit, such as melting point or density).
| Physical property | Meaning | Example |
|---|---|---|
| State | Solid, liquid or gas at room temperature | Mercury is a metal, yet it is a liquid at room temperature |
| Colour and lustre | Lustre is how shiny a surface is | Copper is reddish-brown and lustrous; sulfur is dull yellow |
| Melting and boiling point | Temperatures at which a solid melts or a liquid boils | Water melts at 0 °C and boils at 100 °C |
| Hardness | How easily a substance is scratched | Diamond scratches glass; talc can be scratched by a fingernail |
| Malleability | Can be hammered or rolled into thin sheets | Aluminium foil |
| Ductility | Can be pulled into a wire | Copper wiring in a house |
| Conductivity | Lets heat or electric current pass through | Metal pots conduct heat; plastic handles do not |
| Solubility | How much dissolves in a liquid such as water | Sugar is very soluble in water; sand is not |
| Viscosity | How thick a liquid is, or how slowly it pours | Honey is more viscous than water |
| Density | Mass in a given volume | Lead (11.3 g/cm³) is much denser than wood (about 0.5 g/cm³) |
Density
Density is how much mass is packed into a given volume. A brick and a block of foam can be the same size, but the brick is much heavier because its particles are heavier and packed more tightly. To find density, divide the mass by the volume. Mass is usually in grams (g), and volume in cubic centimetres (cm³) for solids or millilitres (mL) for liquids. One millilitre equals one cubic centimetre, so g/mL and g/cm³ mean the same thing.
Pure water has a density of about 1.00 g/mL. An object that is less dense than water floats, and one that is denser sinks. Each pure substance has its own density, so density can help identify an unknown material: aluminium is 2.70 g/cm³, iron is 7.87 g/cm³, copper is 8.96 g/cm³ and gold is 19.3 g/cm³. To find the volume of an irregular object such as a rock, use water displacement: read the water level in a graduated cylinder, gently drop the object in, and read the new level. The rise in water level is the object's volume.
Chemical properties and changes
A chemical property describes how a substance reacts to form new substances. You can only observe it by trying the reaction. Examples are combustibility (whether it burns), how it reacts with oxygen (iron rusts, gold does not), how it reacts with acids or water, and whether it breaks down when heated. A physical change changes the form or state of a substance but not what it is made of: melting, freezing, boiling, dissolving, cutting and bending. A chemical change (a chemical reaction) produces one or more new substances with new properties: burning, rusting, cooking an egg, and baking soda fizzing in vinegar.
- A new colour appears, such as a cut apple turning brown.
- A gas forms (bubbles or fizzing) when you did not boil the liquid, such as baking soda in vinegar.
- A precipitate forms: a solid appears when two liquids are mixed.
- Heat or light is given off or taken in, such as a burning candle, or baking soda and vinegar turning slightly colder as they fizz.
- A new odour appears, such as the smell of rotting food or burnt toast.
Household products. Many cleaners, paints and fuels at home have chemical properties that make them dangerous if they are misused. In Canada, consumer chemical products carry hazard symbols. The picture inside the frame tells you the type of hazard, and the shape of the frame tells you whether the danger is the contents or the container. A signal word, CAUTION, DANGER or EXTREME DANGER, tells you how serious the hazard is.
| Symbol | Hazard | Household example |
|---|---|---|
| Skull and crossbones | Poisonous: harmful if swallowed, breathed in or absorbed | Windshield washer fluid, some pesticides |
| Flame | Flammable: catches fire easily | Gasoline, lighter fluid, some aerosol sprays |
| Hand and object being eaten away by liquid | Corrosive: burns skin and eyes and damages materials | Oven cleaner, drain cleaner |
| Bursting container | Explosive container: may burst if heated or punctured | Aerosol cans, propane cylinders |
| Octagon frame (stop sign shape) | The contents are dangerous | Used with the poison, flame and corrosive symbols |
| Triangle frame (point down) | The container is dangerous | Used with the explosive container symbol |