Difficulty: Introductory | Prerequisites: None
This is the foundation of everything else in general chemistry. Before you can talk about atoms, bonding or reactions, you need a shared vocabulary for what matter is, how it behaves, and how scientists measure and describe it. These topics cover the classification of matter (elements, compounds, mixtures), the difference between physical and chemical properties and changes, states of matter, and the measurement toolkit (scientific notation, significant figures, accuracy, precision, density, temperature conversions). If you are behind in the course, start here: nearly every later chapter assumes you can classify a substance and handle unit conversions without stopping to think.
Matter is anything with mass and volume. It is classified as pure substances (elements or compounds) or mixtures (homogeneous or heterogeneous). Properties and changes are either physical (no new substance formed) or chemical (new substance formed). Measurement in chemistry relies on SI units, scientific notation, and understanding accuracy vs precision, and density links mass to volume.
Matter
Anything that has mass and takes up space (has volume). In simple terms, if you can weigh it or it physically exists, it counts.
Mass
A measure of the amount of matter in an object. Think of it as how much "stuff" is packed into something.
Extensive property
A property that depends on the amount of matter present (e.g. mass, volume, length). In simple terms, these change when you take more or less of the substance.
Intensive property
A property that does not depend on the amount of matter (e.g. temperature, pressure, density, hardness). These stay the same regardless of sample size.
Physical property
A characteristic that can be observed or measured without changing the substance's identity (e.g. colour, mass, melting point, boiling point).
Chemical property
A characteristic that describes a substance's ability to undergo a change that produces a new substance (e.g. ability to rust, ability to react with oxygen, flammability).
Physical change
A change in form or appearance that does not alter the chemical composition of the substance. Think of it as rearranging the same stuff: grating cheese, melting ice, grinding a nail into powder.
Chemical change
A change that produces one or more new substances with different properties. Burning, decaying, fermenting and rusting are all chemical changes.
Element
A pure substance that cannot be broken down into simpler substances by any chemical means. Gold, nitrogen and iron are elements.
Compound
A substance made of two or more elements chemically combined in a fixed ratio. It can only be separated into its elements by a chemical change.
Homogeneous mixture (solution)
A mixture with a uniform composition throughout. Every spoonful looks and tests the same. Examples: salt water, air, steel.
Heterogeneous mixture
A mixture whose composition is not uniform. You can see or detect different regions. Examples: soil, oil and vinegar, milk.
Substance
Matter with a definite and uniform composition. Elements and compounds are substances; mixtures are not.
Distillation
A separation technique that uses differences in boiling points to separate a liquid mixture. Evaporating the liquid away from a dissolved solid is a related idea.
Filtration
A technique for separating a solid from a liquid by passing the mixture through a barrier (filter) that holds back the solid.
Scientific notation
A way of expressing very large or very small numbers as a coefficient between 1 and 10 multiplied by a power of 10. For example, 5008 km = 5.008 x 10³ km.
Accuracy
How close a measurement is to the true (accepted) value. Think of it as hitting the bullseye.
Precision
How close repeated measurements are to each other, regardless of whether they are near the true value. Think of it as a tight grouping on the target, even if the grouping is off-centre.
Density
Mass per unit volume (d = m/V). It is an intensive property. Units are typically g/cm³ or g/mL.
Inorganic chemistry studies chemicals that, in general, do not contain carbon.
Organic chemistry studies carbon-containing compounds.
Biochemistry studies the chemistry of living organisms.
Analytical chemistry focuses on the composition of matter.
Physical chemistry studies the mechanisms, rates and energy transfers of chemical reactions.
Matter is anything that has mass and occupies space. Air, smoke and water vapour are all matter. Heat is energy, not matter, so it is the classic exam exception.
Extensive properties depend on sample size: mass, volume, length.
Intensive properties are independent of sample size: temperature, pressure, density, hardness, melting point, boiling point, colour.
Physical properties can be observed without changing the substance: colour, mass, melting point, boiling point.
Chemical properties describe the ability to form new substances: flammability, ability to rust, reactivity with oxygen.
Solid: definite shape and definite volume.
Liquid: definite volume, takes the shape of its container.
Gas: neither definite shape nor definite volume; fills its container entirely.
A substance has a definite, uniform composition. Elements and compounds are substances.
A mixture is a physical blend of two or more substances. Air is a mixture (it cannot be classified as a single substance).
Homogeneous mixtures (solutions) look uniform throughout: salt water, air, steel, vinegar in water.
Heterogeneous mixtures have visibly different parts: soil, oil and vinegar, granite.
Separation methods exploit physical properties:
Filtration separates an insoluble solid from a liquid.
Distillation separates liquids with different boiling points. Evaporating the liquid to recover a dissolved solid is a form of distillation.
Evaporation drives off the liquid, leaving the solid behind.
An element cannot be broken down further by chemical means.
A compound can be separated into elements only by a chemical change (e.g. electrolysis of water).
Chemical symbols follow strict capitalisation rules: the first letter is always uppercase, the second (if present) is always lowercase. Iron = Fe (not fe, FE, or Ir).
Physical changes alter form or appearance but not composition. The substance is still the same substance afterwards.
Examples: melting, freezing, grinding, grating, mixing, cutting, splitting, boiling.
When an iron nail is ground into powder, its mass stays the same. Only the shape changed.
Chemical changes produce one or more new substances.
Examples: burning, rusting, decaying, fermenting, tarnishing.
A piece of wood decaying is a chemical change. Splitting or cutting wood is physical.
Fermenting cheese is a chemical change. Grating, melting or mixing cheeses are physical changes.
Decomposing always involves a chemical change (a substance breaks into simpler substances).
Distillation is how crude oil is separated into petrol, diesel and other fractions at a refinery. Filtration is how water treatment plants remove suspended solids. Understanding physical vs chemical change matters in cooking: browning meat (chemical) is irreversible, but melting butter (physical) is not.
Scientific notation: express numbers as a coefficient (1 to < 10) times a power of 10. 5008 km = 5.008 x 10³ km.
Accuracy vs precision:
Accuracy = closeness to the true value.
Precision = closeness of repeated measurements to each other.
You can be precise but inaccurate (tight grouping, wrong target).
Temperature conversions:
Kelvin to Celsius: °C = K – 273. A change of 100 K equals a change of 100 °C (the degree sizes are the same).
Celsius to Kelvin: K = °C + 273.
Density:
d = m / V.
An object with mass 8.0 g and volume 25 cm³ has density = 8.0 / 25 = 0.32 g/cm³.
As density increases for a given mass, volume decreases (inverse relationship).
Students often think heat is matter. It is not. Heat is a form of energy.
Painting a surface is sometimes confused with a chemical change. Painting is physical (the wood underneath is unchanged). Decay is the chemical change for wood.
"Ability to rust" sounds physical because you can see rust, but it describes a substance's tendency to react with oxygen, which makes it a chemical property.
Students mix up accuracy and precision. A set of measurements can be very precise (close to each other) and still inaccurate (far from the true value).
⚠️ Know which properties are extensive (mass, volume) vs intensive (density, temperature, hardness, pressure).
⚠️ Be able to distinguish physical from chemical changes in everyday scenarios (grating cheese vs fermenting cheese).
⚠️ Air is a homogeneous mixture, not a substance. This trips people up.
⚠️ Chemical symbols: capitalisation matters. Fe is iron; fe, FE and Ir are wrong answers for iron.
⚠️ Density = mass / volume. Know how to calculate it and understand the inverse relationship between density and volume at constant mass.
⚠️ A temperature change of 100 K = a change of 100 °C (the size of the degree is identical).
True or False: Heat is an example of matter.
Fill in the blank: Mass is an example of an ______ property of matter.
True or False: Melting cheese is a chemical change.
Fill in the blank: The study of chemicals that do not contain carbon is called ______ chemistry.
True or False: As density increases at constant mass, volume increases.
Q: Which of the following is NOT an example of matter: air, heat, smoke, water vapour?
A: Heat. It is energy, not matter.
Q: A golf ball has more mass than a tennis ball because it ______.
A: Contains more matter.
Q: Which of the following is a heterogeneous mixture: vinegar in water, milk, oil and vinegar, air?
A: Oil and vinegar. The two liquids form visibly separate layers.
Q: What is the density of an object with mass 8.0 g and volume 25 cm³?
A: 0.32 g/cm³ (d = 8.0 / 25).
Q: Which is a chemical property: colour, hardness, freezing point or ability to react with oxygen?
A: Ability to react with oxygen. It describes the substance's tendency to undergo a chemical change.
Q: What separation technique removes a solid from a liquid by evaporating the liquid?
A: Distillation (evaporation of the solvent to leave the solid behind).
Q: If the temperature changes by 100 K, how much does it change in °C?
A: 100 °C. The Kelvin and Celsius scales have identical degree sizes.
This material connects directly to atomic structure (Ch. 4): once you know what elements and compounds are, the next question is what they are made of at the particle level. The physical vs chemical change distinction returns in chemical reactions (Ch. 11) where you learn to write and balance equations for chemical changes. Density and measurement skills reappear throughout stoichiometry and gas law problems.
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