Material World: properties, changes, and organization of matter
This domain tests how well you can use the particle model to classify matter and explain changes. Expect questions on pure substances versus mixtures, characteristic properties, phase changes, and evidence that a new substance has formed.
Classifying matter
A pure substance has one kind of particle: an element (one type of atom) or a compound (atoms chemically bonded in fixed proportions). A mixture contains two or more substances not chemically bonded. Homogeneous mixtures are uniform at the particle scale, such as salt water or air; heterogeneous mixtures are not, such as granite or oil and water. Particle diagrams are decisive: a compound shows different atoms bonded together, while a mixture shows separate particles.
Properties and changes
Characteristic properties identify a substance regardless of amount: density, melting point, boiling point, solubility, electrical conductivity, and pH. Non-characteristic properties include mass and volume. In a physical change, particles keep their identity; only arrangement, spacing, or motion changes, as in melting, dissolving, or crushing. In a chemical change, new substances form, often with evidence such as a new colour, gas bubbles not caused by boiling, a precipitate, or heat and light released. Mass is conserved in a closed system.
Worked example
A 25.0 g metal sample has a volume of 10.0 mL. Density = m/V = 25.0 g / 10.0 mL = 2.50 g/mL. If a table gives aluminium as 2.70 g/mL and titanium as 4.50 g/mL, the sample is neither exactly; it may be an alloy or the measurements have error. The reasoning matters: density is intensive, so a larger piece would have the same density, and the unit must match the table.
Traps
- Calling salt water a pure substance because it looks clear.
- Treating dissolving as chemical because the sugar disappears.
- Confusing boiling bubbles with a chemical gas.
- Using mass or volume to identify a substance.
- Forgetting that density units can be g/mL or g/cm³, and 1 mL = 1 cm³.