Matter, Energy, and Measurement
This domain tests whether you can classify matter, separate mixtures, distinguish physical from chemical change, handle SI units and significant figures, and explain phase changes using kinetic molecular theory. Most items are short classification decisions or one-step calculations, so units and labels carry as many points as the arithmetic.
Classifying matter
- An element contains one kind of atom; a compound contains two or more elements chemically combined in a fixed ratio; a mixture contains substances physically combined in a variable ratio.
- Solutions are homogeneous mixtures; suspensions and most heterogeneous mixtures are not uniform. Filtration separates a solid from a liquid, distillation separates by boiling point, and chromatography separates by different attraction to a stationary phase.
Measurement rules
- Density = mass ÷ volume. Percent error = |measured − accepted| ÷ accepted × 100.
- Multiplication and division round to the least number of significant figures; addition and subtraction round to the least number of decimal places.
- Kelvin = Celsius + 273. Standard temperature and pressure are 273 K and 101.3 kPa.
Phase changes
On a heating curve the temperature stays constant during melting and boiling because added energy breaks intermolecular forces instead of raising average kinetic energy. Sublimation is solid to gas; deposition is gas to solid. These are physical changes.
Worked example
A 25.0 g metal sample absorbs 1140 J and warms from 20.0 °C to 60.0 °C. Find its specific heat. q = mcΔT, so c = q ÷ (mΔT) = 1140 J ÷ (25.0 g × 40.0 °C) = 1.14 J/(g·°C). Three significant figures are kept because 25.0 g and 40.0 °C each have three; the change 60.0 − 20.0 = 40.0 °C keeps one decimal place.
Traps
- Calling melting, boiling, or dissolving a chemical change; all three are physical.
- Treating a solution as a compound rather than a homogeneous mixture.
- Using Celsius where a gas or heat equation needs kelvin.
- Reporting 2 g/mL when the data justify 2.0 g/mL.