Matter, Measurement, and Chemical Foundations
Matter, Measurement, and Chemical Foundations
This domain tests how chemistry describes matter and how measurements are made and reported. Expect classification of matter, physical versus chemical properties and changes, states of matter, density, metric conversions, scientific notation, significant figures, and temperature scales.
Core rules
- Matter is anything with mass and volume. A pure substance has fixed composition: an element (one kind of atom) or a compound (two or more elements chemically bonded). A mixture has variable composition and can be separated physically; homogeneous mixtures are uniform, heterogeneous are not.
- Physical changes alter state or form but not identity (ice melting, salt dissolving). Chemical changes produce new substances (rusting, burning) and are hard to reverse.
- Density = mass/volume. Common units are g/mL for liquids and solids and g/L for gases. Density is an intensive property, so it does not depend on sample size.
- Metric prefixes: kilo- 10^3, centi- 10^-2, milli- 10^-3, micro- 10^-6. Convert by moving the decimal or using dimensional analysis.
- Significant figures: all nonzero digits count; trapped zeros count; leading zeros do not; trailing zeros count only with a decimal point. Multiplication/division keeps the least number of sig figs; addition/subtraction keeps the least number of decimal places.
- K = °C + 273.15; °F = (°C × 9/5) + 32.
Worked example
A metal sample has mass 12.50 g and volume 4.00 mL. Find its density and decide whether it will sink in water (density 1.00 g/mL). Density = 12.50 g ÷ 4.00 mL = 3.125 g/mL. The volume has 3 sig figs, so report 3.13 g/mL. Because 3.13 g/mL > 1.00 g/mL, the metal is denser than water and sinks. If the question asks for kg/L, convert: 3.13 g/mL × (1 kg/1000 g) × (1000 mL/1 L) = 3.13 kg/L.
Traps that cost marks
Do not call dissolving a chemical change unless a reaction occurs; NaCl dissolving is physical. Do not confuse mass with weight: mass is amount of matter (kg), weight is gravitational force (N). When converting °C to K, do not multiply; just add 273.15. In sig figs, do not round intermediate steps too early. And in density, match the volume unit to the mass unit before dividing.