Substances and their properties (aine ja selle omadused)
Domain 1 of the Estonian basic-school chemistry exam - aine ja selle omadused - tests whether you can describe a substance by measurable properties and then use those properties to identify an unknown sample, choose a separation method, or justify a safety rule. The tasks are practical: a table of melting points, a described observation, or an apparatus list.
Physical versus chemical properties
- Physical properties - colour, smell, density, melting and boiling point, hardness, conductivity, solubility - are measured without changing the substance's identity.
- Chemical properties describe how a substance reacts: burning, rusting, reaction with oxygen or with acids.
- Density ρ = m/V. Use g/cm³ for solids and liquids, g/dm³ (identical to g/L) for gases; water is 1.00 g/cm³ at 4 °C.
- A pure substance melts and boils at a fixed temperature; impurities lower the melting point and widen the melting range, so a range signals a mixture.
Mixtures and separation
- In a mixture the components keep their own properties; in a compound the elements no longer do.
- Filtration removes an insoluble solid; evaporation or crystallisation recovers a dissolved solid; distillation separates liquids with different boiling points (ethanol–water, crude oil fractions); a separating funnel separates two immiscible liquids.
Worked example
A white solid does not conduct electricity when dry, its aqueous solution conducts, and it melts sharply at 801 °C. What is it and which property settles the question? Sharp melting point → pure substance. No dry conductivity rules out a metal. Conductivity only after dissolving is the signature of an ionic solid: ions are locked in the lattice and become mobile in water. Answer: an ionic compound such as sodium chloride; the decisive evidence is conduction by the solution, not the melting point alone.
Traps that cost marks
- Reading a melting range as a fixed point - a range means impure.
- Using the wrong density unit, e.g. g/cm³ for a gas.
- Claiming filtration separates a dissolved salt.
- Answering with a chemical property (burns, reacts with acid) when physical properties were asked.
- Forgetting that conductivity needs mobile charge carriers: electrons in metals, ions in solution or melt.