Study Notes
The criteria of purity involve understanding how to determine if a substance is pure or impure, often using techniques like chromatography. Pure substances have fixed melting and boiling points, while impure substances do not.
- Pure substance — a single element or compound not mixed with any other substance Example: Pure milk
- Impure substance — a substance that contains more than one element or compound Example: A mixture of salt and sand
- Chromatography — a method used to separate mixtures and identify different substances Example: Separating ink into its component dyes
- Rf value — a ratio used in chromatography to identify substances Example: Distance traveled by substance divided by distance traveled by solvent
- Melting point — the temperature at which a solid becomes a liquid Example: Ice melts at 0°C
- Boiling point — the temperature at which a liquid becomes a gas Example: Water boils at 100°C
Exam Tips
Key Definitions to Remember
- Pure substance: a single element or compound not mixed with any other substance
- Impure substance: a substance that contains more than one element or compound
- Chromatography: a method used to separate mixtures and identify different substances
- Rf value: a ratio used in chromatography to identify substances
Common Confusions
- Confusing pure substances with mixtures
- Misinterpreting the Rf value in chromatography
Typical Exam Questions
- What is a pure substance? A pure substance is a single element or compound not mixed with any other substance.
- How can you determine if a substance is pure? By checking if it has a fixed melting and boiling point.
- What does an Rf value indicate in chromatography? It indicates the ratio of the distance traveled by the substance to the distance traveled by the solvent.
What Examiners Usually Test
- Understanding of the differences between pure and impure substances
- Ability to interpret chromatograms and calculate Rf values
- Knowledge of the significance of melting and boiling points in determining purity