Chemistry: Pure Substances and Mixtures

Pure Substances

• Pure substances are composed of only one type of material.
• Their composition is fixed and constant.
• Properties such as melting point, boiling point, and density are fixed and constant.
• Components cannot be separated by physical processes.
• Examples include elements and compounds.
• To identify if a substance is pure, measure its melting or boiling point. Impurities lower the melting point and raise the boiling point.

Elements

• Elements are pure substances that cannot be broken down into simpler substances by ordinary physical or chemical processes.
• Elements are made of only one type of atom.

Compounds

• Compounds are pure substances formed from two or more different elements chemically bonded together in fixed proportions.
• Their properties are different from the properties of the individual elements.

Mixtures

• Mixtures consist of two or more substances (elements and/or compounds) physically combined in variable proportions.
• Each component retains its own properties.
• Composition can vary.
• Properties are variable; they are the average of the substances in the mixture.
• Components can be separated by physical means.
• Homogeneous mixtures are uniform throughout (e.g., solutions).
• Heterogeneous mixtures are not uniform (e.g., suspensions, colloids).

Comparing Compounds and Mixtures

Example: Iron and Sulfur

• Iron and sulfur mixture: mixture of yellow and silver spots, can be separated by magnet, sulfur dissolves in solvent.
• Iron sulfide compound: brown-black, cannot be separated by magnet, does not dissolve.
• Heat is involved in forming a compound, but not in a mixture.
• Different gases are released when compounds, like iron sulfide, react with acid compared to mixtures, like iron and sulfur.