Lesson 5: Pure Substances and Mixtures

Pure Substances vs. Mixtures

• Distilled water is a pure substance, consisting of only hydrogen and oxygen; it contains no impurities like salts or silt.
• Rainwater, while derived from pure water vapor, is a mixture due to tiny dust particles it collects as it falls.

Mixtures

• When two compounds are mechanically combined, such as by stirring, they form a mixture rather than a pure substance.
• Sand and water can be separated by allowing the sand to settle and then draining the water, or by filtering the mixture.

Chemical Changes

• Common examples include burning wood, rusting iron, and dissolving metals in strong acids, indicating transformation of substances.

Separation Processes

• Electrolysis is a method that separates water into hydrogen and oxygen using electric current.
• Mixtures can also be physically separated by methods such as straining, filtering, magnetizing, and allowing components to settle.

Homogeneous and Heterogeneous Mixtures

• Homogeneous mixtures are uniform throughout; examples are iced tea, soda without bubbles, saltwater, coffee, and vinegar.
• Heterogeneous mixtures have visible components; examples include salad dressing, vegetable soup, and pizza.
• Colloids, although heterogeneous, do not allow their particles to settle out.

Colloids

• Examples of colloids containing a gas and liquid are fog and mist; aerosols like hairspray contain chemicals and a gas propellant.
• Colloids containing a gas and solid include smoke, smog, and air pollution, which consist of small particles suspended in air.

Definitions and Characteristics

• A pure substance is defined as an element or compound that cannot be separated by physical changes.
• Powdered drink mixes and saltwater are considered mixtures because they do not consist of a single element or compound.
• A compound is formed when atoms or ions of different elements are chemically bonded.
• Hydrogen and salt are pure substances: hydrogen as an element and salt as a chemically bonded compound, separable only through chemical processes.