Elements & Compounds

Pure Substances

• Definition: A pure substance has a fixed composition and fixed properties; its constituent parts cannot be separated by physical methods.
• Example: Water ($H_{2}O$) is a pure substance.

Types of Pure Substances

1. Elements

   • Definition: An element is the simplest form of matter that can exist; it cannot be broken down into a simpler substance.
   • Smallest Unit: The smallest particle of an element that retains its properties is called an atom.
   • Elements Overview:
   • Over $100$ known elements exist.
   • Majority occur naturally; some are synthetic, created in labs.
   • All known elements are listed on The Periodic Table.
   • Metaphor: Think of elements as the Lego blocks of the natural world; everything is made up of these blocks.

2. Compounds

   • Definition: A compound is formed by the chemical combination of atoms of different elements in fixed ratios.
   • Example: Water ($H_{2}O$) is a compound made from two hydrogen atoms and one oxygen atom.
   • Separation: Elements in a compound cannot be separated by physical methods.
   • Property Differences: Compounds often have properties that differ from those of their constituent elements.
   • Example: Hydrogen and oxygen are both gases at room temperature, while water is a liquid.

Mixtures

• Definition: A mixture is a physical combination of two or more pure substances and can be in any ratio.
• The individual components in a mixture retain their unique properties.
• Can be separated using physical methods.
• Example: A mixture of salt and water can be separated by physical means (evaporation, filtration).

Classification of Mixtures

1. Homogeneous Mixtures

   • Definition: Uniform composition and properties throughout; individual components cannot be distinguished.
   • Examples:
   • Solutions (like salt dissolved in water).
   • Air (nitrogen as the solvent, with oxygen and carbon dioxide as solutes).

2. Heterogeneous Mixtures

   • Definition: Non-uniform composition where individual components can be distinguished.
   • Examples:
   • Suspensions (sand in water or dust in air).
   • Colloids (milk, fog).

Solutions

• Definition: Solutions are homogeneous mixtures consisting of a solvent and one or more solutes dissolved within it.
• Solvents can be solid, liquid, or gas.
• Examples:
  • Solid solute in liquid solvent: Salt dissolved in water.
  • Liquid solute in liquid solvent: Ethanol in water.
  • Gas solute in liquid solvent: Carbonated water (carbon dioxide in water).
  • Air (nitrogen and minor gas components).
  • Solid solutions (alloys) e.g., brass (copper and zinc).

Suspensions

• Definition: Heterogeneous mixtures where visible particles are dispersed in another substance (usually a liquid).
• Characteristic: The dispersed particles are large enough to be seen and can settle down upon standing.
• Examples: Sand in water, chalk dust in water.

Colloids

• Definition: Heterogeneous mixtures where the dispersed particles are smaller than those in suspensions but larger than dissolved particles in solutions.
• Characteristics: Particles do not settle upon standing; too small to see under a microscope.
• Examples:
  • Smoke: Solid particles in gas.
  • Fog: Liquid particles in gas.
  • Milk and Mayonnaise: Liquid particles in liquid (emulsions).
  • Gelatin and Jelly: Solid particles in liquid (gels).

Comparison of Mixtures