Compounds and Mixtures

A compound is a substance made up of two or more elements that are chemically combined. It is formed through a chemical change and cannot be separated by physical means.

Examples of Compounds and Their Elements

Compound Constituent Elements
Water Hydrogen, Oxygen
Sand Silicon, Oxygen
Limestone Calcium, Carbon, Oxygen
Common Salt Sodium, Chlorine
Ethanol Carbon, Hydrogen, Oxygen

Properties of a Compound

  1. It has different properties from its component elements.
  2. Its formation usually involves a large amount of heat.
  3. It cannot be separated by physical methods.
  4. Its elements are combined in a fixed ratio by mass.

Formulae of Compounds

When an element exists as a molecule, the number of atoms is written as a subscript after its symbol (e.g., H2, O2).

A compound's formula shows the number of atoms of each element it contains, using whole numbers.

Steps for Writing Molecular Formulas

  1. Write the symbols of all the elements in the compound close together.
  2. Add a subscript to indicate the number of atoms of each element.

Examples of Compound Formulae

Compound Formula
Hydrochloric Acid HCl
Water H2O
Ammonia NH3
Carbon(IV) Oxide CO2
Lead(II) Chloride PbCl2
Calcium Trioxonitrate(V) Ca(NO3)2

Writing Formulas from Valencies

To derive the formula of a compound from the valencies of its elements or radicals, follow these steps:

  1. Write the symbols of the elements or radicals.
  2. Write their valencies below the symbols.
  3. Exchange the valencies between the elements.
  4. Write the new formula, combining the symbols and adjusted subscripts.

Mixtures

A mixture contains two or more substances physically combined. The components can be separated by physical methods and are not in fixed proportions.

Examples of Mixtures and Their Constituents

  1. Air: Oxygen, Carbon(IV) oxide, Nitrogen, Rare gases, Dust, Moisture
  2. Soil: Sand, Clay, Humus, Water, Air, Mineral salts
  3. Urine: Urea, Water, Mineral salts
  4. Coca-Cola: Water, Sugar, Cola flavor, CO2
  5. Milk: Water, Sugar, Fat, Protein, Mineral salts, Vitamins

Differences Between Mixtures and Compounds

Property Mixture Compound
Appearance Can be homogeneous or heterogeneous Always homogeneous
Separation Can be separated by physical methods Cannot be separated by physical methods
Composition Components can be in any proportion Components are in a fixed ratio by mass
Chemical Formula Cannot be represented by a chemical formula Can be represented by a chemical formula
Properties Properties are the sum of those of the components Properties differ from those of the components