How to Identify a Compound Experimentally Using Tests

To experimentally prove that a substance is a compound, you must demonstrate two main things: (1) it has a fixed, definite composition, and (2) it cannot be separated by physical means because its components are chemically bonded.

1. Test for Fixed Melting/Boiling Point
   • Procedure: Heat the substance and measure its melting point (for solids) or boiling point (for liquids) using a thermometer.
   • Result: A pure compound will melt or boil at a single, sharp, and specific temperature (e.g., pure water boils at 100°C at 1 atm). A mixture will typically melt or boil over a range of temperatures.
2. Attempt Physical Separation
   • Procedure: Try to separate the substance using physical methods such as filtration, distillation, magnetism, or chromatography.
   • Result: A mixturecan be separated by one or more of these methods. A compoundcannot; its components are chemically bonded and will not separate by physical means.
3. Test for Chemical Decomposition (e.g., Electrolysis)
   • Procedure: Apply a strong energy source, such as electricity (electrolysis) or intense heat (thermal decomposition), to try and break the chemical bonds.
   • Result: A compoundwill break down (decompose) into its simpler constituent elements or simpler compounds. A mixturecannot be decomposed this way; it only separates physically.
   • Classic Example: The electrolysis of water (a compound) breaks it down into hydrogen gas and oxygen gas, proving it was composed of hydrogen and oxygen.
4. Check for Fixed Proportions (Composition Analysis)
   • Procedure: Decompose several samples of the substance (from different sources) and measure the mass of each element produced.
   • Result: A compound will always yield the same elements in the same mass ratio (this is the Law of Definite Proportions). A mixture, however, can vary in composition from sample to sample.