AQA GCSE Chemistry

Revision Notes

2.2.5 Properties of Small Molecules

Melting & Boiling Points: Small Molecules

  • Small molecules are compounds made up of molecules that contain just a few atoms covalently bonded together
  • They have low melting and boiling points
  • Most covalent compounds are insoluble in water as they tend to be non-polar but can dissolve in organic solvents
  • Some do dissolve in water by forming intermolecular attractions with the water molecules
    • Examples include sucrose (table sugar C12H22O11) and iodine I2
  • As the molecules increase in size, the melting and boiling points generally increase

Exam Tip

The atoms within covalent molecules are held together by covalent bonds while the molecules in a covalent substance are attracted to each other by intermolecular forces.

Electrical Conductivity: Small Molecules

  • They are poor conductors of electricity as there are no free ions or electrons to carry the charge.
  • Most covalent compounds do not conduct at all in the solid state and are thus insulators
  • Common insulators include the plastic coating around household electrical wiring, rubber and wood
  • Electrical wire insulator, IGCSE & GCSE Chemistry revision notes

The plastic coating around electrical wires is made from covalent molecules that do not allow a flow of charge

Intermolecular Forces Vs. Covalent Bonds

  • Small molecules have covalent bonds joining the atoms together, but intermolecular forces that act between neighbouring molecules
  • They have low melting and boiling points as there are only weak intermolecular forces acting between the molecules
  • These forces are very weak when compared to the covalent bonds and so most small molecules are either gases or liquids at room temperature
    • Often the liquids are volatile
  • As the molecules increase in size the intermolecular forces also increase as there are more electrons available
  • This causes the melting and boiling points to increase

Intermolecular Forces Vs Covalent Bonds, downloadable IGCSE & GCSE Chemistry revision notes

The bonds between hydrogen and oxygen in water are COVALENT, and the attractions between the molecules are INTERMOLECULAR FORES which are about one tenth as strong as covalent bonds

 

Exam Tip

When a covalent molecule melts or boils the covalent bonds do not break, only the intermolecular forces. If you think about it, when you boil a kettle full of water you are not generating large volumes of hydrogen and oxygen gas in your kitchen – this might give you an interesting unwanted chemical reaction ! Boom !

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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