Module 3: Water (Summary)

1. Water is distributed on Earth as a solid, liquid and gas

Students learn to: • Define the term solute, solvent and solution A solution is a homogeneous mixture in which the dispersed particles are so small (molecules or ions) that they never settle out and cannot be seen by a microscope. Solute is the substance which is dissolved in a solution. Solvent is the substance which dissolves other substances. Reference: textbook page 18

• Identify the importance of water as a solvent Water is an excellent solvent for a wide range of substances, and so it is the medium in which many chemical reactions occur – both natural ones and human-induced ones. In all living matter, most of the chemical reactions responsible for ‘life’ occur in water solutions, or aqueous solutions as they are called. Water dissolves nutrients from soil and carries them to living cells. Water also carries waste products away from cells. Drinks, bleaches, disinfectants, cleaning agents and some medicines are aqueous solutions. Industry uses aqueous solutions of acids and alkalis and often extracts products from aqueous solutions (such as chlorine, common fertilisers, zinc and in the purification of copper). Reference: textbook pages 202, 184, 188

• Compare the state, percentage, and distribution of water in the biosphere, lithosphere, hydrosphere and atmosphere
|Zone Of The Earth |Water Present As |% That Is Water |
|Atmosphere |Vapour or tiny liquid droplets (in clouds) |0.5 – 5% |
|Hydrosphere |Liquid (with other substances dissolved in it) |>95% in oceans; >99% in polar ice caps, lakes and rivers |
|Lithosphere |Ground water and ‘water of crystallization’ in many | O > N ≈ Cl > Br > C ≈ S ≈ I > P ≈ H > Si The C–H bond is regarded as non-polar for practical purposes. A pair of equal and opposite charges separated in space as in the H–O–H molecule is called a dipole. Polar molecules are ones which have a net dipole. To determine whether a molecule is polar: o Diatomic molecules which have a polar covalent bond are polar. o Polyatomic molecules (more than two atoms) with polar bonds can be polar or non-polar, depending on the shape of the molecule, because the two or more dipoles in the one molecule may cancel each other out. o If the central atom has lone pairs (pairs of non-bonded electrons), the molecule is polar. Water has two dipoles (the hydrogen end of the molecule is slightly positive) which do not cancel each other out (because its shape is bent). Therefore it is a polar molecule. Reference: textbook pages 192-193, text box at the top of this section

• Describe the attractive forces between polar molecules as dipole-dipole forces Because polar molecules have positive and negative ends, they are able to line up so that the positive end of one molecule attracts the negative end of another molecule. Therefore electrostatic attraction holds the molecules to one another more strongly than would be the case for non-polar molecules. These electrostatic attractions are called dipole-dipole interactions. Reference: textbook page 193, text box at the top of this section

• Explain the following properties of water in terms of its intermolecular forces: surface tension; viscosity; boiling and melting points Surface tension is a measure of the resistance of a liquid to increasing its surface area. The higher the value, the greater the tendency of the liquid to form a spherical drop rather than spread out as thin film. A molecule well inside a liquid experiences intermolecular attractions from other molecules all around it, whereas a molecule in the surface layer of a liquid experiences intermolecular