Solutions
The structure of its molecules makes water act as a very good solvent of a large number of substances. Because of the distribution of the four non-bonding electrons, there is a distribution of charge across the molecule, known as a dipole moment. As a result of this, positive and negative ions are easily dissolved into water, as each will be attracted to on end of a water molecule. (See Cyberlab: Dissolving ionic salts.) A good example of the ability of water to act as a solvent is sea water, in which as at least forty elements (including gold and silver) can be found. The average concentration of dissolved salts in sea water is around 35 grams per litre. In the Dead Sea (which is in fact a lake), the constant influx of a salt water and a high rate of evaporation have lifted the concentration to more than double the usual.
A solution has a number of defining characteristic:
- The solution is homogeneous, in that the solvent and solute are so well mixed that the are not visually distinguishable and all parts of the solution have the same concentration.
- The solute will not separate from the solvent if left to stand.
- A solution will be normally be fairly transparent in visible light.
The types of substances that can dissolve in water include polar molecular compounds such as alcohols and ionic compounds such as sodium chloride. The different structures and weights of ionic compounds mean that some are more easily dissolved than others. The solubility of a compound is a measure of how much can be dissolved per weight of water. For example, at 18°C, 100 grams of water can hold 35 grams of NaCl in solution, but less than a milligram of CaC03. At higher temperatures, this saturation point is increased.
Filtration of solutions through an inactive medium (such as filter paper) generally does nothing to separate the solute from a solution, as the individual ions and molecules are far too small to be blocked and pass through the relatively large holes unhindered.
Gases too can be dissolved in water, although in relatively small concentrations. Dissolved oxygen and carbon dioxide are essential for aquatic plants and animals. Interestingly, gases are more soluble at lower temperatures, as the reduced motion of the particles means that bubbles are less likely to form. Heat , as a from of pollution, is detrimental to aquatic ecosystems as the ability of water to sustain this life is reduced as there is less dissolved CO2 and O2 available for photosynthesis and respiration.
Suspensions
A suspension is formed when particles of an insoluble compound are mixed into water and spread throughout. This is deferent to a solution as on a molecular scale the particles in suspension are many thousands of times larger than the water molecules. The particles in a suspension can therefore be more easily separated than a solution.
River water may often contain suspended particles of clay, giving the water a murky appearance. Over time some sediment may settle, but the motion of particles and molecules is enough to keep suspended particles in the water.