Iso, Iso, Iso…

isobars

A recent Winchester paper had a question about isomers. It has made me very aware of the prefix iso-. The Greek word isos means equal – according to Wiktionary. In common usage it means the same so isotonic means “the same concentration”; isobars are regions of equal pressure; isomers have the same chemical formulae; and isotopes have the same number of protons.

If you have read this post on osmosis you will know that solutions of different concentrations, separated by a semi-permeable membrane, force water molecules to pass through the membrane until the concentrations either side are equal. This is the principle behind preserving fruit in syrup (jam); the high concentration of the sugar solution draws the water across the cell membrane of any bacteria that might try to spoil the fruit. The sugar solution (syrup) is hypertonic relative to the bacteria (higher concentration), or you could say that the fluids inside the bacteria are hypotonic relative to the syrup. Hyper means more or over whilst hypo means less or under. Water moves from the more dilute solution, across the semi-permeable membrane to the more concentrated side until the two solutions are isotonic, or the same concentration.

Weather maps have lines representing regions of equal air pressure called isobars. You may remember that the device for measuring air pressure is called a barometer – from baros, the Greek for weight. Changes in air pressure are indications of a change in the weather. A rapid drop in pressure, known as a low pressure system, often results in rain. When the air pressure increases rapidly you should get blue skies. Normal air pressure (at sea level) is 1 atmosphere (atm) which is equivalent to 101,325 Pascal (Pa), or 1013.25 millibar or hectopascal. The numbers on weather maps are in millibar or hectopascal. The pressure unit Pascal is one Newton per m2 (N/m2). In school we normally use N/cm2 as our unit of pressure, and since there are 10,000 cm2 in one m2, then we can say that normal atmospheric pressure is 10.1325 N/cm2 or about 10 N/cm2 keeping it simple.

Isomers are molecules that have the same chemical formula but a different arrangement of atoms. The different arrangement sometimes causes them to have slightly different properties. A good example of a pair of isomers can be found in the molecule propanol. Propanol is a colourless organic solvent that burns very rapidly in air, so we use it for the bottle rockets. Its chemical formula is C3H7OH – the OH at the end showing you that it belongs to the group of molecules called alcohols, which includes ethanol (C2H5OH).  Unlike ethanol though, propanol can arrange itself in two different ways.

also known as n-propyl alcohol
also known as isopropyl alcohol

Each of these isomers is a volatile, colourless liquid. Interestingly, propan-1-ol has a boiling point of 97.5 °C whilst propan-2-ol boils at 82.6 ºC. The tiny differences in the ways that the molecules interact with each other result in this variation. We use the more volatile isomer for the whoosh bottle demonstration and it is excellent in bottle rockets. Isopropyl alcohol is what we use for cleaning the electrical connectors in Train Club

I have also written about isotopes here.

Questions…

  1. Apart from a cell membrane, what other cellular features would you expect a bacterium to have?
  2. How many cm3 are there in one m3?
  3. What products would you expect to form when propanol burns in plenty of air?
  4. What does ‘volatile’ mean?
  5. Butanol has four carbon atoms. What would its chemical formula be?
  6. How many isomers of butanol could there be?