There are numerous chemicals that are used in science practical work but which only get the briefest of mentions. I thought it might be useful to make a list of some here so that you can learn what they all do.
Solvents
We use various solvents (to dissolve things) with water being the most obvious. Sometimes things are not water-soluble, so then we need to use an organic solvent of some sort. The most obvious is ethanol, which is the alcohol found in beer and wine. We often use it in the form of methylated spirits, which has had methanol added to make it toxic, discouraging people from drinking it. We also use surgical spirit (or rubbing alcohol) which can be made from ethanol or isopropyl alcohol. In either case it has things added to it to make it poisonous and unpleasant to drink.
This follows the same naming convention as the alkanes (methane, ethane and propane). Methanol has one carbon atom and the OH group is what makes it an alcohol. Ethanol has two carbon atoms and an OH group. Propanol has three carbon atoms and the OH group is attached to the second carbon so it is propan-2-ol. If it was on the end it would just be propanol. These alcohols share the property of being highly flammable and useful for dissolving things that won’t easily dissolve in water.
Acetone/propanone
Another common solvent is called propanone (it used to be called acetone), which is nail varnish remover. My colleagues often come and ask for some when they have written on their whiteboards with permanent marker pens – something that happens remarkably often!
Propanone is closely related to propanol – it is a ketone rather than an alcohol. It has three carbon atoms and the central one is linked to an oxygen atom with a double bond.
It has quite a strong smell and is highly volatile – meaning that it has a low boiling point (56°C). It is also highly flammable.
Indicators
Universal Indicator is a solution of various indicators in ethanol. Each one changes colour within a different pH range and it is the combination of colours that produces the familiar spectrum of colours that UI provides. Litmus indicator is made from various ground up lichens found in the Netherlands. It is pink in an acid and blue in an alkali. It is one of the oldest and simplest indicators. A lichen is an interesting organism that is really a partnership between an alga (a primitive plant) and a fungus. The fungus provides a protective home for the alga and the alga photosynthesises to feed the fungus.
Food Tests
Tincture of iodine is an orange/red solution of the non-metal element iodine dissolved in ethanol. It reacts with starch to produce a blue-black colour. We also sometimes use iodine solution as a stain for simple plant cells, such as onion cells, adding a little colour for contrast instead of the original colourless cells.
Glucose (which is known as a reducing sugar) can be identified by using something called Benedict’s reagent. This is a blue solution containing copper sulfate and other substances. When it is heated with glucose it turns a brick red colour. Other carbohydrates do not cause this colour change.
Biuret reagent can identify protein. Biuret is also blue and again contains copper sulfate and also sodium hydroxide, amongst other things. It turns a purple colour when it is mixed with a solution containing protein.
Stains for Microscope Slides
As mentioned above, iodine solution can be used as a very simple stain for colourless plant cells like onion cells. A slightly more sophisticated stain is methylene blue which is particularly good at binding to the nucleus, turning them a strong blue colour. The classic example of this is when observing human cheek cells with a microscope.
Questions…
- What two products would be produced if isopropyl alcohol were burned in plenty of air?
- What is meant by the word organic in the second paragraph?
- What are the raw materials for photosynthesis?
- Starch is made from a long chain of what chemical?
- Suggest a likely pH for Biuret reagent.
- What is contained within the nucleus of a human cheek cell?
- In Benedict’s reagent, what is likely to be the solvent?
- Name one solute from Benedict’s reagent.
Comments