Purity, Formulations, Chromatography and Identification of Common Gases
A pure substance is either a single element, or compound, that is not mixed with any other substance.
In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, e.g. pure milk.
A mixture consists of two or more elements/compounds, that are not chemically combined together and can be separated by physical processes.
Pure substances will melt and boil at specific temperatures, so we can use the temperature that a substance melts/boils at to tell how pure it is. Impure substances will melt over a range of
temperatures. If a substance is impure it will generally also have a lower melting point and a higher boiling point.
A formulation is a mixture that has been designed as a useful product. Many products are complex mixtures in which each chemical has a particular purpose. Formulations are made by mixing the components in carefully
measured quantities to ensure that the product has the required properties. Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
It is important that formulations keep the same ingredients, so that the formulation is always produced the same way. People buy these products for continuity, especially paints - it would be pointless buying the same
shade of paint that was actually different colours.
Fertilisers are formulated so that they add nutrients to the soil for different purposes. NPK fertilisers contain the following chemicals:
- nitrogen (N) is added for plant growth
- phosphorus (P) is added for root growth
- potassium (K) is added to help withstand drought and disease
Chromatography is a method for separating dissolved substances from one another. It works because some of the coloured substances dissolve in the solvent better than others, so they travel further
up the paper. This technique is often used to separate out inks, or food colourings.
Paper chromatography consists of two stages:
- the stationary phase (a.k.a. the phase that doesn't move - this is the paper)
- the mobile phase (a.k.a. the phase that moves - this is the solvent)
Separation by chromatography produces a chromatogram, and can be used to distinguish between a pure substance (that produces one spot), and a mixture (impure) which produces multiple spots. Different substances
will move at different rates through the paper.
The Rf value of a spot can be used to identify unknown chemicals if they can be compared to a range of reference substances. The Rf value will always be the same for each substance (when the same solvent is used). It
can be calculated by using:
Rf value = distance moved by substance ÷ distance moved by solvent
Different compounds have different Rf values in different solvents, which can be used to help identify the compounds. The compounds in a mixture may separate into different spots depending on the solvent, but a pure
compound will produce a single spot in all solvents.
When setting up the experiment you must make sure to draw the baseline in pencil, otherwise the ink from a pen would also dissolve in the solvent.
Test for Gases
The test for hydrogen uses a burning splint held at the open end of a test tube of the gas. Hydrogen burns rapidly with a pop sound.
This test requires you to allow a second, or two, for oxygen gas from the air to mix with the hydrogen first, as the test is actually showing the chemical reaction between hydrogen and oxygen to make water.
The test for carbon dioxide uses an aqueous solution of calcium hydroxide (limewater). When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy).
This test is actually a reaction between the carbon dioxide and limewater, to make limestone (calcium carbonate).
The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.
The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white.