2.7-9 Flashcards
Sources of water
In the UK, there are a number of sources of water which can be purified provide us with potable water (water that is fit to drink). The main sources of water in the UK are:
* Surface water: from lakes, rivers, and reservoirs. In much of England and Wales, these sources start to run dry during the summer months.
* Ground water: from aquifers (rocks that trap water underground). In parts of south-east England, where surface water is very limited, as much as 70% of the domestic water supply comes from ground water.
* Wastewater: from water that’s been contaminated by a human process, e.g., as a by-product from some industrial processes. Treating wastewater is often better for the environment than simply disposing of it, as wastewater can contain pollutants. How easy wastewater is to treat depends on the levels of contaminants in it.
Water purification
Water from some sources is dirtier than from others, and so requires a lot more treatment to make it drinkable. Ground water from aquifers is normally quite clean, but wastewater and surface water need far more treatment.
The process of purification
No matter the source, all water must go through a basic process of purification to make it potable. This process includes:
1. Filtration - a wire mesh screens out large twigs, etc., and then gravel and sand beds filter out any solid bits.
2. Sedimentation iron sulphate or aluminium sulphate is added to the water, which makes fine particles clump together and settle at the bottom.
3. Chlorination - chlorine gas is bubbled through to kill harmful bacteria and other microbes.
Distilling sea water
In some dry countries, e.g., Kuwait, there’s not enough surface water or ground water, so instead sea water must be distilled to provide potable water. The sea water is boiled to produce steam, which is then condensed - this separates the water from the dissolved salts.
Distillation requires a lot of energy, so it’s really expensive, especially if it’s used to produce large quantities of fresh water. Countries don’t tend to use this if there are other sources of water available.
Deionised water
A lot of analytical chemistry involves carrying out experiments to work out what something is, or how a substance will react. For experiments that involve mixing or dissolving something in water, you should use something called deionised water.
Deionised water is water that has had the ions (such as calcium, iron, and copper ions) that are present in normal tap water removed. These ions, although present in small amounts and harmless in tap water, can interfere with reactions. So, using normal water could give your experiment a false result.
What is Chromatography
Chromatography is a method used to separate a mixture of soluble solids and identify them.
mobile phase
a liquid solvent , where the molecules can move (e.g. water or ethanol)
stationary phase
chromotography paper - it is where the molecules can’t move
chromatography method
use of paper chromatography to separate different dyes in an ink:
- Draw a line near the bottom of a sheet of filter paper. This is known as the baseline. (Use a pencil to do this-pencil marks are insoluble and won’t move with the solvent.)
- Spot the ink by placing a small amount on the line in a single place. Then put the sheet upright in a beaker of solvent, e.g. water. The solvent used depends on what’s being tested. Some compounds dissolve well in water, but sometimes other solvents, like ethanol, are needed. Make sure the ink isn’t touching the solvent initially - you don’t want it to be washed away.
- Place a watch glass on top of the beaker to stop the solvent evaporating.
- The solvent will start to move up the paper. When the chemicals in the ink dissolve in the solvent, they will move up the paper too.
- The different dyes in the ink will move up the paper at different rates, so the dyes will separate out and form spots in different places. If any of the dyes in the ink are insoluble (won’t dissolve) in the solvent you’ve used, they’ll stay on the baseline.
- When the solvent front has nearly reached the top of the paper, take the paper out of the beaker, draw a line with a pencil along the solvent front and leave to dry.
- The end result is a pattern of spots called a chromatogram.
solvent front
The point the solvent has reached as it moves up the paper is known as the solvent front.
factors affecting the distance a compound moves
- How soluble they are in the solvent.
- How attracted they are to the paper.
Molecules with a higher solubility in the solvent, and which are less attracted to the paper, will spend more time in the mobile phase- and they’ll be carried further up the paper.
chromatogram
The result of a chromatography experiment is called a chromatogram
Rf values
Each substance on a chromatogram has an Rf value.
The further through the stationary phase a substance moves, the larger the Rf value. You can calculate Rf values using the formula:
Rf values - e.g.
Identifying a substance using a reference
Paper chromatography is often carried out to see if a certain substance is present in a mixture. To do this, you run a pure sample of that substance (a reference) alongside the unknown mixture. If the R values of the reference and one of the spots in the mixture match, the substance may be present.
If the R, values match in one solvent, you can check to see if the chemicals are the same by repeating with a different solvent. If they match again, there’s a greater chance that they’re the same.
example of Identifying a substance using a reference
Chromatography was carried out on a reference dye and an ink at the same time. The reference dye is the substance on the left, the ink is the substance on the right. As the R, values of B and D are the same, you can suggest that the reference dye may be in the ink. However, this isn’t proof that B and D are the same substance, as other dyes may have the same R, value in this solvent.
