Integrated Assignment 06
Conducting solutions
This assignment builds on students’ experience of investigating the variation of resistance with size for solids, by extending the technique to liquids. They should be familiar with determining resistance of solid conductors by measuring voltage and current. They now use a basically similar technique to measure the resistance of water samples, in order to investigate the effect of ionic solutes.
The overall task is to produce a presentation which explains how resistance indicates the purity of water and the concentration of ionic solutes present, who uses such information, and why it is important. If you have a water deioniser with a built-in conductivity meter, this could be shown to students as an example of using resistance / conductivity to monitor water purity.
Students should be encouraged to be thinking about their presentation from the start. This will then encourage them to gather appropriate amounts of information and to begin to organise it. The importance of a balance of conciseness and completeness in the presentation should be stressed.
Students are only required to measure resistance, but need to understand the differences between resistance, resistivity and conductivity. They should appreciate that resistance depends on the resistivity of the particular material and the size of the sample, similar to the way that mass depends on density and size, (though the mathematical relationship is different).
Practical tips
Task 1: Internet search
This section could be usefully reinforced by local examples relating to the occupations limited, or others of local significance. The point about selecting and extracting relevant information should be reinforced. Students may need help in identifying what they should be searching for.
Task 2: Comparing resistivities of water samples
Standard procedure SP 0009:2005 can be used for reference for tasks 2, 3 and 4, although this relates to wires and not liquids.
The resistivities of the water samples differ by a factor of 104. So, therefore will the current between the electrodes. Using a 6V supply and electrodes 5 cm apart, with an immersed area of 10 cm2 (e.g. 5x2 cm), the expected currents are a few microamps for distilled water, around 50 milliamps for 0.2% sodium chloride, and over 2.5 A for 5%. Ammeters and shunts with suitable ranges (or multimeters) are thus required. If multiple shunts for each meter are unavailable, stations for each solution could be set up, each with the appropriate range meter.
The range of samples could be extended to include seawater or river water if available.
It is important that the circuit is switched on only briefly, just to take a reading, since electrolysis produces bubbles on the electrodes, which affects the resistance. This problem is avoided by using a low-voltage AC supply if available. The circuit should still not be left on for a prolonged period, since the heating effect will also alter the resistance.
The difference between the readings may be made clearer by expressing them all in amps. It is not intended that students should calculate actual resistances at this stage.
Task 3: The effect of adding a solute
This task is a preliminary to task 4. The objective is to gauge the size of solute portions to use. In deciding how much solute to try, students should consider their results for distilled water and 0.2% sodium chloride in task 2. This change was produced by dissolving only 0.2 g of salt in 100 cm3 of distilled water.
A change of ammeter range is likely to be needed.
Task 4: Quantitative investigation of the effect
Depending on time, this could be done accurately, by weighing the solute, or semi-quantitatively by simply counting equal-sized ‘portions’. Again, a change of ammeter scale will be needed. All readings should be expressed in amps, so that all resistances are calculated in ohms.
As before, the circuit should be switched on only when taking readings.
Tasks 5 and 6: Investigating the effects of size
It is convenient to perform these at the end of task 4, while the equipment is set up. Students should use their knowledge of the resistance of wires to predict the results.
The investigation can be done qualitatively very quickly, by simply moving the electrodes while watching the meter. However, if time allows, quantitative measurements of separation and immersed area should be taken.
Task 7: Qualitative ionic analysis
The tests in standard procedure SP 0006:2005 are those required for GCSE Applied Science, and may not cover all the ions present in the mineral water. If solids form around the edge during evaporation, they should be redissolved in the concentrated sample. Any solid that will not redissolve (e.g. calcium or magnesium carbonate formed by decomposition of hydrogencarbonate on boiling) should be tested separately.
Students should check their results against the mineral contents declared on the label.
Health and safety note
It is advisable to use ammeters with overload protection, since they are likely to be subjected to currents beyond their nominal range.
Silver nitrate solution stains skin and clothing black. The stain is metallic silver and will not wash out. Protective clothing and disposable gloves should be worn.
Concentrated hydrochloric acid is corrosive – but only a few drops are needed for cleaning the flame test wire.
Apparatus and reagents
- Standard Procedures:
- SP0006:2005 Parts 1,2 and 3 Chemical tests for identifying cations and anions in minerals
- SP0009:2005 Method for measuring the resistance of metal wires
- SP 0010:2005 Method for measuring changes in resistance with size
For task 2
- 6 V battery (or low voltage supply)
- voltmeter capable of measuring 6 V DC
- ammeter(s) with ranges capable of measuring from 20μA to 5A DC
- connecting wires
- switch
- 100 cm3 beaker
- 2 copper electrodes, approx 2 x 6 cm
- distilled or deionised water
- mineral waterapprox. 0.2% sodium chloride solution
- river water and/or sea water (optional)
For tasks 3 and 4
- apparatus as above
- access to balance weighing to 0.01 g
- spatula
- stirring rod
- distilled or deionised water
- sodium chloride or copper(II) sulphate (small crystals)
- or tasks 5 and 6
- apparatus as for task 2
- wider beaker, or trough, to allow larger separation of electrodes
- • cm/mm ruler
For task 7
- rack of 5 test tubes
- 9 droppers (unless reagent bottles have built-in droppers)
- watch glass
- nichrome wire with small loop at one end
- concentrated hydrochloric acid
Caution: corrosive - nitric acid (approx 1 mol dm-3)
Caution: corrosive - limewater
- silver nitrate solution (approx 0.1 mol dm-3)
Caution: stains clothes and skin black - ammonia solution (approx 1 mol dm-3)
- • barium chloride solution (approx 0.1 mol dm-3)
Caution: harmful - sodium hydroxide solution (approx 1 mol dm-3)
Caution: corrosive - potassium thiocyanate solution (approx 0.1 mol dm-3)