Integrated Assignment 06
Conducting solutions
Like solids, some liquids conduct electricity better than others. Measurements of how well a liquid conducts electricity are used by a number of people who carry out analyses as part of their work:
- public analysts, testing the purity of water;
- environmental monitoring officers, testing the purity of water;
- oceanographers studying seawater;
- building surveyors studying dampness in walls and floors;
- archaeologists looking for underground remains of buildings;
- doctors testing blood or urine samples.
You can tell how well something conducts electricity by measuring its resistance. If it conducts well, it has low resistance. If it doesn’t, it has high resistance.
Resistance depends on the material it’s made from – its resistivity. It also depends on:
- for a solid, its cross-sectional area and length;
- for a liquid, the area of the two electrodes and the distance between them.
If
you keep the shape and size of the specimens the same, you can compare
the resistivity of different solids simply by measuring their resistances.
So, if you keep the size and separation of the electrodes the same,
you can also compare the resistivity of different liquids by measuring
resistances.
Your task is to explain why measuring resistance provides information about the purity of a sample of water. You should work in a pair to make a presentation.
What you have to do
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Carry out an internet search, using sites from the UK only. Obtain information about how people at work use resistivity measurements. You could choose one or more of the professions listed above.
Make a very brief report on what you find out. Your report should simply specify what jobs you have explored and what they use the measurements for.
Never just copy information from the Internet. Select only the information that you need.
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Your teacher will give you four labelled samples of water. They are:
- distilled water (or deionised water);
- mineral water (bottled water);
- salt solution (0.2%);
- tap water.
Compare the resistivities of these different water samples by measuring their resistances.
Use the same
circuit to test each of the four samples. For each sample, close the switch only for just as long as
you need to make a measurement. (If left ‘on’, electrolysis decomposes the liquid and alters the
resistance.) Your ammeter needs to have a variable range, to measure from a few microamps up to 200 mA.Fill in the information in a table like the one below. Remember that small current means high resistance and large resistivity (a poor conductor). Use descriptions such as low, moderate, and relatively high.
Sample type Resistivity description How would the information that you have gathered be useful to either an oceanographer or a public analyst?
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Ions are charged particles in solutions that allow electric current to flow. They are too small to see. But there are ways to identify different kinds of ion. Obtain a copy of Standard Procedure SP 0006:2005. Use it to find out the names of some of the ions in a bottle of mineral water. This has a low concentration of ions. The test will be clearer if you increase the concentration. You can do this by boiling away most (about 80%) of the water, in a clean evaporating basin.
Use SP 0006-1:2005 to identify anions. (They are ions with negative charge.) Use SP 0006-2:2005 to carry out a flame test for cations (which are ions with positive charge). You will probably find that one particular kind of ion produces a strong effect. It will hide the colours of other cations, even if they are present. Use SP 0006-3:2005 to identify cations.
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To make your full investigation, start with some new distilled water. Add one portion of solute and dissolve it completely before taking readings of the microammeter and voltmeter. Add further equal-sized portions (about 0.05 g in 100 cm3 water), taking readings after each addition. Use a table like the one below to record your measurements
Note: If the current is too large to measure in microamps then use milliamps instead.
Amount of solute added* Microammeter reading Current I in amps Voltage V in volts Resistance V ÷ I Plot a graph of resistance (on the y axis) against amount of solute added (on the x axis).
Your graph shows how concentration affects resistance (and resistivity).
Notes:
* You will need to write in a unit here. The amount of solute is related to the concentration of the solution. You could measure the mass of solute using a balance, measuring in grams. For a less exact investigation, you could simply add the solute using a spatula. If you add the same amount each time then you could measure the amount of solute in ‘spatula loads’.
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Obtain a copy of Standard Procedure SP 0010:2005. Use it to investigate whether changing the distance between electrodes has the same effect as changing the length of a wire.
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Use SP 0010:2005 to investigate whether the depth of immersion of the electrodes has the same effect as increasing the diameter of a wire.
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Ions are charged particles in solutions that allow electric current to flow. They are too small to see. But there are ways to identify different kinds of ion. Obtain a copy of Standard Procedure SP 0006:2005. Use it to find out the names of some of the ions in a bottle of mineral water. This has a low concentration of ions. The test will be clearer if you increase the concentration. You can do this by boiling away most (about 80%) of the water, in a clean evaporating basin.
Use SP 0006:2005-Part 1 to identify anions. (They are ions with negative charge.) Use Part 2 to carry out a flame test for cations (which are ions with positive charge). You will probably find that one particular kind of ion produces a strong effect. It will hide the colours of other cations, even if they are present. Use Part 3 to identify cations.
What you need to think about
Seawater and sweat are salty, and they are much better than distilled water at conducting electricity. Some lie detectors work by using the change in resistivity of skin when a person sweats nervously when they lie.
When you increase the concentration of a solution you increase the number of ions in it. In the solution, it is the movement of ions that makes an electric current. The more ions there are, the more electric current there can be.
Presenting your conclusions
Prepare your presentation. This could be a poster or a PowerPoint sequence.
The presentation should include:
- an example of use of resistivity measurement by a person at work;
- what resistivity measurements indicate about the purity of a water sample;
- an explanation of why resistivity decreases when a solution becomes more concentrated;
- an explanation of why you must keep the electrodes the same distance apart when you are comparing the resistivities of solutions;
- an explanation of why you must keep the electrodes immersed to the same depth when you are comparing the resistivities of solutions;
- simple illustrations of some different kinds of ions.
A good presentation will:
- have a title to explain what it is about;
- not be too long;
- not have too many words;
- explain the main ideas in pictures as well as words.