What it shows
Pure water barely conducts, but dissolved salt gives free ions that carry current. The current through a salt solution rises with concentration at first and then levels off as the ions crowd each other.
The science
Current in a liquid is carried by ions, not electrons. Sodium chloride dissolves into Na+ and Cl- ions, and the more ions per litre, the more charge can move between the electrodes each second, so the current rises with concentration. With a fixed cell voltage and a fixed electrode spacing, current is a direct measure of conductance (I/V). At high concentration the ions interfere with each other’s motion, so the rise slows down and the graph bends over.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two carbon rods pulled from dead zinc-carbon cells (an adult must open them) or two graphite pencil leads
- Common salt, a teaspoon and a kitchen weighing scale
- A 250 ml measuring jug and five identical glasses
- Three AA cells in a holder
- A digital multimeter
- Crocodile clip leads and a cardboard strip to hold the rods 3 cm apart
Safety
For this project: Cells only. Do not use the solutions afterwards for anything else. Wash hands after handling the carbon rods and wipe up spills, since salt water corrodes metal.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Make five solutions: 0, 2, 4, 8 and 16 g of salt each dissolved in 200 ml of water, and label the glasses.
Fix the two carbon rods through holes in the cardboard strip exactly 3 cm apart so the spacing stays the same for every reading.
Connect the cells, the rods and the meter on milliamps in series, and dip the rods to the same depth mark in the plain water first.
Record the current, rinse the rods in plain water, dry them, then repeat for each solution from the weakest to the strongest.
Record a table with columns for mass of salt, concentration in g per litre, current in milliamps, and I/V in millisiemens.
Plot current against concentration and describe the shape of the curve.
As a check, test tap water, mineral water and a sugar solution and explain why sugar water gives almost no current.
Look for tiny bubbles at the rods and explain that the current also causes a small amount of electrolysis.
What you should see
Plain water gives almost zero current, the current climbs steeply up to about 20 g per litre and then flattens. Sugar water conducts almost nothing, showing that only ionic solutes carry current.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does sugar solution not conduct?
Sugar dissolves as neutral molecules, not ions, so there are no charge carriers.
2What carries the current in a salt solution?
Sodium and chloride ions moving in opposite directions.
3Why keep the rods at a fixed spacing and depth?
Resistance depends on the length and area of the liquid path, so these must stay constant for a fair test.
4What is the unit of conductance?
Siemens, the reciprocal of ohm.
Take it further
Repeat at 10, 25 and 40 degrees Celsius for one concentration to show that conductivity of a solution rises with temperature.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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