What it shows
Water conducts electricity only through dissolved ions. Two probes in a liquid feed a tiny current into a transistor, which amplifies it to light an LED. Distilled water gives nothing, tap water a faint glow and salt water a bright one.
The science
Pure water has very few charged particles, so it hardly conducts. Dissolving salt splits it into sodium and chloride ions, which drift towards the probes and carry the current; sugar dissolves but does not form ions, so sugar water behaves like pure water. The probe current flows through the 10 kΩ resistor into the base of the BC547, which multiplies it by about 200 in the collector circuit holding the LED. The 470 Ω resistor keeps the LED under 20 mA when the transistor is fully on.
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.
- One BC547 transistor
- A 470 Ω resistor and a 10 kΩ resistor
- One red LED
- A 9 V battery with a snap connector
- Two probe wires with 1 cm bare ends taped 1 cm apart on a stick
- Four cups: distilled or rain water, tap water, sugar water and salt water
- A breadboard and jumper wires
Safety
For this project: Battery power only. Dry your hands before handling the circuit, and rinse the probes between cups.
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.
Place the BC547 on the breadboard and connect its emitter to the ground rail with the battery negative.
Connect the LED short leg to the collector and the LED long leg through the 470 Ω resistor to the 9 V rail.
Connect one probe wire to the 9 V rail and the other probe wire through the 10 kΩ resistor to the base.
Snap on the battery and check the LED is off while the probes are in the air.
Dip the probes in distilled or rain water: the LED stays off or just flickers.
Dip them in tap water: a faint glow. Dip them in sugar water: little or nothing.
Dip them in salt water: a bright glow. Add more salt and watch it brighten slightly.
Write a table of each liquid and the brightness, then rank them from best to worst conductor.
What you should see
The LED brightness ranks the liquids: salt water brightest, tap water faint, sugar water and distilled water dark.
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 water not light the LED?
Sugar dissolves as whole molecules, not ions, so there is nothing to carry the charge.
2What role does the transistor play?
It amplifies the tiny probe current into a larger current that can light the LED.
3Why keep the probe spacing the same for every test?
The resistance of the liquid depends on the spacing, so a fair test needs it fixed.
4Why does the 470 Ω resistor go in the collector circuit?
It limits the LED current when the transistor is fully switched on.
Take it further
Add a multimeter in the base lead and record microamperes for each liquid instead of judging brightness by eye.
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.
Free ebooks for this project
Hardware & repair
Using a Digital Multimeter
Measure voltage, current, resistance and continuity safely