What it shows
A weighted straw floats lower in a lighter liquid and higher in a denser one. Marking it in liquids of known density turns it into a hydrometer.
The science
A floating body displaces its own weight of liquid. If the hydrometer has mass m and cross-section area A, and sinks to a depth h in a liquid of density ρ, then m = ρ A h, so h = m / (ρ A). The depth is therefore inversely proportional to the density: a lighter liquid lets the straw sink deeper. By measuring h in liquids of known density you can draw a scale, and the scale is closer together at the high-density end.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A plastic drinking straw about 20 cm long, sealed at the bottom with clay
- Lead shot, sand or small nuts as ballast
- Salt and a kitchen scale
- A tall glass or measuring cylinder
- A ruler and a waterproof marker
- Water, milk and cooking oil to test
Safety
For this project: An adult should supervise if sharp objects are used to cut the straw. Do not drink anything used in the experiment.
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
7 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.
Seal the lower end of the straw with clay and add ballast through the top until the straw floats upright with about 4 cm above the surface of water.
Float it in plain water and mark the water level on the straw as the 1.00 g/mL point.
Dissolve known amounts of salt in 200 mL of water to make solutions of density about 1.05, 1.10 and 1.15 g/mL; find each density by weighing a known volume.
Float the straw in each solution and mark the level for every density, labelling the marks.
Plot density against depth h, or 1 / h against density, and draw a smooth line.
Use your hydrometer to find the densities of milk, cooking oil and another liquid.
Check the milk reading by weighing 100 mL; a value of 1.03 g/mL for pure milk suggests no added water.
What you should see
The straw sinks about 4 percent less in a 1.04 solution than in water, and the scale marks are closer together at higher densities. Oil, being lighter than water, makes the straw sink to the bottom unless more ballast is added.
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 the hydrometer sink deeper in a lighter liquid?
It must displace more liquid to equal its own weight.
2What is the relation between the depth h and the density ρ?
h = m / (ρ A), so h is inversely proportional to ρ.
3Why is the scale not evenly spaced?
Because h varies as 1 / ρ, not as ρ.
4Where are hydrometers used?
To check milk, battery acid strength and the sugar content in wine and juices.
Take it further
Make a lactometer scale and test milk diluted with 10 and 20 percent water to see how the reading changes.
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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