What it shows
Sealed cans of regular and diet soft drinks look the same, but in a tub of water the regular one usually sinks and the diet one floats. The sugar dissolved in the regular drink raises its density above that of water.
The science
An object floats if its average density is less than the density of water, about 1 g per cm3. A regular 330 ml drink has about 35 g of sugar dissolved in it, which makes the liquid about 4 percent denser than plain water (about 1.04 g per cm3). The diet version has only a tiny amount of sweetener, so its density is close to that of plain water and the air gap in the can makes the total less than water.
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 sealed metal can of a regular fizzy drink (not a plastic bottle, which floats either way)
- One sealed can of the diet or sugar-free version of the same brand
- A large tub or bucket of water
- A kitchen scale if available
- A notebook
- Several cups, 3 spoons of sugar and 3 spoons of salt
- A small sealed plastic bottle with a little water or clay inside so that it just floats
Safety
For this project: Keep the cans sealed and do not shake them. Dry the floor if water spills. Do not drink the test water.
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.
Step 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.
Fill a tub with water deep enough for a can to float and let it stand.
Place the regular can gently into the tub and note whether it sinks or floats.
Place the diet can in and note what happens. If both cans float, note which one sits lower in the water, because that also shows the density difference. Remove both cans.
Read the labels and note the grams of sugar per 100 ml for each and write them in a table.
If you have a scale, weigh both cans and write the mass of each. The regular can should be heavier by roughly 10 to 15 g, because the sugar adds mass but also takes up some space.
Make a sugar solution at home: dissolve 3 level spoons of sugar in a cup of water and test whether a small sealed bottle with a little water floats higher in it than in plain water.
Test with salt water in the same way and note which makes it float higher.
Draw a diagram showing the cans in the water with the downward weight and upward buoyant force.
What you should see
The regular can usually sinks (or floats much lower) while the diet can floats higher, and the label shows about 10 g of sugar per 100 ml in the regular one and almost none in the diet one.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01What makes the regular can sink?
The dissolved sugar makes its average density greater than that of water.
02What is density?
Mass per unit volume, for water about 1 g per cm3.
03Why do two cans of the same size have different masses?
The regular drink carries about 35 g of dissolved sugar and the diet one only a tiny amount, so the same volume of regular drink has more mass.
04Why is the experiment kept with sealed cans?
Opening a can lets the gas escape and changes the mass and the volume.
Take it further
Make a hydrometer from a straw and clay and rank four drinks by density.
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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