IGCSE Physics / Density and pressure
Practical: measuring density
Edexcel International GCSE Physics (4PH1), chapter 18: Density and pressure, specification point 5.4. Free lesson.
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Density is found by dividing mass by volume. The method for measuring volume depends on whether the substance is a regular solid, an irregular solid or a liquid.
How to measure a regular solid
For a rectangular block, measure its mass on a balance and measure its dimensions. Zero the balance before taking the mass. For a small block, callipers give a more precise length than a ruler because they grip the block. Measure the length, width and height in a few places, then take the mean of each measurement. Volume = length × width × height. Density = mass ÷ volume. If mass is in grams and volume is in cm³, density is in g/cm³. Name the instrument as a balance, not a scale.
How to measure an irregular solid
A stone does not have simple dimensions to measure, so find its volume by displacement. The stone pushes water aside, and the rise in the water level gives the volume of the stone. Measure the stone's mass on a balance. For a large stone that will not fit in a measuring cylinder, use a eureka can. Fill it to the spout, lower the stone in and collect the overflow. The volume of the overflow is the stone's volume. Lower the stone gently. Splashing water out makes the measured volume too small.
How to measure the density of a liquid
The measuring cylinder has its own mass, so you must remove that from the combined mass before calculating the liquid's density. Read the liquid volume at the bottom of the meniscus, the curved surface of the liquid, with your eye level with it. Mass of liquid = mass of cylinder and liquid − mass of empty cylinder. Density = mass of liquid ÷ volume of liquid.
How to improve accuracy
Each method has possible errors. Choose an improvement that matches the source of the error, and repeat measurements where one reading might be unreliable. A ruler reads only to the nearest millimetre, which can be a large fraction of a small block's length. Use callipers instead. Reading the meniscus from above or below gives the wrong volume. Read its bottom at eye level. Air bubbles on a stone add to its measured volume. Tap the stone gently to free them. Repeat readings, ignore any anomaly and take a mean.