Week 3 · 1-3 Physical Properties of Water (A): density, relative density, the hydrometer, what changes the density of ocean water, ocean currents, the 4 °C maximum density and why lakes freeze from the top · 3 lessons
1
Density and how a hydrometer reads it
Integrated Science Sec 1 Term 1, Unit 1: pp. 16–19
GoalCalculate density from mass and volume, use relative density, and explain how a hydrometer shows density.
New words
density · mass divided by volume: how much matter is packed into each unit of volumerelative density · the density of a substance divided by the density of pure water at 4 °C; it has no unithydrometer · a weighted, sealed floating tube with a scale, used to read the density of a liquid
Explanation
Density = mass ÷ volume. It is measured in kg/m³ or g/cm³. For pure water at 4 °C, 1 cm³ has a mass of 1 g, so its density is 1 g/cm³, which is 1000 kg/m³. Relative density compares a substance with this water: relative density = density of the substance ÷ density of water at 4 °C. It has no unit, so a liquid of density 1.2 g/cm³ has relative density 1.2. A hydrometer reads the density of a liquid. It floats upright, and it sinks deeper in a less dense liquid and floats higher in a denser one. So the scale shows the highest density at the bottom and the lowest at the top, which looks upside down. Do not mix up density with weight: a big bucket of water weighs more than a pebble, but the pebble is denser.
Worked examples
A block has a mass of 460 g and a volume of 400 cm³. Its density is 460 ÷ 400 = 1.15 g/cm³, and since water at 4 °C is 1 g/cm³, its relative density is 1.15. It is denser than water, so it would sink.
Use density to find mass and volume of pure water at 4 °C (1000 kg/m³). A tank of 3.2 m³ has a mass of 3.2 × 1000 = 3200 kg. A mass of 750 kg fills 750 ÷ 1000 = 0.75 m³.
A hydrometer floats with more of its tube under the surface in liquid A than in liquid B. Which liquid is denser? A less dense liquid lets it sink deeper, so A is less dense and B is denser.
Class activity
Home-made hydrometer, with a grown-up. An adult cuts a drinking straw and you put a ball of modelling clay at the bottom so that it floats upright in a tall glass of tap water. Mark the water line with a pen. Then float it in a glass of strongly salted water (a grown-up stirs in the salt) and mark the new line. Describe where the marks are, and say which liquid is denser. Then weigh 250 mL of tap water and 250 mL of salty water with a kitchen scale and compare. Do not taste.
Practice
1
A liquid has a mass of 600 g and a volume of 500 cm³. Find its density.
600 ÷ 500 = 1.2 g/cm³.
2
What is the mass of 5 m³ of pure water at 4 °C?
5 × 1000 = 5000 kg.
3
An oil has a density of 850 kg/m³. What is its relative density, and will it float on water?
850 ÷ 1000 = 0.85. It is less dense than water, so it floats.
4
A hydrometer floats lower in liquid X than in liquid Y. Which is denser?
Y is denser, because the hydrometer floats higher in it.
