Density

Density is a fundamental physical property that describes how much mass is contained in a given volume of a substance. It is calculated by dividing the mass of an object by its volume. Density determines many everyday phenomena, from whether objects float or sink to how hot air rises.

Discover FizziQ

How to measure it in class

With the FizziQ app, you can explore density concepts by measuring related properties such as atmospheric pressure (which depends on air density) or by conducting experiments that demonstrate density effects.

Steps:

Open FizziQ and select the barometer for atmospheric pressure measurements
Measure pressure at different altitudes to observe how air density decreases with height
Create a density column using liquids of different densities (honey, water, oil)
Predict and test which objects will float at each layer
Measure the mass and volume of various objects to calculate their densities
Compare measured densities with standard values

Scientific activities on this topic

Explore density concepts through these FizziQ activities:

Study gas behavior and density changes: https://www.fizziq.org/en/activities/gas-law/
Measure altitude using pressure and air density: https://www.fizziq.org/en/activities/altimeter/

Learn more

Archimedes’ principle:

The ancient Greek scientist Archimedes discovered that an object immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces. This principle explains floating and sinking:

Object density vs fluid	Result
Object density < fluid density	Object floats
Object density = fluid density	Object is neutrally buoyant
Object density > fluid density	Object sinks

Buoyancy and floating:

When an object floats, it displaces exactly enough fluid to support its weight. A ship floats because its overall density (including the air-filled hull) is less than water. Submarines control their buoyancy by filling or emptying ballast tanks.

Density of common materials:

Material	Density (kg/m3)
Air (sea level)	1.2
Wood (oak)	750
Water	1000
Seawater	1025
Aluminum	2700
Iron	7870
Gold	19300

Temperature and pressure effects:

Density varies with temperature and pressure. Most substances expand when heated, decreasing their density. This is why hot air rises. For gases, density is directly proportional to pressure and inversely proportional to temperature, as described by the ideal gas law.

Atmospheric pressure and density:

Air density decreases with altitude because there is less air above to compress it. At sea level, air density is about 1.2 kg/m3. At 5000 meters altitude, it is approximately half that value. This affects aircraft performance, breathing, and weather patterns.

Formula

Density:

rho = m / V

where:

rho: density (kg/m3)
m: mass (kg)
V: volume (m3)

Buoyant force (Archimedes’ principle):

F_b = rho_fluid x V_displaced x g

where:

F_b: buoyant force (N)
rho_fluid: density of the fluid (kg/m3)
V_displaced: volume of fluid displaced (m3)
g: gravitational acceleration (9.81 m/s2)

For floating objects in equilibrium:

rho_object / rho_fluid = V_submerged / V_total

Application examples

Ice floats on water because ice is less dense (917 kg/m3) than liquid water (1000 kg/m3)
Ships are designed with large volumes of air space to reduce average density below that of water
Hot air balloons rise because heated air inside is less dense than surrounding cool air
Oil floats on water, allowing oil spills to be skimmed from the surface
Hydrometers measure liquid density to determine alcohol content in beverages or battery acid strength
Layered cocktails are created by carefully pouring liquids of decreasing density

FAQ

Q: Why does ice float on water? A: Water is unusual in that it expands when it freezes. Ice has a density of 917 kg/m3, less than liquid water at 1000 kg/m3. This is why ice cubes and icebergs float.

Q: How do submarines control their depth? A: Submarines have ballast tanks that can be filled with water to increase density (sink) or filled with compressed air to decrease density (rise). This allows precise control of buoyancy.

Q: Why is it easier to float in the Dead Sea? A: The Dead Sea has very high salt content (about 34%), making its water density approximately 1240 kg/m3. This higher density provides greater buoyant force, making floating effortless.

Q: How does density affect sound speed? A: Sound travels faster in denser media because molecules are closer together. Sound travels about 4 times faster in water than in air, and even faster in solids like steel.

Q: Why do hot air balloons work? A: Heating the air inside the balloon reduces its density. When the air inside is less dense than the outside air, the balloon experiences net upward buoyancy and rises.

Archimedes’ Principle - Buoyancy - Mass - Volume - Atmospheric Pressure - Floating and Sinking - Ideal Gas Law - Temperature