Matter and Energy · 3 min read
Density
What Density Means
Density is the amount of mass in a given volume, or how tightly matter is packed. A golf ball and a table-tennis ball are about the same size, but the golf ball has much more mass packed into that space, so it is denser.
Measure mass with a balance. For a regular solid, volume = length × width × height. For an irregular object, use water displacement: the rise in water level in a graduated cylinder equals the object's volume. Density does not depend on the size of the sample; a small piece of aluminum and a large block of aluminum have the same density.
Worked Examples
Example 1: A Metal Block
Example 2: An Irregular Rock
Example 3: Finding Mass from Density
Floating and Sinking in Different Fluids
Whether an object floats depends on its density compared with the fluid it is in, not just on water. An object less dense than the fluid floats; an object denser than the fluid sinks. Liquids that do not mix stack in layers the same way, with the densest at the bottom.
| Fluid | Density (g/mL) | Will a 0.85 g/cm³ plastic bead float? |
|---|---|---|
| Rubbing alcohol | about 0.79 | No: 0.85 > 0.79, so it sinks |
| Vegetable oil | about 0.92 | Yes: 0.85 < 0.92 |
| Fresh water | 1.00 | Yes: 0.85 < 1.00 |
| Salt water (seawater) | about 1.03 | Yes: 0.85 < 1.03 |
| Corn syrup | about 1.38 | Yes: 0.85 < 1.38 |
Salt water is denser than fresh water, which is why swimmers and ships float a little higher in the ocean. A rubber piece with a density of 1.20 g/cm³ sinks in water but floats on corn syrup, so in the column it rests where the water meets the syrup.
- Density = mass ÷ volume, in g/cm³ or g/mL (1 mL = 1 cm³).
- Less dense than the fluid → floats; denser than the fluid → sinks.
- The same object can float in one fluid and sink in another.
- Cutting an object in half does not change its density.
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