01 // What density means
Density is how much mass is packed into a given volume — it answers "how heavy is this stuff for its size," not "how heavy is this object." A small lead ball can be much denser than a large foam block, even though the foam block weighs more overall.
Density is a property of a material, not of a specific object — a gram of aluminum and a ton of aluminum have the same density, 2.70 g/cm³. That is why density is useful for identifying an unknown material: measure it once, then compare to a reference table.
02 // Worked example
A student finds the volume of an irregular rock by water displacement, then weighs it on a balance.
| Mass (triple beam balance) | 47.5 g |
| Water level before rock | 50.0 mL |
| Water level after rock | 68.0 mL |
Formula density = mass ÷ volume
03 // Why it matters
- Density decides whether something floats: an object floats in a liquid if its density is lower than the liquid's density, and sinks if it is higher.
- 1 mL and 1 cm³ are the same volume, which is why water displacement (measured in mL) can be used directly in a density formula that wants cm³.
- Water has a density of 1.0 g/cm³ at 4 °C, which is why it is a common comparison point — anything denser than 1.0 g/cm³ sinks in water.
04 // Questions people ask
What is the formula for density?
Density equals mass divided by volume (D = m ÷ V). Mass is usually measured in grams and volume in cubic centimeters or milliliters, giving density in g/cm³ or g/mL, which are equal units.
How do you find the volume of an irregular object?
Use water displacement: record the water level in a graduated cylinder, lower the object in completely, and record the new level. The difference in volume is the object's volume.
Why does an object float or sink based on density?
An object floats when it is less dense than the liquid around it, because it displaces a weight of liquid equal to its own weight before it is fully submerged. An object denser than the liquid cannot displace enough liquid to support its own weight, so it sinks.