Density tells how much mass is packed into a given volume, so it helps identify substances and predict whether objects sink or float. In chemistry, density is often reported in g/mL for liquids and g/cm^3 for solids. Because density connects mass and volume, it is also a useful conversion factor in lab calculations.
Specific gravity compares a substance's density to the density of water, giving a unitless way to describe relative heaviness.
Key Facts
- Density formula: ρ = m/V
- Mass from density: m = ρV
- Volume from density: V = m/ρ
- For water displacement: object volume = final water volume - initial water volume
- Specific gravity: SG = density of substance/density of water
- At about 4 °C, density of water = 1.00 g/mL, so SG often has the same numerical value as density in g/mL
Vocabulary
- Density
- Density is the mass of a substance divided by its volume.
- Mass
- Mass is the amount of matter in an object, commonly measured in grams or kilograms.
- Volume
- Volume is the amount of space an object or substance occupies.
- Water displacement
- Water displacement is a method for finding the volume of an irregular object by measuring how much the water level rises when it is submerged.
- Specific gravity
- Specific gravity is the ratio of a substance's density to the density of water at a specified temperature.
Common Mistakes to Avoid
- Using mass divided by volume in the wrong order, such as V/m, gives the reciprocal of density instead of density.
- Forgetting units makes the answer incomplete because density units, such as g/mL or g/cm^3, show what quantities were divided.
- Reading the graduated cylinder at an angle gives an inaccurate volume because the bottom of the meniscus should be read at eye level.
- Treating specific gravity as having units is wrong because it is a ratio of two densities, so the units cancel.
Practice Questions
- 1 A metal cube has a mass of 96.0 g and a volume of 12.0 cm^3. What is its density in g/cm^3?
- 2 A rock is placed in a graduated cylinder and the water level rises from 35.0 mL to 48.5 mL. If the rock has a mass of 37.8 g, what is its density?
- 3 Two liquids have densities of 0.79 g/mL and 1.26 g/mL. Explain which one would float on water and which one would sink, and relate your answer to specific gravity.