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.
Understanding Chemistry: Density and Specific Gravity
Density is called an intensive property because it does not depend on the sample size. A small bead of pure copper and a large copper pipe have different masses and volumes, yet they have nearly the same density under the same conditions. This makes density useful for checking material identity.
It is not perfect proof by itself, since different substances can have similar values. Chemists combine it with observations such as color, melting behavior, or chemical tests.
Density can change when temperature changes. Most materials expand when warmed, so the same mass takes up more space and its density falls.
Accurate density work depends on careful measurement. A balance measures mass, while a graduated cylinder, pipette, or volumetric flask can measure liquid volume. Read the bottom of the curved liquid surface, called the meniscus, at eye level.
Looking from above or below gives a volume error. For an irregular solid that does not dissolve, water displacement finds volume. Lower the object gently into water and use the rise in water level.
Remove air bubbles first because trapped air makes the measured volume too large. Then the calculated density becomes too small.
Floating is controlled by buoyancy. Water pushes upward on an immersed object because water pressure is greater lower down than higher up. If an object has a lower average density than the liquid, the upward push can support it before it fully sinks.
If it has a greater average density, it sinks. Average density matters for hollow objects.
Steel is denser than water, yet a steel ship floats because its hull contains a large volume of air. A coin sinks because its compact shape gives it a much higher average density.
Specific gravity is especially practical when comparing liquids. Since it has no unit, it gives a quick relative value near the reference temperature. A hydrometer measures it by floating at different depths.
In a denser liquid, the hydrometer floats higher. Car mechanics use this idea when testing battery electrolyte, and brewers use it to track dissolved sugar in a liquid.
Students should watch the temperature listed with a density value, keep units consistent, and avoid rounding too early. A density result with several decimal places is not trustworthy if the balance or volume reading was only measured roughly.
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.