Section 5: DensityChapter 2: Measurement and Problem Solving

Learning ObjectivesCalculate the density of a substance.

Use density as a conversion factor.

DensityThe density of a substance is the ratio of its mass to its volume.

DensityMeasuring mass: use a balanceMeasuring volume:

Formula (l x w x h) (πr2h)

Water displacement (Vf-Vi)

DensityUnits of density:

Solid substance = g/cm3

Liquid substance = g/mL

ExampleA sample of liquid has a volume of 22.5 mL and a mass of 27.2 g.

DensityD = m/Vm =V =

ExampleFor a liquid substance with a

density of 1.32 g/cm3, what volume should be measured to deliver a mass of 68.4 g?

ExampleA steel cylinder has a volume of

246 cm3 and a density of 7.93 g/cm3. What is its mass in kilograms?

DensityDensity is an intensive property

It does not change, no matter the size of a sample

▪ 1 kg of gold has the same density as 1 g, or 1 lb, or 1000 kg, etc.

DensityEvery substance has a unique density you can identify an unknown substance by calculating its density and comparing it to known values

Temperature and DensityDensity changes with temperature For most substances, an increase in temperature results in a decrease in density

Temperature and Density▪ As temperature increases, molecules move further apart. This increases the volume of the substance while the mass remains the same – thereby decreasing the density.

▪ So solids are more dense than liquids, which are more dense than gases

Density of WaterThe density of liquid water is 1.00 g/mL Anything more dense than water will sink

Anything less dense than water will float

Density of WaterWater is one of a unique group of substances that is actually less dense as a solid than a liquid Ice floats in water!

Practice A jeweler offers to sell a ring to a woman and

tells her that it is made of platinum. Noting that the ring feels a little light, the woman decides to perform a test to determine the ring’s density. She places the ring on a balance and finds that it has a mass of 5.84 g. She also finds that the ring displaces 0.556 cm3 of water. Is the ring made of platinum? The density of platinum is 21.4 g/cm3.

PracticeMass = 5.84 g Volume = 0.556 cm3

Practice The woman takes the ring back to the jewelry

shop, where she is met with endless apologies. The jeweler had accidentally made the ring out of silver rather than platinum. The jeweler gives her a new ring that she promises is platinum. This time when the customer checks the density, she finds the mass of the ring to be 9.67 g and its volume to be 0.452 cm3. Is this ring genuine?

PracticeMass = 9.67 g Volume = 0.452 cm3