Density

What is Density? Density = Mass Volume Density is defined as mass per unit volume. Density = Mass Volume

Density describes how matter is packed into space. Density is a derived unit. It is calculated from other measurements.

Density is a physical constant. A physical constant  a physical property that can be described with a number and a unit. A physical constant is the same for all samples of a substance regardless of sample size or shape.

1 drop of pure water has the same density as 1 liter of pure water. Density does not change with sample size.

Physical Constants We use physical properties such as density to identify substances. Some other physical constants include melting point, boiling point, heat capacity, heat of vaporization, and heat of fusion.

All samples of pure gold have the same density.

What are the units of density? The most common units of density are grams per cubic centimeter (g/cm3) or grams per milliliter (g/mL). By the way, 1 cm3 = 1 mL.

Density and Phase For most substances, the solid is the densest phase and the gas is the least dense. S  L  G Water is Weird! Water is at its densest at 4.0C, when it’s a liquid. L  S  G

The density of water is 1.0 g/mL (At 4.0°C). The relative density of any substance can be determined by adding it to water. If the substance floats, it’s density is less than the density of water: d < 1.0 g/mL If the substance sinks, it’s density is greater than the density of water: d > 1.0 g/mL 0.80 g/mL 1.0 g/mL 1.2 g/mL source

Density Column

Density of Gases Balloonx.mov Balloony.mov Flow of CO2

What is the density of …? Substance Density (g/cm3) water 1.00 seawater 1.025 ice .92 aluminum 2.70 gasoline .70 oil 0.90 lead 11.30 gold 19.30 white pine .50 rubber 1.10 cork .24 bone 1.80 iron 7.90 human fat mercury 13.60 helium .000178

What is the density of some selected elements? source

Can you explain this? Picture 1 Insert V. 2, CCA

What variables affect density? Temperature, T Pressure, P Liquid or solid: Even large changes in P & T  small changes in density. Gas: Even small changes in P & T  large change in density.

How does Temperature affect Density? Most materials expand when they are heated. Mass stays the same (it’s constant), but volume increases. So the matter isn’t packed as tightly. Density must decrease. (Hot air balloons!)

When the material expands, the density decreases.

Temperatue & Density As T , liquid becomes less dense. Spheres sink. As T , liquid becomes more dense. Spheres rise. Source: http://www.cogifts.com/products/execgifts/galileo.jpg

How does Pressure affect Density? If you  the pressure on an object, the volume . If the volume , the matter must be packed more tightly & density must .

When you apply pressure to an object, the density increases!

Remember: Water is weird! Ice Bomb Insert V. 2, CCA

Density Problems Algebra-type problems. The density equation has 3 variables. In any given problem, you are given information about 2 of them and you solve for the 3rd. You might have to rearrange the equation.

Problem-solving Rubric Given: Identify the given info. Unknown: Identify the unknown. Equation: Identify the correct equation and rearrange it as necessary. Substitute: substitute numerical values in for the letters. Solve: Do the math!

You can’t ignore reciprocals! 1 can’t magically become V. V

M D = V M = D X V or M V = D

Algebra Helper You might remember the density triangle from earth science: M D V

Density Problems An aluminum block 1.25 cm X 1.25 cm X 1.25 cm has a mass of 5.27 g. What is its density? Find the volume! 2.70 g/cm3

Density Problems Mercury has a density of 13.60 g/ml. What is the mass of 25 ml of mercury? 340 g

Density Problems Silver has a density of 10.5 g/cm3. What is the volume of 210 g? 20 cm3

Density Measurements Mass – Use a balance. Volume of regularly-shaped objects: direct measurement of length, width, & height. Volume of irregularly-shaped objects: use water displacement.

Mass is measured on a balance.

Direct measurement is used to find the volume of simple objects. Water displacement is used to find the volume of oddly shaped objects.