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Density

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**What is Density? Density = Mass Volume**

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

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**Density describes how matter is packed into space.**

Density is a derived unit. It is calculated from other measurements.

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**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.

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**1 drop of pure water has the same density as 1 liter of pure water.**

Density does not change with sample size.

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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.

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**All samples of pure gold have the same density.**

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**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.

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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 4.0C, when it’s a liquid. L S G

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**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

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Density Column

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

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**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

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**What is the density of some selected elements?**

source

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

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**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.

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**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!)

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**When the material expands, the density decreases.**

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**Temperatue & Density As T , liquid becomes less dense. Spheres sink.**

As T , liquid becomes more dense. Spheres rise. Source:

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**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 .

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

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**Remember: Water is weird!**

Ice Bomb Insert V. 2, CCA

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**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.

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**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!

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**You can’t ignore reciprocals!**

1 can’t magically become V. V

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

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

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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

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

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

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**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.

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**Mass is measured on a balance.**

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**Direct measurement is used to find the volume of simple objects.**

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

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