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Density

Density. What is Density?. 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.

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Density

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  1. Density

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

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

  4. Density is a physical constant. • A physical constant  aphysical 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.

  5. 1 drop of pure water has the same density as 1 liter of pure water. Density does not change with sample size.

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

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

  8. 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.

  9. Density and Phase • For mostsubstances, 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

  10. 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

  11. Density Column

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

  13. What is the density of …?

  14. source What is the density of some selected elements?

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

  16. 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.

  17. 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!)

  18. When the material expands, the density decreases.

  19. 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

  20. 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 .

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

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

  23. 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.

  24. 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!

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

  26. M D = V M = D X V or M V = D

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

  28. 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

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

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

  31. 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.

  32. Mass is measured on a balance.

  33. 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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