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Density Ms Fedor Arts and College Preparatory Academy

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What do you think? In the year 250 BCE, King Hiero commissioned a goldsmith to make him a crown out of pure gold. However, when he received the crown, he suspected that the goldsmith had taken some of the gold and replaced it with a cheaper metal, even though it sill weighed the same. He asked Archimedes to determine whether the crown was solid gold. How do you think Archimedes determined that the crown was real gold?

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Activity: All That Glitters Goal: Determine if the gold-colored penny is really gold 1. Calculate the density of each object you measured yesterday. 2. Make some conclusions about how density relates to the type of substance 3. Use density to determine if the penny is gold

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Density A measurement of the amount of matter in a unit of volume. Typically g/mL or g/cm 3 1 mL = 1 cm 3 We can calculate density with the following formula: D = m/V

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Density is an intensive property Intensive property: a characteristic of a material that does not change, even if the amount of material, size, or shape are different. We can use intensive properties to identify a substance because they do not change.

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Extensive properties cannot be used to identify a substance Extensive property: a characteristic of a material that changes if the amount of a substance, size, or shape changes. Examples: mass and volume

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Exceptions Temperature causes the density of a substance to change, so the density measurement must be at the same temperature. Some substances, like cooking oil and plastic, come in many varieties. These can have different densities because their composition is different.

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Densities of common metals CopperZincGoldAluminumBrass 9.0 g/mL7.l g/mL19.3 g/mL2.7 g/mL8.4 g/mL Is the penny gold?

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ChemCatalyst A 68 g bar of gold is cut into 3 equal pieces. How does the density of each piece compare to the density of the original bar?

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Reminder Lab practical tomorrow. Know your lab equipment! Know how to read a graduated cylinder!

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What property of cranberries allows them to float on water, rather than sinking? Cranberries have a lower density than water. This means that a 1 cm 3 of cranberry has less mass (matter) than 1 cm 3 of water.

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Density of common materials Examine the densities of some common materials. Why do we fill balloons with helium? What other gas might we use?

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Calculating Density Remember that density is the amount of material (usually in g) per unit volume (usually in cm 3 or mL) Density = grams per cm 3 Density = g/cm 3 or g 1 cm 3 (remember that the “/” means “per”)

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Density triangles What happens if you have to calculate the mass from the density? Or the volume? You can use a density triangle. m D V You multiply Density x Volume to get mass That is why they are side by side You divide mass by volume to get density, this is why mass is on top of density. Likewise, you can divide mass by density to get volume.

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Practice: Calculating Density

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