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Density Chapter 3 Scientific Measurements

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I CAN I CAN define DENSITY and explain how it is calculated and determine the volume both a regular object (mathematically) and and irregular object (Archimedes' Principle).

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DENSITY Which weighs more? A pound of lead or a pound of feathers? Neither…they each weigh a pound! Which takes up more space? Why? The feathers do because they are not as tightly packed as the particle in the lead.

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What is DENSITY? Density COMPARES the MASS of an object to its VOLUME. Density is a UNIQUE PROPERTY of a substance; it can be used to help IDENTIFY UNKNOWN SUBSTANCE. DENSITY is defined as MASS PER UNIT OF VOLUME.

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Calculating Density The mathematical equation for finding density is: DENSITY = MASS VOLUME -any MASS and VOLUME unit can be used, but commonly in science GRAMS are used for mass and MILLILITERS (ml) and CUBIC CENTIMETERS (cm 3 ) are used for volume.

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mass and volume units cannot cancel each other; the final unit for density is the MASS UNIT over the VOLUME UNIT. For example g/ml or kg/l

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Pure water has a density of 1 g/ml. When we compare a substance’s density to that of water, we refer to the value as SPECIFIC GRAVITY. If an substance’s density is LESS THAN that of water, it will FLOAT in water; if MORE THAN water’s density, it will SINK!

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Practice Problems What is the density of a substance if 25 grams has a volume of 5.0 ml? Density = Mass = 25 grams = 5g/ml Volume 5.0 ml What is the density of a substance if 60 grams has a volume of 100 mL? Density = Mass = 60 grams = 0.60g/ml Volume 100 ml

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Mass and Volume Finding the MASS of an object is easy….place it on a balance and get the value. But what about VOLUME? Volume can be found in two ways: –A. Mathematically –B. Archimedes Principle

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Calculating Volume The volume of regular objects, such as cubes or boxes is found by this formula: –VOLUME = Length X Width X Height –What would the units be? –Since you are multiplying the same unit by itself three times, the final unit is CUBED!

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Sample Problem A student measure a cube of wood and gets the following results: L= 5 cm W = 4 cm H = 3 cm What is the volume of the cube? V = L X W X H V = 5cm X 4 cm X 3 cm = 60 cm 3

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Archimedes’ Principle Archimedes’ Principle says that an object will displace a volume of water equal to its own volume. Used to determine the volume of oddly shaped object, such as a stone.

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Using Archimedes’ Principle 1. Fill a graduated cylinder with enough water to cover the object. 2. Record the INITIAL water level. 3. Carefully add the object and record the FINAL water level. 4. The difference between the two numbers is the object’s volume.

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Practice Problem A student is asked to find the volume of a 44 g stone in lab. She fills a graduated cylinder to the 50 ml mark. She carefully adds the stone, then records the final volume of water, which is now 62 ml. What is the stone’s volume? Volume = 62 ml – 50 ml = 12 ml

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What is the stone’s density? D = M / V = 44g / 12 mL = 3.7 g/mL

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Other Calculations How can the density equation be used to find other values, such as Mass or Volume? D = M / V V = M / D M = V x D

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Practice Problems A certain mineral has a D = 2.1 g / cm3 What is the MASS of a sample having a V= 8cm 3 ? A piece of wood has a D=2.3 g / mL A piece of this wood having a M=22.6 g would have a Volume of?

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