Density Bethany Faint - 10B Contents • The Discovery of density • Formulas and Units of Density • Examples of the Density rule • Densities of some common substances • Floating and Sinking • Calculations of density for objects • Calculations of density for gas The discovery of density • 250 BC - Ancient Greece • The King of Syracuse gave his goldsmith some gold. “Make a crown”, he said. So the goldsmith did. But the King was suspicious. Perhaps the goldsmith had kept some of the gold. Perhaps he had mixed in cheaper silver to take its place. He asked Archimedes to test the crown. • Archimedes knew that the crown was the correct mass. He knew that silver was less dense than gold. So, if the crown contained any silver it would have more volume than it should. • But how could he check the volume of the crown? Stepping into his bath one day, Archimedes noticed the rise in the water level. Here was the answer to the problem! Put the crown in the water and measure the rise in level. Put an equal mass of pure gold in the water. See if the rise in level was the same. “Eureka!” he shouted. Which is Greek for ‘I have found it’. • Was the crown made of pure gold? Unfortunately, nobody wrote down the result. So we shall never know…… • Density of the measurement of how close and how heavy are the particles of matter in an object. • It is measured in kg/m 3 or g/cm 3 or g/mL • The Rule for density = Mass/Volume Formulas & Units of Density Examples of the Density Rule • Q. A stone has a mass of 10 grams. When it is immersed in a measuring cylinder of water, the water level rises 5mm. What is the density of the stone? • A. Density = Mass/Volume • = 10 grams/5 mL • = 2 g/mL Floating and Sinking • The density of water is g/mL. • Any substance (such as ice, petrol or oil) whose density is less than 1 will float • Any substance (such as lead) whose density of more than 1 will sink Calculations of density for regular objects • Step one: Measure the MASS of the object whether it be liquid or solid (remember to allow for mass of the beaker containing the liquid) using a top pan balance. • Step 2: Measure the VOLUME. For a liquid the level on the scale on the measuring cylinder will give the volume. For a solid the rise in the level of water give the volume of a solid. • Step 3: Calculate the DENSITY: DENSITY= MASS/VOLUME Calculation of density for a gas • Step 1: Find the mass difference between • - the flask with air in it • - flask when air has been removed by a vacuum pump • Step 2: Fill the flask with water and measure the volume of the water • Step 3: Calculate the density Density=Mass/Volume A gas mask used when harmful gas is released into the air Sample Activity . Density Bethany Faint - 10B Contents • The Discovery of density • Formulas and Units of Density • Examples of the Density rule • Densities. Floating and Sinking • Calculations of density for objects • Calculations of density for gas The discovery of density • 250 BC - Ancient Greece • The