SolidLiquidGas  The density of an object is a measure of how tightly packed the matter is in the substance.  As you will remember from Key Stage 3 there are 3 basics states of matter.  However what can you remember about their spacing and their motion?

volume mass In order to calculate the density of the substance we need to measure the volume and the mass. This is easy if it is a regular solid as the volume can be calculated, however if the solid is awkward or irregular you need to use the method as described on the practical sheet. Therefore; Units for Density are Kgm -3

Most Solids in the 1000’s Liquids in the hundreds Gases less than 10 Substance Density Kgm -3 Platinum (3 rd densest element)21,400 Lead11,000 Mercury13, 600 Iron8,000 Aluminium2,700 Glass2,500 Water1,000 Ice917 at -4 o C Ethanol790 Hardwood700 Balsa wood200 CO Air1.3 Methane0.7 Hydrogen0.09 These values are to give you a general idea NOT to be memorised!!

 Rigidity – How easily an object changes it’s shape, other terms are also used such as Malleable and Ductile referring to the ability to be reshaped or stretched  Fluidity - How easily an object flows to take up the shape of the container. Gases and Liquids are considered to be fluid, some are termed “superfluid”, since they flow to take up the shape of their container.  Compressibility – How easily a substance can have it’s volume reduced. The force generating a minute change in the volume of a solid could affect a gas hugely. There are no real rules about how these properties link or relate to each other. Generally the denser an object is the less likely it is to flow or bend but it actually depends on how the molecules are bound together.

Measuring Density There are no set practical for measuring the density of solids, liquids and gases but here are methods for each. Solids Record the mass in Kg of the object using an accurate set of scales. Record it’s volume either by calculation or measuring using the method shown.

Liquids Simply measure the mass of the measuring cylinder, add the liquid up to a convenient volume, calculate the mass of the liquid and calculate the density. This can be carried out for an ice cube by dropping the ice cube into a known volume of water, watching as it melts and recording the new volume after it has melted fully.

Gases Slightly harder since the mass of a gas is very small. Have the gas in a flask and record mass of the flask and gas, remove the gas either by heating or pumping it out, seal the tube with a clamp. Air pumped out Re-weigh it, then allow the flask to refill with water by putting the tube into water and measuring the change in the volume of water. You can then calculate the density of the gas.

Useful tip

1.A tank measures 60 cm long and 45 cm wide. If 72 kg of water is enough to fill the tank, how deep is the tank? (Density of water = 1000 kg m -3 ). 2.In the diagram below, the piston contains 0.2 kg of oxygen, which has a density of 1.43 kg m -3. (A) If the plunger is at a height of 40 cm, what is the cross- sectional area of the plunger? (B) The gas is heated and the plunger rises a further 20 cm. What is the density of the oxygen now?

Density = Mass/Volume  A property of the material. Pressure = Force/Area  Depends on the height of the fluid.  Same in all directions.  Units are:  Force/Area = N/m 2.  Pascals  1 Pa = 1 N/m 2.  Atmosphere  1 atm = X 10 5 N/m 2. 11

 Pressure = Force/Area.  p = F/A. 12 Show that p =  gh Remember F = ma and  = m/V

 Fluid or gas exerts a force on materials it touches  For an immersed object, the sum of forces from surrounding material is the buoyancy  If the net force on an object is upward, then it “rises”, if it is downward then it “sinks”, if zero then it “floats” 13

Force exerted by a displaced liquid. 14 F t -F b = B  gAh t -  gAh b = B B = g  A(h t – h b ) = W t - W b = B B =  A(h t – h b ) * g

 Solids are generally denser than liquids  Liquids are generally denser than gases. Therefore ice SHOULD sink 15 In fact, “really” cold water is denser than ice (I.e. water at 1 o C is denser than ice at 0 o C) If this wasn’t true, oceans and lakes would freeze from the bottom up

1. A swimming pool is 20.0 m by m and has a uniform depth of 1.92 m. A) What is the absolute pressure on the bottom of the swimming pool? B) What is the total force on the bottom of that swimming pool? C) What will be the pressure against the side of the pool near the bottom? 2. A dentist’s chair of mass kg is supported by a hydraulic lift having a large piston of cross-sectional area cm 2. The dentist has a foot pedal attached to a small piston of cross-sectional area 76.0 cm 2. What force must be applied to the small piston to raise the chair? 16

 Factsheet 64  S&C Manometer  Calculation sheet 4.8  Practical 4.8 – Determining Density  Application – Density and Pressure