Topic 2 Solids liquids and gases

5.3 Density formula

density = mass/volume (kg/m³)

5.4 Finding the density of a solid

1) Measure its mass by placing it on a top-pan balance(before immersing into water, prenventing wet stone affecting its mass)

2) Regular shaped solid: Measure its dimensions and calculate its volume using a formula, e.g. l x wx h

3) Irregular shaped solid: Fill an eureka tin with water. Put a beaker under the SPOUT. Immerse the solid in water and measure the water displaced into the beaker using a measuring cylinder. Ensure reading is taken at eye-level to avoid parallax error.

4) Calculate density using density = mass/volume

5.4 Finding the density of a liquid

1) Place a measuring cylinder on a top pan balance. Press tare to avoid zero error,

2) Add the liquid to the measuring cylinder

3) Read the volume of the liquid from the measuring cylinder at eye-level to avoid parallax error

4) Read its mass from the top-pan balance

5) Find the density using the formula density = mass/volume

5.5 Formula of Pressure

Pressure is the force applied per unit area.

Pressure = Force/Area

P = F/A

5.6 Pressure in a fluid properties

• acts equally in all directions

• created from the movements of particles as they collide with each other and a surface

5.7 Pressure difference formula

pressure difference = height/depth x density x g

→ Pressure increases with depth: The more fluid there is above, the more weight there is, increasing pressure.

Ideal Gas Molecules

• Gas molecules move rapidly and randomly and collide with each other

• The collisions change their speed and direction (→force)

• They exert a FORCE on the walls and hence a pressure on the walls of a container due to collisions between gas molecules and the walls.

5.20 Pressure and temperature at a constant volume

At a constant volume, pressure increases if temperature increases as:

1) the molecules have more kinetic energy, so they move quicker

2) they collide harder and more frequently with each other and the walls

3) They exert a greater force on the walls, hence greater pressure.(Since pressure = F/A)

Pressure and volume at a constant temperature

At a constant temperature, if volume increases, pressure decreases as:

• The molecules collide less frequently with each other and with the walls since there is more space to travel through.

• They collide over a greater area, and since P = F/A, pressure will decrease

5.16 Kelvin scale and absolute zero

The temperature at which the pressure is 0 is called absolute 0. The molecules have no kinetic energy, and they do not move.

The Kelvin scale defines absolute 0 as 0K, with an increment of 1K equal to an increment of 1^oC.

Temp. in Kelvin = Temp in degrees Celsius + 273

5.19 Kelvin temperature and average kinetic energy

The Kelvin temperature of a gas is directly proportional to the average kinetic energy of its molecules.

Temperature and average speed of molecules

If temperature increases, the average kinetic energy of the molecules increases, so the average speed of the molecules also increases.

5.21 For a gas at fixed mass and constant volume, formula linking pressure and temperature:

P1/T1 = P2/T2

5.22 For a gas at fixed mass and constant temperature, formula linking pressure and volume

p1V1 = p2V2 or PV= constant