density

• DENSITY:


  A measure of how much mass a substance has, per unit of its volume.

  • Units are kg/m3
• The DENSITY of a material depends on how closely its particles are packed.
• A DENSE material means the particles are close together.

  • If the material is compressed, the particles are forced closer, increasing the DENSITY.
• Density (Kg/$m^3$) = Mass (Kg) $\div$ Volume ($m^3$)

  
• A metal cube has sides of 70cm and a mass of 600g.

  Calculate the density of the cube in kg/$m^3$.


  Conversions:

  • 600g $\div$ 1000 = 0.6kg
  • 70cm$\div$ 100 = 0.7m

  Volume of the cube:

  • 0.7 x 0.7 x 0.7 = 0.343$m^3$

  Density = mass / volume

  • 0.6 / 0.343 = 1.75 kg/$m^3$

  
• To find the DENSITY of a regular-shaped object like a box:

  • Measure the LENGTH, WIDTH, and HEIGHT of the box and multiply them for the VOLUME.
  • Use a MASS BALANCE for the MASS.
  • Use the DENSITY EQUATION.

  
• Equipment To find the DENSITY of a regular-shaped object like a box:

  • A ruler (for larger objects)
  • Vernier calliper (for smaller objects)
  • Mass balance

  
• To find the density For objects with an irregular shape (a key) the DISPLACEMENT TECHNIQUE is used:

  • Measure MASS using a MASS BALANCE.
  • Fill a Eureka can with water to the brim.
  • Place a MEASURING CYLINDER under the spout.
  • Put the object in the Eureka can.
  • The water displaced will pour into the measuring cylinder.
  • The VOLUME of water displaced & collected in the measuring cylinder, is the VOLUME of the object.
  • Use the DENSITY EQUATION.

  
• Equipment to find the density of irregular objects:

  • Measuring cylinder
  • mass balance
  • eureka can

  
• As density increases:

  • Mass increases
  • volume decreases
• As density decreases:

  • Mass decreases
  • volume increases
• To find density of liquids:

  • To find Volume of liquid, use measuring cylinder.

  To find mass of liquid:

  • weigh an empty container onto a balance, then "zero" the reading.
  • add the liquid to container & weigh again.
  • The mass is the difference

  Use density equation.
• a density of object < density of water

  • object floats

  b) density of object = density of water

  c) density of object > density of water

  • object sinks

  
• "zero error" is the type of error shown on the balance, as the balance should read zero:

  Subtract the zero error (4.2 g) from the recorded mass to get the correct value for the mass of the cube.

  
• • Read the cylinder at eye level
  • Place the cylinder on flat surface

  
• To use a displacement method to determine an accurate value for the volume of a single coin:

  • Add several coins to the measuring cylinder
  • Measure the change in the water level in the measuring cylinder.
  • Divide by the number of coins added.

  
• methods to reduce the uncertainty in experimental measurements of density:

  • Take multiple measurements & calculate the mean
  • When measuring the density of liquids, use a larger volume of liquid
• 1 g/cm3 = 1000 kg/m3
• To convert g/cm3 to kg/m3 you have to:

  Multiply the g/cm3 value by 1000
• • Maximum density = 2.65 (g/$cm^3$)
  • Minimum density = 2.45(g/$cm^3$)

  
• Add key and measure level of water, subtract original volume.

  
• Repeat readings of volume, need taking of each fruit to show that the readings are close together.

  
• Meter rule has a lower resolution, so is less accurate than the micrometer screw gauge.

  
• power (W) = Energy (J) / Time (s)
• Taking measurements more than once, may improve the accuracy of the value as:

  A mean can be calculated, which reduces the effect of random errors.
• Metal:

  • has a high thermal conductivity, which increases the rate of energy transfer from the mixture.

  Plastic:

  • has a low thermal conductivity, which reduces the rate of energy transfer from the surroundings to the liquid coolant at –20°C.

  
• The liquid coolant has a freezing point below –20 °C, another property that the liquid coolant should have:

  A high specific heat capacity, so it can absorb a large amount of energy with only a small temperature change.

  
• Total energy = Energy of specific heat capacity + Energy of specific latent heat

  • 165 kJ = 165000 J

  E of specific heat capacity = mcΔ0:

  • E of shc = m × 3500 × 21.5 = 75250 m

  E of specific latent heat = m x Lf:

  • E of slh = m × 255000

  165000 = 75250 m + 255000 m

  • 165000 = 330250 m

  m = 165000 / 330250

  • m = 0.50 kg (2 d.p)