Density
Density is how concentrated is matter in the object –
it is the mass per unit volume
calculated by dividing the mass of the object by its volume.
The density of water at 4°C: 1 g/cm3 or 1 g cm-3 or 1 kg/l (one kilogram per litre).
Mandatory Practical Activity 21
Measure the mass and volume of a variety of solids and liquids and hence determine their densities.
(i) Measure the Mass and Volume of a Variety of Solids
The mass of solids is easily measured by using a top pan balance.
Zero the balance first then place the solid on the pan and record the digital reading of its mass.
(a) Measuring the Volume of a Small Irregular Solid – use a measuring cylinder.
Pour some water into a measuring cylinder and record the level e.g. 40 cm3 .
Gently lower the solid into the water by a thread until it is completely submerged – if the solid is less dense than water then push it under with a long thin needle.
Record the new higher level of water e.g. 65 cm3 .
Subtract the readings to find the volume of the solid: 65 cm3 - 40 cm3 = 25 cm3 .
Make sure the measuring cylinder was perfectly vertical while taking the readings.
Remember to read from the bottom of the meniscus curve.
(b) Measuring the Volume of a Large Irregular Solid – use an overflow can.
Place an overflow can at the side of a sink with the spout overhanging the sink.
Overfill the overflow can and allow the excess water to flow out – wait till the dripping stops – the water surface is level with the bottom of the opening in the can.
Place a clean dry measuring cylinder under the spout.
Gently lower the solid into the water by a thread until it is completely submerged – if the solid is less dense than water then push it under with a long thin needle.
The displaced water flows out of the can from the spout into the measuring cylinder.
Wait for the dripping to stop.
The volume water in the measuring cylinder is the volume of the solid.
The densities of each of the solids are calculated by dividing their mass by their volume.
(ii) Measure the Mass and Volume of a Variety of Liquids
The volume of liquids can be determined by using a measuring cylinder and exact volumes of liquid can be delivered by using a pipette or burette.
A two step process is needed to find the mass of a liquid.
First measure and record the mass of a clean dry beaker using a top pan balance e.g. 30 g.
Pour the liquid into the beaker and measure the mass of the beaker plus the liquid e.g. 70 g.
By subtraction the mass of the liquid is calculated: 70 g – 30 g = 40 g.
If the volume of the liquid is known then its density can be calculated by dividing the mass of the liquid by the volume of the liquid.
Density and Floatation
Investigation Investigate Floatation for a Variety of Solids and Liquids in Water and Other Liquids.
a) Investigate Floatation for a Variety of Solids in Water and Other Liquids.
The density of water is 1 g/cc.
Measure the density of a variety of solids and place them in a trough of water.
Record whether they sink or float plus if they float by what proportion are they immersed in the water.
Compare their densities with that of the ‘floating’ results.
The solids that sink are denser than the water.
The solids that float are equal or less than the density of water.
If equal in density they float fully submerged but not touching the bottom.
The less dense the solid the less its proportion below the surface – the less dense the relatively higher it float in the water.
Repeat for other liquids like oil and methylated spirits.
b) Investigate Floatation for a Variety Liquids in Water and Other Liquids.
This investigation can only be carried out with immiscible liquids i.e. liquids that do not mix, liquids that will remain separated e.g. water, oil and mercury.
The density of the different liquids is measured and recorded.
Place liquids in the same container and try to mix them.
Allow the liquids to settle.
Note the order of the layers of liquid – mercury on the bottom, water in the middle and oil on the top.
The less dense liquid floats on the denser liquid.