Figure 2 shows a rock found by a student on a beach.
To help identify the type of rock, the student took measurements to determine its density.
i- Describe a method the student could use to determine the density of the rock.
[6 marks]
SOLUTION
Both methods are acceptabl, you need to write any ine of them. And don’t forget to mention the fromula of density at the end to anlyse the result.
Method # 01 (Using Measuring Cylinder)
Make sure the rock to be used fits easily into the measuring cylinder.
Measure the mass of the rock using a balance.
Put enough water into the measuring cylinder to submerge the rock. Measure the volume of water in the measuring cylinder.
Tilt the measuring cylinder and slide the rock in.
Measure the new position of the water surface in the measuring cylinder. Then calculate the volume of the rock.
The Volume of rock = final volume − initial volume.
Use mass and volume to calculate density of rock.
Method # 02 (Using Displacement/Eureka Can)
Measure the mass of the rock using a balance.
Fill displacement can till water starts dripping from the top.
Tie a small string around the rock and slowly lower it into the can to make sure no water splashes.
Collect the displaced water in measuring cylinder and measure it. This is the volume of the can.
Use mass and volume to calculate density of rock.
ii- The student determined the density of the rock to be 2.55 ± 0.10 g/cm3.
What are the maximum and minimum values for the density of the rock?
[1 mark]
SOLUTION
Table 1 gives the density of five different types of rock.
iii- Which two types of rock in Table 1 could be the type of rock the student had?
[1 mark]
SOLUTION
Which ever falls in the range of rock that is is considered as the type of the the rock.
As we can see that there are only two type of rock in the above table which have density overlaping the range of rock that the student has.
The student only took one set of measurements to determine the density of the rock.
iv- Explain why taking the measurements more than once may improve the accuracy of the density value.
[2 marks]
SOLUTION
By taking multiple reading, the student may calculate the mean density of the rock which will be more accurate. It will reduce any random if present.
(Remember, a Random Error is a type of error which is unpredictable. Repeating the measurement and then working out a mean reduces the effect of a random error. In a graph, data that is scattered about the line of best fit shows a random error. )
