3.2 Required practical: density
9 exam questions worth 22 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Describe how to find the volume of a small stone using a eureka can and a measuring cylinder.
Mark scheme
fill the eureka can until water is level with the spout / just overflowing (1); lower the stone in and collect the displaced water in a measuring cylinder; the volume of water collected is the volume of the stone (1)
Model answer
Fill the eureka can until the water just reaches the spout and stops dripping. Lower the stone in carefully and collect the displaced water in a measuring cylinder. The volume of this water equals the volume of the stone.
Describe how to find the density of a liquid using a balance and a measuring cylinder.
Mark scheme
measure the mass of the empty measuring cylinder (1); add liquid, read the volume from the bottom of the meniscus at eye level and measure the total mass (1); subtract to find the mass of the liquid, then density = mass / volume (1)
Model answer
Measure the mass of the empty measuring cylinder. Pour in some liquid and read its volume at eye level from the bottom of the meniscus, then measure the mass of the cylinder and liquid. Subtract the two masses to get the mass of the liquid, then divide by the volume to find the density.
A block is 4.0 cm long, 3.0 cm wide and 2.0 cm high and has a mass of 62 g. Calculate the density of the block in g/cm³.
Mark scheme
volume = 4.0 × 3.0 × 2.0 = 24 cm³ (1); ρ = m / V = 62 / 24 (1); 2.6 g/cm³ (1)
Model answer
Volume = 4.0 × 3.0 × 2.0 = 24 cm³. Density = 62 / 24 = 2.6 g/cm³.
Explain why the reading on a measuring cylinder should be taken at eye level.
Mark scheme
to avoid parallax error / reading too high or too low (1); to get an accurate / correct volume (1)
Model answer
Reading at eye level avoids a parallax error, so the volume is read accurately.
State one source of error when using a eureka can to find the volume of an object, and explain how it affects the calculated density.
Mark scheme
some displaced water stays in the spout / is spilt and not collected (1); the volume measured is too small, so the calculated density is too large (1)
Model answer
Some of the displaced water may stay in the spout and not be collected. The measured volume is too small, so the calculated density is too large.
Which piece of apparatus is used to measure the mass of an object?
Choose one answer.
Correct Not quite: the answer is Top-pan balance
Mark scheme
C: top-pan balance
Model answer
A top-pan balance (C).
A stone has a mass of 156 g. When it is lowered into a eureka can, 60 cm³ of water flows out. Calculate the density of the stone.
Mark scheme
volume of the stone = volume of water displaced = 60 cm³ (1); ρ = m ÷ V = 156 ÷ 60 (1); 2.6 g/cm³ (1)
Model answer
Volume = 60 cm³. Density = 156 / 60 = 2.6 g/cm³.
A student uses a ruler to measure the sides of a cube of 10 mm. Another student measures a cube of 50 mm. Explain why the percentage uncertainty in the volume is greater for the smaller cube.
Mark scheme
the uncertainty in each length (about ± 0.5 mm) is the same for both (1); this is a much larger percentage of a 10 mm length than of a 50 mm length (1); the uncertainties in three lengths combine, so the volume has a larger percentage uncertainty (1)
Model answer
The ruler has the same uncertainty of about 0.5 mm for each length, but this is a much bigger percentage of 10 mm than of 50 mm. The volume uses three lengths, so the percentage uncertainty is bigger for the small cube.
A student finds that the density of a salt solution is 0.95 g/cm³, but it should be more than 1.0 g/cm³. Suggest two possible causes of the error.
Mark scheme
the balance was not zeroed / the mass of the empty cylinder was not subtracted correctly (1); the volume was read incorrectly / not at eye level / parallax (1); some of the liquid was spilt (1)
Model answer
The balance may not have been zeroed, giving a mass that was too low. The volume may have been read from the wrong level on the measuring cylinder, giving a value that was too high.
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