Pure and Impure SubstancesIB MYP Chemistry: Topic test

20 questions, 54 marks

IB MYP Chemistry

Pure and Impure Substances topic test

Total 54 marks

Name

Class

Date

1
A student in Hanoi is given samples of copper wire, pure water, table salt (sodium chloride) and sea water. She has to classify each sample as an element, a compound or a mixture.
(a)
Which sample is an element?
[1 mark]
APure water
BTable salt
CSea water
DCopper wire
(b)
Which statement about the sea water is correct?
[1 mark]
AIt is a compound because the water and salts are joined by chemical bonds
BIt is a mixture because the substances in it are not chemically joined and can be separated by physical methods
CIt is an element because it is a single liquid
DIt is a compound because it has a fixed composition
(c)
Explain why pure water is a compound and not an element or a mixture.
[2 marks]
Total for question 1: 4 marks
2
A silversmith in Mexico City makes jewellery from sterling silver, an alloy that is 92.5% silver and 7.5% copper by mass. Pure silver is too soft for rings and bracelets, so a little copper is added.
(a)
What is an alloy?
[1 mark]
AA compound of a metal with oxygen
BA pure metal made of one type of atom
CA mixture of a metal with one or more other elements
DA solution of a metal in water
(b)
Why is sterling silver a mixture and not a compound?
[1 mark]
AThe silver and copper are not chemically bonded and the proportions can vary
BIt contains more than one element
CIt melts at a single temperature
DIts properties are completely different from silver and copper
(c)
A sterling silver ring has a mass of 40.0 g. Calculate the mass of copper in the ring.
[2 marks]
Total for question 2: 4 marks
3
A student in Tunis has two tasks in the laboratory. For the first task she must separate a muddy mixture of sand and clay particles in water. For the second task she must obtain pure water from a bottle of sea water.
(a)
Describe how she can separate the sand and clay particles from the water in the first task.
[3 marks]
(b)
Describe how she can obtain pure water from the sea water in the second task.
[4 marks]
Total for question 3: 7 marks
4
A textile factory in Dhaka releases waste water that contains dyes into a river. An environmental scientist collects a sample of river water downstream and compares it with three known dyes, A, B and C, using paper chromatography. The solvent front travels 12.0 cm from the start line. The known dyes have Rf values of 0.30, 0.62 and 0.85 respectively. The river sample separates into two spots, with Rf values of 0.30 and 0.62. A new treatment plant for the factory's waste water would cost millions of dollars but would remove most of the dye. The factory manager says the dyes in the river may come from other sources.
(a)
Interpret the results of the chromatography, calculate how far the spot with an Rf value of 0.62 moved from the start line, and evaluate how far they show which dyes are in the river water.
[6 marks]
(b)
Evaluate whether the factory should be required to install a treatment plant that removes dyes from its waste water.
[6 marks]
Total for question 4: 12 marks
5
A pharmacist in Nairobi tests a sample of aspirin powder. A data book says that pure aspirin melts at exactly 135 °C. When the pharmacist heats the sample slowly, it starts to melt at 128 °C and is completely liquid by 133 °C.
(a)
What would be expected when the melting point of a pure solid is measured?
[1 mark]
AIt melts at one sharp temperature that matches the data-book value
BIt melts slowly over a range of several degrees
CIt melts at a temperature below the data-book value
DIt melts at a temperature above the data-book value
(b)
What is the effect of impurities on the melting point of a solid?
[1 mark]
AThey raise the melting point and make it sharper
BThey have no effect on the melting point
CThey lower the melting point and make the solid melt over a range of temperatures
DThey raise the melting point and make the solid melt over a range
(c)
Explain why the pharmacist concludes that the sample is not pure aspirin.
[2 marks]
Total for question 5: 4 marks
6
A technician in Lima adds sodium chloride to 100 g of water at 25 °C in four portions of 10 g, stirring after each portion. The first three portions dissolve completely. After the fourth portion some solid remains at the bottom of the beaker, even after long stirring. A data book gives the solubility of sodium chloride at 25 °C as 36 g per 100 g of water.
(a)
Which term describes the sodium chloride in this mixture?
[1 mark]
ASolvent
BSolute
CSolution
DSuspension
(b)
Which describes the liquid in the beaker at the end?
[1 mark]
AAn unsaturated solution, because the salt has dissolved
BAn emulsion, because the salt is mixed with water
CA pure substance, because it contains only sodium chloride and water
DA saturated solution, because no more solid can dissolve at this temperature
(c)
Calculate the mass of sodium chloride that remains undissolved.
[2 marks]
Total for question 6: 4 marks
7
A student in Mumbai uses paper chromatography to separate the pigments in a spinach leaf extract. She draws a pencil line 1.0 cm from the bottom of the paper, puts a small spot of extract on the line and stands the paper in a beaker with the solvent level below the pencil line. When the solvent front has moved 9.0 cm from the pencil line, she removes the paper. A yellow-orange spot has moved 7.2 cm from the pencil line and a green spot has moved 4.5 cm. A data book gives the Rf value of carotene, a yellow-orange pigment, in this solvent as 0.80.
(a)
Calculate the Rf value of the green pigment.
[3 marks]
(b)
Explain why the start line is drawn in pencil and above the level of the solvent, and use the data to decide whether the yellow-orange pigment could be carotene.
[4 marks]
Total for question 7: 7 marks
8
A company on the shore of the Dead Sea in Jordan obtains potassium salts and other minerals by letting lake water flow into large, shallow pools, where the sun evaporates the water and crystals form. The company wants to find out how the temperature of the water affects the mass of potassium chloride that dissolves. Its work brings jobs and export income, and potassium salts are used in fertilisers, but the level of the lake is falling and the pools cover large areas of land.
(a)
Plan an investigation to find out how the temperature of the water affects the mass of potassium chloride that dissolves in 100 g of water. Include a hypothesis with a reason, the variables, the method and a safety precaution.
[6 marks]
(b)
Evaluate whether the company should expand its evaporation pools.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).