Pure Substances, Mixtures and Separation — Free IB MYP4 Chemistry Questions

1QuestionDefinition of mixtures and types (homogeneous vs heterogeneous)Concept Practice

2 marks~3 minCriterion A

A technician prepares two samples in a school laboratory. Sample A is a saltwater solution that appears clear and uniform throughout. Sample B is a mixture of olive oil and water, which separates into two distinct visible layers.

a

Identify which sample is a homogeneous mixture and which is a heterogeneous mixture. [1]

b

Explain why Sample B cannot be classified as a homogeneous mixture, even if the layers are briefly shaken together. [1]

Solutions

2QuestionReal-life examples (milk, fog, muddy water)Concept Practice

2 marks~3 minCriterion A

A diagram shows three samples: a glass of milk with a visible light beam passing through it, a beaker of muddy water with particles settled at the bottom, and a jar of fog with a visible light beam passing through it.

a

Identify which sample is a suspension. [1]

b

The diagram shows a light beam passing through both milk and fog. Deduce what this observation reveals about the nature of the particles in these two samples. [1]

Solutions

3QuestionInterpreting solubility curves (basic graph reading)Concept Practice

2 marks~3 minCriterion A

The solubility of

KNO_3

at 50°C is 80 g per 100 g of water.

A student adds 60 g of

KNO_3

(the solute) to 100 g of water (the solvent) at 50°C.

Using the terms solute, solvent, saturated, unsaturated, and solution, explain what happens when the

KNO_3

is added to the water. [2]

Solutions

4QuestionFiltration process (separating insoluble solids)Concept Practice

2 marks~3 minCriterion A

A municipal water treatment plant filters river water through layers of sand and gravel to remove suspended solids such as silt and clay before disinfection.

a

State the physical principle that allows filtration to separate suspended solids from river water. [1]

b

Explain one reason why filtration alone is insufficient to make river water safe for drinking. [1]

Solutions

5QuestionSimple distillation (separating solvent from solution)Concept Practice

3 marks~5 minCriterion B

A chemist needs to recover a solvent from a salt solution using simple distillation. The boiling points of three candidate solvents at standard atmospheric pressure are given below.

Liquid	Boiling point (°C)
Water	100
Ethanol	78
Propanone	56

a

State the trend in boiling points shown in the data. [1]

b

Identify which liquid would be most easily separated from a salt solution by simple distillation. [1]

c

Explain why a lower boiling point makes a liquid easier to separate from a salt solution using simple distillation. [1]

Solutions

6QuestionFractionating column and temperature gradientConcept Practice

2 marks~3 minCriterion A

A fractionating column separates crude oil using a temperature gradient:

350^\circ\text{C}

at the base and

40^\circ\text{C}

at the top. A mixture of hydrocarbon vapours enters at the bottom and rises through the column.

a

Explain how the temperature gradient drives vaporization and condensation as hydrocarbons rise through the column. [1]

b

Explain why different hydrocarbons condense at different levels within the column. [1]

Solutions

7QuestionBasic principle of chromatography (mobile vs stationary phase)Concept Practice

2 marks~3 minCriterion A

A student sets up a paper chromatography experiment to separate the dyes in a food colouring. The chromatography paper is clipped vertically inside a sealed glass beaker containing a small volume of ethanol as the solvent. The paper's lower edge dips into the ethanol, but the food colouring spot remains above the liquid surface.

a

Identify the mobile phase and the stationary phase in this setup. [1]

b

The student seals the beaker before starting the experiment. Explain why a sealed environment is important for obtaining reliable chromatography results. [1]

Solutions

8QuestionDefinition of pure substances (elements vs compounds)Assessment Practice

8 marks~12 minCriterion B

The table below shows the mass of oxygen that combines with 1.00 g of nitrogen in three nitrogen–oxygen compounds.

Compound	$\text{N}_2\text{O}$	$\text{NO}$	$\text{NO}_2$
Mass of O per 1.00 g N	0.571 g	1.14 g	2.28 g

a

Deduce the pattern in the masses of oxygen combining with 1.00 g of nitrogen across the three compounds. [2]

b

Calculate the mass of oxygen that combines with 1.00 g of nitrogen in

\text{N}_2\text{O}_3

, showing all working. [3]

c

Analyse how the oxygen masses for all four compounds —

\text{N}_2\text{O}

,

\text{NO}

,

\text{N}_2\text{O}_3

, and

\text{NO}_2

— support the law of multiple proportions and atomic theory. [3]

Solutions

9QuestionPhysical vs chemical properties in identifying purityAssessment Practice

5 marks~8 minCriterion D

A white powder is discovered near an abandoned factory. It may be pure table salt (

\text{NaCl}

, melting point

801 \,°\text{C}

) or sodium fluoride (

\text{NaF}

, melting point

993 \,°\text{C}

), a toxic industrial compound. A scientist measures the powder's melting point as

801 \,°\text{C}

and observes that melting occurs over a sharp, narrow temperature range.

a

Deduce the identity and purity of the powder from the melting point data. [2]

b

Explain why a melting point measurement alone is insufficient to guarantee the powder is safe for use in food preparation. [3]

Solutions

10QuestionPhysical vs chemical properties in identifying purityAssessment Practice

3 marks~5 minCriterion C

A sample of an unknown white solid is heated steadily. Its temperature rises, holds constant at

80^\circ\text{C}

for several minutes, then rises again, as shown in the graph.

a

Identify what the plateau at

80^\circ\text{C}

represents. [1]

b

Explain why a pure substance produces a plateau at a single fixed temperature, whereas an impure substance does not. [1]

c

Analyse how the graph provides evidence that melting point, as a physical property, can be used to assess the purity of this sample. [1]

Solutions

11QuestionParticle size differences (solution vs colloid vs suspension)Assessment Practice

6 marks~9 minCriterion B

Four mixtures were tested under identical conditions (still water, 20 °C). Results are shown below.

Mixture — Starch in water	Particle diameter (nm): $0.1$	Settling: No settling after 24 h
Mixture — Clay in water	Particle diameter (nm): $100$	Settling: No settling after 24 h
Mixture — Sand in water	Particle diameter (nm): $5000$	Settling: 15 seconds
Mixture — Milk	Particle diameter (nm): $800$	Settling: No settling after 24 h

a

Classify each mixture as a solution, colloid, or suspension. Justify each classification using both particle diameter and settling behaviour. [2]

b

Deduce a general rule linking particle diameter to mixture type, using the pattern shown in the data above. [2]

c

A student dissolves gelatin in hot water and allows it to cool. The gelatin particles have an average diameter of

300

nm. Evaluate whether this mixture will behave as a solution, colloid, or suspension, and identify one condition that, if changed, could cause the mixture to settle visibly. [2]

Solutions

12QuestionDefinition and properties of solutions (homogeneous mixtures)Assessment Practice

3 marks~5 minCriterion C

The graph below shows the solubility of potassium chloride (

KCl

), sodium chloride (

NaCl

), and potassium nitrate (

KNO_3

) in water across a temperature range of 0 °C to 80 °C.

Temperature (°C)	0	20	40	60	80
$KCl$ (g / 100 g $H_2O$ )	28	34	40	45	51
$NaCl$ (g / 100 g $H_2O$ )	35	36	37	37	38
$KNO_3$ (g / 100 g $H_2O$ )	13	32	64	110	169

a

Describe the trend in solubility for each solute as temperature increases. [1]

b

Identify which solute has the greatest solubility at 20 °C and state its value. [1]

c

A chemist claims that a higher solubility at room temperature means a more stable homogeneous mixture can be formed. Evaluate this claim using the data provided. [1]

Solutions

13QuestionReal-life examples (milk, fog, muddy water)Assessment Practice

5 marks~8 minCriterion D

A student hypothesises that milk, fog, and muddy water are all true solutions because they appear uniform to the naked eye. The following data were collected to test this hypothesis.

Sample — Milk — Fog — Muddy water

Particle size (nm): 100	200	5000
Sedimentation time (h): >24	>24	0.5
Filter paper retention: none	none	all particles

a

Define the terms solution, colloid, and suspension using particle size ranges. [1]

b

Using the particle size data, deduce the classification of each sample. [2]

c

Evaluate whether the experimental evidence supports or refutes the student's hypothesis. Justify your conclusion by referring to all three data columns. [2]

Solutions

14QuestionInterpreting solubility curves (basic graph reading)Assessment Practice

2 marks~3 minCriterion B

The solubility of potassium nitrate (

KNO_3

) at selected temperatures is given below.

Temperature (°C)	10	20	30	50	60
Solubility (g per 100 g water)	21	32	46	85	110

The solubility at 40°C is not recorded.

a

Calculate the solubility of

KNO_3

at 40°C by interpolating between the data points at 30°C and 50°C. [1]

b

The data show that solubility does not increase by equal amounts for each 10°C rise. Deduce what this non-linear pattern suggests about the relationship between temperature and solubility of

KNO_3

. [1]

Solutions

15QuestionInterpreting solubility curves (basic graph reading)Assessment Practice

6 marks~9 minCriterion C

The solubility of potassium nitrate (

KNO_3

) in water was measured at regular temperature intervals. Results are shown below.

Temperature (°C): 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100

Solubility (g per 100 g water): 13.3, 20.9, 31.6, 45.8, 63.9, 63.9, 110.0, 138.0, 169.0, 202.0, 246.0

a

Identify the anomalous data point in the table and deduce the solubility value that would be expected at 50°C based on the trend between 40°C and 60°C. [2]

b

Explain one experimental error that could have produced this anomaly, describing how it would affect the measured solubility. [2]

c

A student claims that the solubility of

KNO_3

at 45°C is approximately 75 g per 100 g water. Evaluate this claim using the corrected data trend, showing your reasoning clearly. [2]

Solutions

16QuestionSaturated, unsaturated, and supersaturated solutionsAssessment Practice

2 marks~3 minCriterion D

A reusable hand warmer contains a supersaturated solution of sodium acetate (

\text{NaC}_2\text{H}_3\text{O}_2

). Clicking a metal disc inside the pack triggers rapid crystallization, releasing heat that warms the user's hands. The warmer can be reset by placing it in boiling water until all crystals dissolve.

a

Explain why the crystallization of the supersaturated sodium acetate solution releases heat energy. [1]

b

Evaluate one limitation of this hand warmer as a practical heat source for emergency situations. [1]

Solutions

17QuestionCrystallization for obtaining pure solidsAssessment Practice

3 marks~5 minCriterion B

A student purifies copper(II) sulfate by crystallization, varying the cooling rate of a saturated solution. The results are shown below.

Cooling rate (°C/min)	1	5	15	30
Mass of purified crystals (g)	42.5	38.0	30.5	22.0
Percentage yield (%)	85.0	76.0	61.0	44.0

In each trial, 50.0 g of crude copper(II) sulfate was used.

a

Describe the trend in percentage yield as cooling rate increases. [1]

b

Explain why slower cooling produces larger crystals. [1]

c

Explain why slower cooling produces purer crystals, and evaluate whether maximising crystal purity always means maximising percentage yield. [1]

Solutions

18QuestionChoosing appropriate method based on mixture typeAssessment Practice

6 marks~9 minCriterion D

A mining company is evaluating two methods for extracting lithium from brine in a desert region where freshwater is scarce and the local population depends on groundwater for drinking and farming.

Method A (solar evaporation): 5000 hectares of land; 2 million

\text{m}^3

of water lost annually; carbon footprint of 10 kg

\text{CO}_2

per kg of lithium; 50 000 tonnes of salt waste per year.

Method B (forced evaporation and crystallization): 10 hectares of land; 0.5 million

\text{m}^3

of water annually (most recycled); carbon footprint of 200 kg

\text{CO}_2

per kg of lithium; 10 000 tonnes of salt waste per year, requiring chemical treatment.

a

Identify one environmental advantage and one environmental disadvantage of each method. [2]

b

Evaluate the societal impacts of each method on the local community, considering water security, health risks, and economic opportunities. [2]

c

Evaluate the trade-offs between the two methods and reflect on the limitations of concluding that one method is universally better without considering local conditions. [2]

Solutions

19QuestionFiltration process (separating insoluble solids)Assessment Practice

5 marks~8 minCriterion C

A student investigates how filter paper pore size affects the mass of sand residue collected during filtration. In each trial, 100 mL of a sand–water mixture is filtered for exactly 2 minutes.

Pore size ( $\mu$ m)	2	5	10
Mass of residue (g)	15.2	12.8	9.5

The student expects mass of residue to increase as pore size decreases.

a

Identify the anomalous data point and suggest one experimental cause. [2]

b

Describe the relationship between pore size and mass of residue collected, and explain the particle-level reason for this trend. [2]

c

Evaluate whether the 2

\mu

m filter paper would be the most suitable choice for routine laboratory filtration of sand–water mixtures. [1]

Solutions

20QuestionBoiling point differences in separationAssessment Practice

6 marks~9 minCriterion D

A biofuel plant uses fractional distillation to separate ethanol (boiling point

78\,^{\circ}\text{C}

) from a fermentation broth containing water (boiling point

100\,^{\circ}\text{C}

) and other organic compounds. The distillation column is heated continuously by burning natural gas.

a

Explain how the difference in boiling points between ethanol and water enables their separation by fractional distillation. [2]

b

Analyse how the energy requirement of the distillation column influences the overall carbon footprint of the biofuel plant. [2]

c

Evaluate whether fractional distillation powered by natural gas can be considered a genuinely sustainable method for biofuel production. [2]

Solutions

21QuestionBoiling point differences in separationAssessment Practice

5 marks~8 minCriterion C

A student performs fractional distillation on a crude oil sample and collects three fractions, recording their boiling point ranges:

Fraction A:

70\text{–}85\ °C

Fraction B:

120\text{–}140\ °C

Fraction C:

340\text{–}400\ °C

Reference boiling points of pure alkanes:
Octane (

C_8H_{18}

):

125.6\ °C

Hexadecane (

C_{16}H_{34}

):

287\ °C

The student hypothesises that Fraction B is pure octane.

a

State the boiling point of pure octane and explain what a fixed, single boiling point indicates about the purity of a substance. [1]

b

Using the data provided, deduce whether Fraction B could contain hexadecane. Justify your answer with reference to the boiling point values. [2]

c

Evaluate the student's hypothesis that Fraction B is pure octane, using the boiling point range of Fraction B as evidence. [2]

Solutions

22QuestionApplications (forensics, food testing, drug analysis)Assessment Practice

6 marks~9 minCriterion B

A forensic scientist examines five ink samples from a forged document using paper chromatography in two solvents: water (polar) and ethanol (less polar).

Spots in water: A = 1, B = 2, C = 1, D = 3, E = 2

R_f

values in water: A = 0.45; B = 0.30 and 0.60; C = 0.70; D = 0.25, 0.50, and 0.55; E = 0.20 and 0.35

Spots in ethanol: A = 1, B = 1, C = 1, D = 2, E = 1

R_f

values in ethanol: A = 0.72; B = 0.55; C = 0.80; D = 0.40 and 0.62; E = 0.68

a

Deduce which ink samples are pure substances and which are mixtures, using evidence from both solvents. [2]

b

Analyse the

R_f

data for ink sample B across both solvents and generalise a rule linking solvent polarity to the effectiveness of chromatographic separation. [2]

c

Evaluate whether hexane (a non-polar solvent) would be suitable for identifying the individual components of ink sample D. Use your rule from part (b) and the data for sample D to support your judgement. [2]

Solutions

23QuestionBasic principle of chromatography (mobile vs stationary phase)Assessment Practice

5 marks~8 minCriterion C

A student separates three ink samples (X, Y, Z) by paper chromatography using water and then ethanol as separate mobile phases. The stationary phase is identical in both runs.

Water as mobile phase —

R_f

values: X = 0.2, Y = 0.7, Z = 0.2

Ethanol as mobile phase —

R_f

values: X = 0.6, Y = 0.3, Z = 0.6

The student hypothesises: "All three inks contain the same dye."

a

State the principle that explains why different compounds travel different distances in chromatography. [1]

b

Deduce, using both sets of

R_f

data, which inks could contain the same dye and which could not. [2]

c

Evaluate the student's hypothesis, referring to the role of both mobile phases in supporting your conclusion. [2]

Solutions

24QuestionInterpreting chromatograms (Rf values – basic idea)Assessment Practice

6 marks~9 minCriterion D

A pharmaceutical company uses paper chromatography to assess the purity of a batch of ibuprofen. A reference standard of pure ibuprofen gives an

R_f

value of

0.52

under standard conditions. A technician tests a sample from the new batch and records an

R_f

value of

0.45

for the spot believed to be ibuprofen. The technician relies solely on this

R_f

value to confirm the compound's identity and approve the batch for patient use.

a

Explain why the difference between

R_f = 0.45

and

R_f = 0.52

may indicate the presence of an impurity in the batch. [2]

b

Discuss the risks to patient safety if the compound at

R_f = 0.45

is incorrectly approved as pure ibuprofen. [2]

c

Evaluate the reliability of using

R_f

values alone to confirm the identity of a compound in a pharmaceutical purity test. [2]

Solutions

Pure Substances, Mixtures and Separation — Free MYP4 Chemistry Practice Questions