You're viewing free preview questions. Upgrade to access more MYP5 questions.Upgrade

Pure Substances, Mixtures and Separation

Pure Substances, Mixtures and Separation — Free MYP5 Chemistry Practice Questions

1QuestionCharacteristics of pure substances (fixed melting/boiling points)Concept Practice

2 marks~3 minCriterion A

The heating curve below shows a sample of pure water heated from

-10^\circ\text{C}

120^\circ\text{C}

. A flat plateau is observed at

0^\circ\text{C}

Identify what the plateau at

0^\circ\text{C}

represents, and state what this fixed temperature indicates about the purity of the sample. [1]

Explain why the temperature remains constant during this plateau, referring to energy transfer and particle behaviour. [1]

Solutions

2QuestionCharacteristics of colloids (Tyndall effect – light scattering)Concept Practice

2 marks~3 minCriterion A

A laser beam is directed through three beakers in sequence.

Beaker X contains salt water (a true solution).
Beaker Y contains milk (a colloid).
Beaker Z contains muddy water (a suspension).

The laser beam is clearly visible as a bright path inside one beaker only.

Identify which beaker shows the Tyndall effect and state the name of the optical phenomenon responsible for the visible beam path. [1]

Explain why the laser beam is not visibly scattered in Beaker X, even though salt is dissolved in the water. [1]

Solutions

3QuestionDefinition of solubility (maximum solute in solvent)Concept Practice

2 marks~3 minCriterion A

A beaker contains water and excess solid potassium nitrate (

KNO_3

) at 60°C. Undissolved solid is visible at the bottom of the beaker.

Identify the term that describes the maximum mass of

KNO_3

that can dissolve in 100 g of water at 60°C. [1]

The beaker is heated to 80°C and the solid disappears. Deduce what this observation tells you about how temperature affects the dissolving of

KNO_3

. [1]

Solutions

4QuestionChoosing appropriate method based on mixture typeConcept Practice

2 marks~3 minCriterion A

A student sets up a filtration apparatus to separate sand from a sand–water mixture. The diagram shows the complete setup, including a retort stand, a glass rod, a beaker, and the filtration components.

State the name of the piece of equipment that directly supports the filter paper during filtration. [1]

Explain why filtration is a suitable separation method for this mixture rather than evaporation. [1]

Solutions

5QuestionApplications (petroleum refining, water purification)Concept Practice

2 marks~3 minCriterion A

A fractionating column used in petroleum refining maintains a temperature gradient: approximately 350 °C at the base and 40 °C at the top.

Refer to the diagram of the fractionating column provided.

Identify where in the column the fraction with the highest boiling point is collected. [1]

Explain why this fraction collects at that location rather than higher up the column. [1]

Solutions

6QuestionApplications (forensics, food testing, drug analysis)Concept Practice

2 marks~3 minCriterion A

A paper chromatogram of a sports drink shows three spots labelled A, B, and C. The solvent front travelled 8.0 cm from the baseline. Spot A travelled 2.4 cm, spot B travelled 3.6 cm, and spot C travelled 6.0 cm.

A reference table states that caffeine has an

R_f

value of

0.45

under identical conditions.

Calculate the

R_f

value for each spot. [1]

Deduce which spot corresponds to caffeine, justifying your answer using your calculated

R_f

values. [1]

Solutions

7QuestionDefinition of mixtures and types (homogeneous vs heterogeneous)Assessment Practice

3 marks~5 minCriterion B

Four mixtures are analysed in a food-science laboratory. Their properties are recorded below.

Mixture — Salt water / Sand in water / Air / Oil and vinegar

Appearance	Clear / Cloudy / Clear / Two distinct layers
Number of phases	1 / 2 / 1 / 2

Particle size — Molecular / Visible grains / Molecular / Visible droplets

State the classification (homogeneous or heterogeneous) of each mixture. [1]

Deduce the relationship between the number of phases observed and the type of mixture. [1]

Evaluate whether the number of phases alone is a sufficient criterion for classifying a mixture, using evidence from the data above to support your reasoning. [1]

Solutions

8QuestionCharacteristics of pure substances (fixed melting/boiling points)Assessment Practice

3 marks~5 minCriterion C

A student heats a liquid sample and records its temperature every minute. The temperature rises steadily until it reaches

78^\circ\text{C}

, where it remains constant for several minutes before rising again.

Identify the phase change occurring at

78^\circ\text{C}

. [1]

Explain why a pure substance produces a plateau at a single fixed temperature rather than boiling over a temperature range. [1]

A second liquid sample boils between

78^\circ\text{C}

and

84^\circ\text{C}

with no clear plateau. Evaluate what this difference reveals about the purity of each sample. [1]

Solutions

9QuestionCharacteristics of pure substances (fixed melting/boiling points)Assessment Practice

2 marks~3 minCriterion D

A pharmaceutical company tests a batch of paracetamol for purity. Pure paracetamol has a melting point range of

169\,^{\circ}\mathrm{C}

171\,^{\circ}\mathrm{C}

Describe how the company could use melting point data to assess whether the batch is pure. [1]

The measured melting point of the batch is

163\,^{\circ}\mathrm{C}

. Evaluate one health risk this result poses to a patient taking the tablets. [1]

Solutions

10QuestionParticle 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

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

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

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

11QuestionCharacteristics of colloids (Tyndall effect – light scattering)Assessment Practice

3 marks~5 minCriterion C

A laser beam is directed through two beakers. Beaker A contains a clear aqueous sodium chloride solution; Beaker B contains a dilute milk–water mixture. The beam is invisible inside Beaker A but traces a bright, visible path through Beaker B.

Identify the phenomenon responsible for the visible beam in Beaker B. [1]

Explain, with reference to particle size, why the beam is visible in Beaker B but not in Beaker A. [1]

A student claims that adding more salt to Beaker A would eventually make the beam visible. Evaluate this claim. [1]

Solutions

12QuestionCharacteristics of colloids (Tyndall effect – light scattering)Assessment Practice

6 marks~9 minCriterion D

A student shines a green laser pointer through three water samples collected from a local river: one taken upstream of an industrial site, one at the site, and one downstream. A visible beam appears in the at-site and downstream samples but not in the upstream sample.

Identify the type of mixture present in the at-site and upstream samples, based on the laser observations. [1]

Explain how the presence or absence of a visible laser beam in each sample indicates whether colloidal pollutants are present. [2]

Analyse the limitations of using the Tyndall effect alone to determine whether the colloidal particles detected at the industrial site pose a risk to human health. [3]

Solutions

13QuestionInterpreting 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.

Calculate the solubility of

KNO_3

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

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

14QuestionDefinition of solubility (maximum solute in solvent)Assessment Practice

6 marks~9 minCriterion D

A town applies road salt (sodium chloride,

\text{NaCl}

) to icy roads each winter. Meltwater carries dissolved salt into a nearby freshwater lake. The solubility of

\text{NaCl}

in water at

0^\circ\text{C}

is approximately

357 \text{ g per kg of water}

Explain how

\text{NaCl}

dissolving in the meltwater increases the salinity of the lake. [2]

Apply your understanding of solubility to explain why the solubility value alone does not allow accurate prediction of the salt concentration in the lake during summer. [2]

Discuss the limitations of using solubility data alone to assess the long-term environmental impact of salt runoff on the lake ecosystem, considering both physical and biological factors. [2]

Solutions

15QuestionDefinition of solubility (maximum solute in solvent)Assessment Practice

3 marks~5 minCriterion C

The table below shows the solubility of potassium nitrate (

KNO_3

) in water at four temperatures.

Temperature (°C)	20	40	60	80
Solubility (g per 100 g water)	32	64	110	169

State the trend in solubility of

KNO_3

as temperature increases from 20°C to 80°C. [1]

Explain how increasing temperature affects the kinetic energy of particles and their ability to overcome intermolecular forces. [1]

A student claims that doubling the temperature always doubles the solubility of

KNO_3

. Using the data, evaluate this claim. [1]

Solutions

16QuestionCrystallization 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.

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

Explain why slower cooling produces larger crystals. [1]

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

Solutions

17QuestionEvaporation to obtain dissolved solidsAssessment Practice

3 marks~5 minCriterion C

A student monitors the volume of water remaining in two open containers — a wide beaker and a narrow conical flask — over six hours. Both containers start with 50 mL of water and are left in identical conditions. The graph shows two curves, A and B, plotting volume (mL) against time (hours).

Describe the trend shown by both curves. [1]

Identify which curve corresponds to the wide beaker and which to the narrow conical flask. Justify your answer using the concept of surface area. [1]

A student claims that temperature, not surface area, is the main factor causing the difference between curves A and B. Evaluate this claim. [1]

Solutions

18QuestionChoosing appropriate method based on mixture typeAssessment Practice

6 marks~9 minCriterion D

A paint manufacturer tests two solvents to dissolve a pigment: water and a volatile organic compound (VOC). The water-based mixture is evaporated in an open tank, releasing only water vapour. The VOC-based mixture must be evaporated inside a closed system fitted with a fume hood to capture toxic vapours; this system carries higher operational and maintenance costs. VOC vapours that escape contribute to ground-level ozone formation and smog.

Explain why evaporation is a suitable separation method for recovering the pigment from either mixture. [2]

Discuss the environmental and health implications of choosing the VOC solvent rather than water. [2]

Evaluate the limitation of relying on evaporation alone to protect workers from hazardous solvent vapours in the workplace. [2]

Solutions

19QuestionApplications (petroleum refining, water purification)Assessment Practice

5 marks~8 minCriterion B

The table below shows data for five fractions obtained from the fractional distillation of crude oil.

Fraction	A	B	C	D	E
Boiling point range (°C)	30–100	100–175	175–275	275–350	350–400
Main carbon chain length (C atoms)	$\text{C}_4$ – $\text{C}_8$	$\text{C}_6$ – $\text{C}_{12}$	$\text{C}_{11}$ – $\text{C}_{16}$	$\text{C}_{15}$ – $\text{C}_{20}$	$\text{C}_{19}$ – $\text{C}_{30}$

Kerosene, used as jet fuel, has a boiling point range of 150–275 °C and contains hydrocarbons with 10 to 16 carbon atoms.

Deduce which fraction (A–E) is most likely to be kerosene. [1]

Explain, using the data in the table, how boiling point is related to carbon chain length in these hydrocarbon fractions. [2]

Explain, at the molecular level, why this relationship between carbon chain length and boiling point exists. [2]

Solutions

20QuestionFractionating column and temperature gradientAssessment Practice

6 marks~9 minCriterion D

During the fractional distillation of a crude oil mixture, temperatures were recorded at five heights along the fractionating column.

Height from base (cm)	0	20	40	60	80
Temperature (°C)	350	280	210	150	40

Known boiling points of hydrocarbon fractions:
Fuel oil: 340 °C
Diesel: 290 °C
Kerosene: 180 °C
Gasoline: 70 °C

Deduce which hydrocarbon fraction condenses at a height of 20 cm. [2]

The temperature at 40 cm is 210 °C. Justify why no single fraction condenses cleanly at this height, and explain what this indicates about the separation occurring there. [2]

Analyse the temperature reading of 40 °C at 80 cm. Identify the anomaly and evaluate one specific flaw in the column setup that could account for it. [2]

Solutions

21QuestionSimple distillation (separating solvent from solution)Assessment Practice

6 marks~9 minCriterion C

A student performed a simple distillation of seawater to obtain pure water. She recorded the temperature of the vapour every 30 s. The data are shown below.

Time (s): 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330, 360

Temperature (°C): 22, 48, 72, 89, 98, 100, 100, 100, 100, 100, 103, 105, 107

Identify the time interval during which the temperature remained constant and explain why it remained constant at that value. [2]

Explain why the temperature rose above 100 °C after 270 s, referring to the composition of the liquid remaining in the distillation flask. [2]

Analyse the full data set to determine whether the distillate collected over the entire distillation is pure water. Justify your conclusion with reference to specific values from the data. [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

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

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]

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

23QuestionApplications (forensics, food testing, drug analysis)Assessment Practice

8 marks~12 minCriterion C

A forensic chemist investigates a suspect document by analysing three ink samples (X, Y, Z) using thin-layer chromatography (TLC). The stationary phase is silica gel; the mobile phase is hexane:ethyl acetate (9:1). Reference

R_f

values: caffeine

0.12

, paracetamol

0.28

, aspirin

0.55

, codeine

0.47

. The solvent front travels

8.0

cm; the codeine reference spot travels

3.76

cm. Observed spots: Sample X — one spot at

R_f\ 0.12

; Sample Y — one spot at

R_f\ 0.45

; Sample Z — two spots at

R_f\ 0.28

and

0.55

Show that the

R_f

value of the codeine reference standard is consistent with

0.47

. [2]

Explain how the polarity of the stationary and mobile phases accounts for the relative positions of caffeine (

R_f\ 0.12

) and aspirin (

R_f\ 0.55

) on the chromatogram. [2]

Justify whether Sample Y can be reliably identified as codeine, using

R_f

evidence, the possibility of co-elution, and at least one confirmatory strategy. [4]

Solutions

24QuestionApplications (forensics, food testing, drug analysis)Assessment Practice

6 marks~9 minCriterion D

A forensic laboratory uses thin-layer chromatography (TLC) to analyse ink from ransom notes. Polar solvents such as ethyl acetate and methanol are used as the mobile phase; both are toxic and must be disposed of as hazardous waste. Fume hoods operate continuously to protect technicians and consume significant electrical energy. TLC separates ink components by their differing affinities for the stationary and mobile phases, producing an Rf value unique to each component.

Discuss the societal benefits of using TLC in forensic ink analysis. [2]

Analyse the environmental costs arising from solvent use and energy consumption in this procedure. [2]

Evaluate the overall trade-off between the societal benefits and environmental costs of TLC in forensic contexts, considering the limitations of the technique. [2]

Solutions