Density and pressure — Paper 2 · A Level Marine Science 9693

2 A student investigates the properties of water of different salinities. (a) Describe how the student makes water samples of different salinities. Include the equipment they should use. … … … … … … [3] (b) The student investigates the effect of salinity on the freezing point of water. They use a freezer to freeze each of their water samples. The temperature of the freezer can be adjusted to within 0.1 °C, down to –20 °C. (i) Suggest how the student could use the freezer to obtain reliable results. … … … … [2] (ii) The student designed Table 2.1 for recording their results. Table 2.1 salinity of sample temperature / °C Suggest two improvements that could be made to Table 2.1. 1 … … 2 … … [2] (iii) Predict the relationship between salinity and freezing point that the student would find in this investigation. … … [1] (iv) State the factors that cause the salinity of sea water to change. Describe how the salinity will change for each factor. … … … … … … [3] (c) The student investigates the density of their water samples at each salinity. (i) State how the density of the samples can be measured. Include the correct units for calculating the density. … … … … … … [3] (ii) Use the axes below to sketch the relationship you would expect to find between salinity and density. [2] (iii) Explain the significance of different salinities of sea water having different densities. … … … … [2] [Total: 18]

18 marks

Mark scheme: 2(a) any 3 from: different masses of salt (weighed) ; using balance / weighing scales ; volume of water measured / stated ; using measuring cylinder ; salt (fully) dissolved in water ; OR determine salinity of (sea) water sample / use sample of saturated solution of saline ; measure volume of, sea water / saline ; using measuring cylinder ; diluting with known volume of fresh water ; Question Answer Marks 2(b)(i) any 2 from: freezer initially set at higher temperature / suggested temp, e.g. –1°C ; water samples given time to freeze / suitable suggested time, e.g. two hours ; (if freezing all samples at the same temperature) record time taken to freeze (different) samples / record temperature at which each sample freezes ; temperature gradually reduced ; reference to control variables, e.g. same volume ; use of data loggers ; repeating two more times (for reliability) ; 2 2(b)(ii) any 2 from: ref. to lack units for salinity ; ref. to ‘temperature’ being unspecific, e.g. should be ‘freezing point of sample’ / freezing temperature ; ref. to lack of repeated results / add more trials / add more samples ; 2 2(b)(iii) the greater the salinity the lower the freezing point / lower the salinity the higher the freezing point ; 1 2(b)(iv) any 3 from: precipitation will decrease (salinity) ; melting of sea ice will decrease (salinity) / freezing of sea ice will increase (salinity); run-off will decrease (salinity) / glaciers melting ; evaporation will increase (salinity) ; water from hydrothermal vents will increase (salinity) ; upwelling will increase salinity ; 3 2(c)(i) measure the mass and volume OR use the mass and volume of the water sample ; divide mass by volume ; units are, kg / m3 / kg m –3 3 2(c)(ii) both axes labelled ; any line from bottom left towards top right / bar chart with three or more increasing height bars from left to right; 2 Question Answer Marks 2(c)(iii) more saline water is more dense so will sink (below less saline water) ORA ; PLUS any 1 from: (contributes to) formation of ocean layers ; (contributes to) formation of ocean currents ; ref. to salt water wedges in estuaries ; allows halocline to form ; 2

This question in 9693/21 May/June 2022

2 Fig. 2.1 shows the solubility of oxygen at different temperatures and pressures in sea water at the mean ocean salinity of 35 ‰. 60 50 Key 40 400 kPa dissolved 200 kPa oxygen 30 100 kPa / mg per litre 20 10 0 0 5 10 15 20 25 30 35 40 45 50 temperature / °C Fig. 2.1 (a) Use Fig. 2.1 to describe the relationship between: dissolved oxygen and temperature … … dissolved oxygen and pressure … … [2] (b) The mean temperature of the world’s oceans is 17 °C. Using Fig. 2.1, estimate the mean dissolved oxygen concentration when the water pressure is 200 kPa. Include the correct unit. … [2] (c) Fig. 2.2 shows the concentration of oxygen at different depths at two locations, one in the Atlantic Ocean and one in the Pacific Ocean. dissolved oxygen / mg per litre 0 5 10 0 1000 2000 depth / m 3000 Pacific Atlantic 4000 5000 Fig. 2.2 (i) Describe and explain the similarities between the lines for the two oceans in Fig. 2.2. … … … … … … [3] (ii) Describe and explain the differences between the lines for the two oceans in Fig. 2.2. … … … … … … [3] [Total: 10]

10 marks

Mark scheme: 2(a) (as temperature) increases dissolved oxygen decreases ; (as pressure) increases dissolved oxygen increases ; 2(b) estimate between 15–16 ; mg per litre / mg per l ; 2 2(c)(i) any three from ; concentration highest at surface for both ; due to, surface mixing / photosynthesis (by producers) ; both decrease to depth (of 500 m) ; due to, reduction in photosynthesis / impact of respiration ; both increase gradually beyond 1000 m ; due to increased pressure / decreased temperature ; idea of increase reducing beyond 3000 m ; 3 Question Answer Marks 2(c)(ii) any three from: dissolved oxygen always higher in Atlantic Ocean ; due to increased availability of light / increased light penetration / colder water temperature / increased mixing at surface / increased mixing via ocean conveyor belt / greater rate of photosynthesis / number of producers ; oxygen minimum layer / AW, found deeper in Pacific Ocean ; due to increased light penetration / warmer water in layer ; drop in concentration of oxygen in Pacific is greater ; due to lower population of producers / lower rate of photosynthesis / increased respiration ; 3

This question in 9693/22 May/June 2022

2 Fig. 2.1 shows the solubility of oxygen at different temperatures and pressures in sea water at the mean ocean salinity of 35 ‰. 60 50 Key 40 400 kPa dissolved 200 kPa oxygen 30 100 kPa / mg per litre 20 10 0 0 5 10 15 20 25 30 35 40 45 50 temperature / °C Fig. 2.1 (a) Use Fig. 2.1 to describe the relationship between: dissolved oxygen and temperature … … dissolved oxygen and pressure … … [2] (b) The mean temperature of the world’s oceans is 17 °C. Using Fig. 2.1, estimate the mean dissolved oxygen concentration when the water pressure is 200 kPa. Include the correct unit. … [2] (c) Fig. 2.2 shows the concentration of oxygen at different depths at two locations, one in the Atlantic Ocean and one in the Pacific Ocean. dissolved oxygen / mg per litre 0 5 10 0 1000 2000 depth / m 3000 Pacific Atlantic 4000 5000 Fig. 2.2 (i) Describe and explain the similarities between the lines for the two oceans in Fig. 2.2. … … … … … … [3] (ii) Describe and explain the differences between the lines for the two oceans in Fig. 2.2. … … … … … … [3] [Total: 10]

10 marks

Mark scheme: 2(a) (as temperature) increases dissolved oxygen decreases ; (as pressure) increases dissolved oxygen increases ; 2(b) estimate between 15–16 ; mg per litre / mg per l ; 2 2(c)(i) any 3 from ; concentration highest at surface for both ; due to, surface mixing / photosynthesis (by producers) ; both decrease to depth (of 500 m) ; due to, reduction in photosynthesis / impact of respiration ; both increase gradually beyond 1000 m ; due to increased pressure / decreased temperature ; idea of increase reducing beyond 3000 m ; 3 Question Answer Marks 2(c)(ii) any 3 from: dissolved oxygen always higher in Atlantic Ocean ; due to increased availability of light / increased light penetration / colder water temperature / increased mixing at surface / increased mixing via ocean conveyor belt / greater rate of photosynthesis / number of producers ; oxygen minimum layer / AW, found deeper in Pacific Ocean ; due to increased light penetration / warmer water in layer ; drop in concentration of oxygen in Pacific is greater ; due to lower population of producers / lower rate of photosynthesis / increased respiration ; 3

This question in 9693/23 May/June 2022

3 Scientists collected a series of measurements from a ship travelling across the Atlantic Ocean from locations 1 to 12. They recorded the temperature, concentration of nitrate ions (NO3–) and abundance of phytoplankton at five depths from each of the 12 locations shown in Fig. 3.1. 1212 1111 Africa 1010 9 8 7 6 5 4 3 South America 2 1 Fig. 3.1 (a) State the type of sampling used by the scientists, and describe the benefits of this method. … … … … … … [3] (b) Table 3.1 shows the temperatures recorded at location 4. Table 3.1 depth / m temperature / °C 50 28 60 25 75 20 80 15 170 12 Plot the information shown in Table 3.1 as a line graph. [4] (c) Fig. 3.2 shows the analysis of some of the data that the scientists collected at each location. location 1 2 3 4 5 6 7 8 9 10 11 12 0 depth / m 100 200 Key depth of bottom of thermocline / m depth of greatest concentration of nitrate ions / m depth of greatest population of phytoplankton / m Fig. 3.2 Use the information shown in Table 3.1 and Fig. 3.2 and your own knowledge to discuss the reasons for the distribution of phytoplankton. … … … … … … [3] [Total: 10]

10 marks

Mark scheme: 3(a) systematic / (line / belt), transect ; 3 plus any 2 from: samples taken at regular intervals ; not affected by bias / ensures samples cover the full range of ocean sampled ; idea of change in conditions / environmental factors ; 3(b) axes labels with units ; 4 suitable linear scale ; plotted correctly ± ½ small square ; ruled lines linking points OR line of best fit drawn ; 3(c) any 3 from: 3 (depth of greatest proportion of) phytoplankton similar to / always (closely) above the depth of bottom of thermocline ; idea of water above thermocline is warmer which allows greater, growth / rate of photosynthesis ORA ; depth of (greatest proportion of) phytoplankton corresponds to depth of (greatest concentration of) nitrate ; nitrate needed for growth / named correct biological molecule requiring nitrate ; AVP ;

This question in 9693/21 Oct/Nov 2024

3 Scientists collected a series of measurements from a ship travelling across the Atlantic Ocean from locations 1 to 12. They recorded the temperature, concentration of nitrate ions (NO3–) and abundance of phytoplankton at five depths from each of the 12 locations shown in Fig. 3.1. 1212 1111 Africa 1010 9 8 7 6 5 4 3 South America 2 1 Fig. 3.1 (a) State the type of sampling used by the scientists, and describe the benefits of this method. … … … … … … [3] (b) Table 3.1 shows the temperatures recorded at location 4. Table 3.1 depth / m temperature / °C 50 28 60 25 75 20 80 15 170 12 Plot the information shown in Table 3.1 as a line graph. [4] (c) Fig. 3.2 shows the analysis of some of the data that the scientists collected at each location. location 1 2 3 4 5 6 7 8 9 10 11 12 0 depth / m 100 200 Key depth of bottom of thermocline / m depth of greatest concentration of nitrate ions / m depth of greatest population of phytoplankton / m Fig. 3.2 Use the information shown in Table 3.1 and Fig. 3.2 and your own knowledge to discuss the reasons for the distribution of phytoplankton. … … … … … … [3] [Total: 10]

10 marks

Mark scheme: 3(a) systematic / (line / belt), transect ; 3 plus any 2 from: samples taken at regular intervals ; not affected by bias / ensures samples cover the full range of ocean sampled ; idea of change in conditions / environmental factors ; 3(b) axes labels with units ; 4 suitable linear scale ; plotted correctly ± ½ small square ; ruled lines linking points OR line of best fit drawn ; 3(c) any 3 from: 3 (depth of greatest proportion of) phytoplankton similar to / always (closely) above the depth of bottom of thermocline ; idea of water above thermocline is warmer which allows greater, growth / rate of photosynthesis ORA ; depth of (greatest proportion of) phytoplankton corresponds to depth of (greatest concentration of) nitrate ; nitrate needed for growth / named correct biological molecule requiring nitrate ; AVP ;

This question in 9693/22 Oct/Nov 2024

3 Scientists collected a series of measurements from a ship travelling across the Atlantic Ocean from locations 1 to 12. They recorded the temperature, concentration of nitrate ions (NO3–) and abundance of phytoplankton at five depths from each of the 12 locations shown in Fig. 3.1. 1212 1111 Africa 1010 9 8 7 6 5 4 3 South America 2 1 Fig. 3.1 (a) State the type of sampling used by the scientists, and describe the benefits of this method. … … … … … … [3] (b) Table 3.1 shows the temperatures recorded at location 4. Table 3.1 depth / m temperature / °C 50 28 60 25 75 20 80 15 170 12 Plot the information shown in Table 3.1 as a line graph. [4] (c) Fig. 3.2 shows the analysis of some of the data that the scientists collected at each location. location 1 2 3 4 5 6 7 8 9 10 11 12 0 depth / m 100 200 Key depth of bottom of thermocline / m depth of greatest concentration of nitrate ions / m depth of greatest population of phytoplankton / m Fig. 3.2 Use the information shown in Table 3.1 and Fig. 3.2 and your own knowledge to discuss the reasons for the distribution of phytoplankton. … … … … … … [3] [Total: 10]

10 marks

Mark scheme: 3(a) systematic / (line / belt), transect ; 3 plus any 2 from: samples taken at regular intervals ; not affected by bias / ensures samples cover the full range of ocean sampled ; idea of change in conditions / environmental factors ; 3(b) axes labels with units ; 4 suitable linear scale ; plotted correctly ± ½ small square ; ruled lines linking points OR line of best fit drawn ; 3(c) any 3 from: 3 (depth of greatest proportion of) phytoplankton similar to / always (closely) above the depth of bottom of thermocline ; idea of water above thermocline is warmer which allows greater, growth / rate of photosynthesis ORA ; depth of (greatest proportion of) phytoplankton corresponds to depth of (greatest concentration of) nitrate ; nitrate needed for growth / named correct biological molecule requiring nitrate ; AVP ;

This question in 9693/23 Oct/Nov 2024

4 Fig. 4.1 shows a Southern flounder, a species of fish that lives and reproduces in estuaries. Fig. 4.1 (a) Scientists investigated the effect of the size of sediment in an estuary on the distribution of Southern flounder. Samples of sediment were taken from three locations, A, B and C, in the estuary. Each sediment sample was then analysed. The sediment types found were: clay (smallest particle size) silt sand stones (largest particle size). The percentage of the different sediment types in each location was calculated. The results are shown in Fig. 4.2. 100 Key clay 80 silt sand percentage 60 stones sediment type 40 20 0 A B C location Fig. 4.2 (i) Use Fig. 4.2 to compare the permeability of samples from locations A, B and C. … … … … [2] (ii) Suggest why particle size affects the permeability of sediment types found in locations A, B and C. … … … … [2] (b) Southern flounders were caught using nets dragged along the bottom of the estuary at each location. The mean population density of Southern flounder was recorded. Fig. 4.3 shows these results. 0.07 0.06 0.05 mean population 0.04 density / number per m2 0.03 0.02 0.01 0.00 A B C location Fig. 4.3 (i) Compare the effect of sediment type on the distribution of Southern flounder at locations A, B and C. Use the bar charts in Fig. 4.2 and Fig. 4.3 to support your answer. … … … … … … … … [4] (ii) Suggest three reasons why the type of sediment found in locations A, B and C may affect the distribution of Southern flounder. 1 … … 2 … … 3 … … [3] (c) Particle size affects the permeability of sediments. Fig. 4.4 shows apparatus that could be used to find the permeability of sediment samples. tube containing sediment porous material keeping sediment in tube vessel to catch water passing through Fig. 4.4 Describe how the apparatus in Fig. 4.4 can be used to determine the permeability of sediment samples from locations A, B and C. Include any additional laboratory equipment that may be needed. … … … … … … [3] [Total: 14]

14 marks

Mark scheme: 4(a)(i) any two from: 2 location C would have lowest (permeability) ; B will have slightly higher (permeability) than A / location A and B has more (permeability) than C / A and B are the highest ; samples from location A and B would have similar (permeability) / A is almost as permeable as B ; 4(a)(ii) larger particles have larger spaces in between / ORA ; 2 idea of permeability as, space for / flow of, water through material e.g. allowing greater permeability for water / increased movement of water / ORA ; 4(b)(i) any four from: 4 1 flounder have greatest density on substrates with large particles ORA ; 2 (mean) population / density, of flounder is higher when (percentage of) sand in sediment is higher / ORA ; 3 (mean) population / density, of flounder is lower when (percentage of) clay in sediment is higher / ORA ; 4 (mean) population / density, of flounder is lower when (percentage of) silt in sediment is higher / ORA ; 5 comparison of correct data e.g. location A and B both have population density of 0.05 AND have percentage of sand close to 80% ; 6 (idea that) similar percentage of stones at all three locations has no / little / unknown effect on the (mean) density of flounder ; 4(b)(ii) any three from: 3 (may affect) ability of flounder to, avoid / hide from / camouflaged from, predators ; (may affect) ability of flounder to, hide from / camouflaged from, prey ; (may affect) species / abundance, of, prey / predators, present ; (may affect) ability of flounder to reproduce / better nursery conditions ; (may affect) presence of competitor species ; AVP ; 4(c) any three from: 3 idea of same, mass / volume, of sediment from each sample (placed in tube) ; water poured in at constant rate / equal, mass / volume, of water passed through each sample ; measure, volume / mass / depth, of water passing through in set time OR time taken for set, volume / mass / depth, of water to be collected ; relevant measuring equipment suggested ; e.g. stop clock / timer / measuring cylinder / balance / graduated cylinder. idea of repeats for each sample and mean calculate ;

This question in 9693/21 May/June 2025

3 (a) State three conditions that affect the density of sea water. 1 … 2 … 3 … [2] (b) (i) The density of small, irregular-shaped rocks was investigated. Outline a method that could be used to collect the data needed to calculate the density. … … … … … … [3] (ii) Draw a table to record the data needed to calculate the density from the method planned in 3(b)(i). Include full headings and units in the results table. Do not write in any results. [2] (c) Pumice is one type of rock produced by volcanic activity along a mid-ocean ridge. (i) Explain how a mid-ocean ridge is formed. … … … … … … [3] Fig. 3.1 shows a pumice rock. Pumice contains large air spaces. These spaces are created by bubbles of gas in the lava when the rock forms. 1 cm Fig. 3.1 A student investigated the density of pumice. The results are shown in Table 3.1. The data contains an anomalous result. Table 3.1 sample density / kg m–3 1 806 2 954 3 910 4 923 5 946 mean … (ii) Calculate the mean density for the samples. Do not include the anomalous result. Write your answer in Table 3.1. [1] (iii) Suggest two reasons why the measurements for the density of pumice samples shown in Table 3.1 show a wide variation. 1 … … 2 … … [2] (iv) Pure water has a density of 998.2 kg m–3 at 20 °C. Use Fig. 3.1 and your answer to 3(c)(ii) to explain why recording accurate measurements of the density of the samples of pumice in Table 3.1 was difficult. … … … … … … … … [4] (d) Pumice is found as rounded porous rocks on shorelines and beaches at many locations around the world. (i) Explain why pumice from the mid-ocean ridge in the Atlantic is found on beaches all around the world. … … … … [2] (ii) Suggest why pumice rocks are rounded in shape and found on beaches. … … … … … … [3] [Total: 22]

22 marks

Mark scheme: 3(a) temperature ; 2 salinity ; pressure ; 3(b)(i) (use a balance to) measure the mass ; 3 (use a measuring cylinder) to measure the volume ; use of measuring cylinder AND (electronic) balance ; 3(b)(ii) headings for mass and volume ; 2 correct units for mass and volume ; 3(c)(i) any three from: 3 idea of convection currents causing plate movement ; idea of (mid-Atlantic / mid-ocean ridge is) divergent (plate) boundary ; idea of molten rock / lava / magma, moving up from mantle ; idea of solidification of, molten rock / magma / lava ; 3(c)(ii) 933.25 1 3(c)(iii) any two from: 2 idea that pores could vary in size ; idea that the conditions in the eruption could be different e.g. temperature or cooling rates ; idea that samples may come from different eruptions / contain different minerals ; idea that water may not enter all the pores ; idea that magma contains variable volumes of gas ; 3(c)(iv) any four from: 4 idea that density of samples is, less / lower, than density of water ; pumice will float ; idea that it is difficult to fully submerge the samples ; idea of may not be dry / water may be in the holes in the rock ; idea of increased mass recorded ; idea of decreased volume recorded ; idea of temperature not 20 °C AND affecting, density / volume ; 3(d)(i) idea that (horizontal / surface) currents move the pumice ; 2 idea that oceans and sea are all connected ; 3(d)(ii) idea that weathering has occurred ; 3 description of physical weathering as stones / rocks banging against each other (resulting in rounded shape) ; idea of deposition of sediment onto beaches ;

This question in 9693/22 May/June 2025

3 (a) State three conditions that affect the density of sea water. 1 … 2 … 3 … [2] (b) (i) The density of small, irregular-shaped rocks was investigated. Outline a method that could be used to collect the data needed to calculate the density. … … … … … … [3] (ii) Draw a table to record the data needed to calculate the density from the method planned in 3(b)(i). Include full headings and units in the results table. Do not write in any results. [2] (c) Pumice is one type of rock produced by volcanic activity along a mid-ocean ridge. (i) Explain how a mid-ocean ridge is formed. … … … … … … [3] Fig. 3.1 shows a pumice rock. Pumice contains large air spaces. These spaces are created by bubbles of gas in the lava when the rock forms. 1 cm Fig. 3.1 A student investigated the density of pumice. The results are shown in Table 3.1. The data contains an anomalous result. Table 3.1 sample density / kg m–3 1 806 2 954 3 910 4 923 5 946 mean … (ii) Calculate the mean density for the samples. Do not include the anomalous result. Write your answer in Table 3.1. [1] (iii) Suggest two reasons why the measurements for the density of pumice samples shown in Table 3.1 show a wide variation. 1 … … 2 … … [2] (iv) Pure water has a density of 998.2 kg m–3 at 20 °C. Use Fig. 3.1 and your answer to 3(c)(ii) to explain why recording accurate measurements of the density of the samples of pumice in Table 3.1 was difficult. … … … … … … … … [4] (d) Pumice is found as rounded porous rocks on shorelines and beaches at many locations around the world. (i) Explain why pumice from the mid-ocean ridge in the Atlantic is found on beaches all around the world. … … … … [2] (ii) Suggest why pumice rocks are rounded in shape and found on beaches. … … … … … … [3] [Total: 22]

22 marks

Mark scheme: 3(a) temperature ; 2 salinity ; pressure ; 3(b)(i) (use a balance to) measure the mass ; 3 (use a measuring cylinder) to measure the volume ; use of measuring cylinder AND (electronic) balance ; 3(b)(ii) headings for mass and volume ; 2 correct units for mass and volume ; 3(c)(i) any three from: 3 idea of convection currents causing plate movement ; idea of (mid-Atlantic / mid-ocean ridge is) divergent (plate) boundary ; idea of molten rock / lava / magma, moving up from mantle ; idea of solidification of, molten rock / magma / lava ; 3(c)(ii) 933.25 1 3(c)(iii) any two from: 2 idea that pores could vary in size ; idea that the conditions in the eruption could be different e.g. temperature or cooling rates ; idea that samples may come from different eruptions / contain different minerals ; idea that water may not enter all the pores ; idea that magma contains variable volumes of gas ; 3(c)(iv) any four from: 4 idea that density of samples is, less / lower, than density of water ; pumice will float ; idea that it is difficult to fully submerge the samples ; idea of may not be dry / water may be in the holes in the rock ; idea of increased mass recorded ; idea of decreased volume recorded ; idea of temperature not 20 °C AND affecting, density / volume ; 3(d)(i) idea that (horizontal / surface) currents move the pumice ; 2 idea that oceans and sea are all connected ; 3(d)(ii) idea that weathering has occurred ; 3 description of physical weathering as stones / rocks banging against each other (resulting in rounded shape) ; idea of deposition of sediment onto beaches ;

This question in 9693/23 May/June 2025

2 A student investigated the effect of salinity on the freezing point of water. They made a stock solution of water with a salinity of 40 ppt. The stock solution was used to make a range of solutions of lower salinity from 10 to 30 ppt. Distilled water was also used with a salinity of 0 ppt. Each solution and the distilled water were placed in a freezer with a temperature probe as shown in Fig. 2.1. The temperature probes were all connected to a data logger and the freezing point recorded. A data logger automatically records the temperature at regular intervals. data logger freezer temperature probe plastic container and lid 0 ppt 10 ppt 20 ppt 30 ppt 40 ppt Fig. 2.1 (a) (i) Explain why the student used plastic containers rather than glass. … … … … [2] (ii) State a method to make the stock solution with a salinity of 40 ppt. … … … … [2] (iii) Describe how to use the stock solution to make 500 cm3 of solution with a salinity of 20 ppt. … … … … [2] (b) Table 2.1 shows the results from the investigation. Table 2.1 salinity of solution / ppt freezing point / °C 0 0.00 10 –0.56 20 –1.18 30 –1.80 40 –2.35 (i) Use the data in Table 2.1 to plot a line graph showing the relationship between the salinity of solution and the freezing point. … 0 0 … [4] (ii) Use the graph to predict the freezing point of water with a salinity of 35 ppt. … [1] (c) Explain why the salinity of sea water may decrease. … … … … [2] (d) The student investigated how salinity affected the pH of the five salinities of water. Explain the advantage of using a pH probe rather than Universal Indicator to obtain the results. … … … … [2] [Total: 15]

15 marks

Mark scheme: 2(a)(i) water expanding as it freezes (due to hydrogen bonding) ; 2 reference to hazard of broken glass / plastic is flexible ; 2(a)(ii) stated mass of salt / sodium chloride ; 2 dissolved in stated volume of water ; 2(a)(iii) add known volume of stock solution to measuring cylinder ; 2 top up with distilled water to correct stated volume ; 2(b)(i) both axes labelled with units ; 4 suitable linear scale ; points, plotted correctly  ½ small square (including 0,0) ; suitable line ; 2(b)(ii) correct reading from their plotted graph ; 1 2(c) addition of fresh water ; 2 plus any one from: from precipitation / rain ; from run-off ; 2(d)(i) Universal Indicator is not sensitive enough to distinguish differences ; 2 pH probe is more precise ;

This question in 9693/22 Oct/Nov 2025