Rate of Reaction, Collision Theory, Catalysts and Reaction Graphs

1. A student reacts marble chips with excess dilute hydrochloric acid and collects carbon dioxide in a gas syringe.

Which change would increase the initial rate but not change the total volume of gas produced?

A use the same mass of larger marble chips
B use the same mass of powdered marble chips
C use twice the mass of marble chips
D use half the volume of acid at the same concentration

2. In an experiment, 1.00 g of magnesium ribbon reacts with excess dilute hydrochloric acid. The hydrogen gas is collected.

Which graph is most likely?

A steep rise, then horizontal line
B straight line rising forever
C horizontal line from the start
D curve rising slowly at first, then becoming steeper

3. A reaction between calcium carbonate and hydrochloric acid produces 120 cm3 of carbon dioxide in 60 s.

What is the average rate of reaction?

A 0.50 cm3/s
B 2.0 cm3/s
C 60 cm3/s
D 7200 cm3/s

4. In a reaction, 80 cm3 of gas is collected in the first 20 s and 120 cm3 is collected by 60 s.

Which statement is correct?

A The rate is constant throughout.
B The reaction is faster between 20 s and 60 s than between 0 s and 20 s.
C The reaction is slower between 20 s and 60 s than between 0 s and 20 s.
D The reaction stops at 20 s.

5. A student reacts sodium thiosulfate solution with hydrochloric acid and measures the time taken for a cross to disappear.

Which result shows the fastest reaction?

A cross disappears in 15 s
B cross disappears in 30 s
C cross disappears in 45 s
D cross disappears in 60 s

6. In the sodium thiosulfate and hydrochloric acid experiment, why does the cross disappear?

A sulfur dioxide forms as a white solid
B sulfur forms as a yellow solid suspension
C sodium chloride forms as an insoluble white precipitate
D hydrogen gas bubbles block the view

7. Which change increases the rate of reaction by increasing the frequency and energy of successful collisions?

A increasing temperature
B decreasing concentration
C decreasing surface area
D removing catalyst

8. Which change increases rate by increasing only the frequency of collisions, not the energy of each particle?

A increasing temperature
B increasing concentration
C adding a catalyst
D cooling the reaction mixture

9. Which statement about a catalyst is correct?

A It increases the energy of reacting particles.
B It lowers activation energy and is chemically unchanged at the end.
C It increases the final amount of product in every reaction.
D It is used up and must be replaced after every collision.

10. A catalyst is added to a reaction that produces gas.

Which graph is correct compared with the uncatalysed reaction using the same amounts of reactants?

A steeper curve with same final volume
B steeper curve with greater final volume
C less steep curve with same final volume
D same curve but greater final volume

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

11. Four experiments react magnesium with hydrochloric acid.

experiment	magnesium	acid concentration	temperature
A	ribbon	0.5 mol/dm3	25°C
B	powder	0.5 mol/dm3	25°C
C	ribbon	1.0 mol/dm3	25°C
D	ribbon	0.5 mol/dm3	45°C

Which experiment is expected to have the fastest initial rate?

A A
B B
C C
D D

12. In a reaction between zinc and excess hydrochloric acid, the mass of the reaction mixture decreases with time.

Why does the mass decrease?

A zinc atoms become lighter
B hydrogen gas escapes
C hydrochloric acid evaporates completely
D zinc chloride is insoluble

13. A student measures loss of mass during a reaction that produces carbon dioxide.

Which graph shape is expected?

A mass decreases rapidly at first, then becomes constant
B mass increases rapidly at first, then becomes constant
C mass decreases at a constant rate forever
D mass remains constant from the start

14. In a gas-collection experiment, the gas volume becomes constant after 90 s.

What does this show?

A the gas has dissolved completely
B all particles have stopped moving
C one reactant has been used up
D the catalyst has been used up

15. Which reaction is suitable for measuring rate by gas syringe?

A hydrochloric acid + sodium hydroxide
B calcium carbonate + hydrochloric acid
C silver nitrate + sodium chloride
D copper(II) oxide + sulfuric acid

16. Which reaction is suitable for measuring rate by disappearance of a cross?

A sodium thiosulfate + hydrochloric acid
B magnesium + hydrochloric acid
C sodium hydroxide + hydrochloric acid
D copper carbonate + sulfuric acid

17. Which reaction is suitable for measuring rate by change in mass?

A magnesium + hydrochloric acid in an open flask
B sodium hydroxide + hydrochloric acid in a closed flask
C silver nitrate + sodium chloride in a covered beaker
D copper(II) sulfate + sodium hydroxide in a sealed tube

18. A reaction has a high activation energy.

Which statement is correct?

A Most collisions are successful at low temperature.
B More particles need high energy before reaction can occur.
C The reaction must be exothermic.
D The reaction cannot be catalysed.

19. A catalyst is added.

Which statement about particles is correct?

A More particles have energy greater than the original activation energy.
B The activation energy is lower, so a greater fraction of collisions are successful.
C The particles move more slowly but collide more effectively.
D The catalyst increases the number of particles in the reaction mixture.

20. The rate of a reaction decreases with time.

Which explanation is correct?

A reactant concentration decreases, so successful collisions become less frequent
B product concentration decreases, so collisions become more frequent
C activation energy increases during the reaction
D temperature always decreases during every reaction

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

21. A reaction produces gas. The results are shown.

time / s	gas volume / cm3
0	0
10	35
20	60
30	75
40	82
50	85
60	85

During which interval is the reaction fastest?

A 0–10 s
B 10–20 s
C 20–30 s
D 40–50 s

22. Using the same data, what is the average rate between 20 s and 40 s?

time / s	gas volume / cm3
20	60
40	82

A 0.55 cm3/s
B 1.1 cm3/s
C 2.0 cm3/s
D 22 cm3/s

23. A student reacts 2.00 g of calcium carbonate with excess hydrochloric acid. The reaction finishes in 100 s. Another student uses 2.00 g of powdered calcium carbonate with the same acid.

Which graph is expected for the powdered calcium carbonate?

A steeper curve, same final gas volume
B steeper curve, lower final gas volume
C less steep curve, same final gas volume
D less steep curve, higher final gas volume

24. Calcium carbonate reacts with hydrochloric acid.

CaCO3 + 2HCl → CaCl2 + H2O + CO2

In experiment 1, acid is in excess. In experiment 2, the same mass of calcium carbonate reacts with a higher concentration of acid, also in excess.

Which statement is correct?

A experiment 2 has higher initial rate and larger final gas volume
B experiment 2 has higher initial rate and same final gas volume
C experiment 2 has same initial rate and larger final gas volume
D experiment 2 has lower initial rate and same final gas volume

25. In another experiment, hydrochloric acid is limiting and calcium carbonate is in excess.

Which change increases the final volume of carbon dioxide?

A use larger pieces of calcium carbonate
B use a catalyst
C use more hydrochloric acid of the same concentration
D increase temperature only

26. A student reacts magnesium with excess acid in four experiments.

experiment	mass of Mg / g	surface area	acid concentration
A	0.50	low	1.0
B	0.50	high	1.0
C	1.00	low	1.0
D	0.50	low	2.0

Which two experiments produce the same final volume of hydrogen but different initial rates?

A A and B
B A and C
C B and C
D C and D

27. A reaction is repeated at a higher temperature using the same amounts of reactants.

Which statement is correct?

A The final amount of product always increases.
B The reaction reaches completion faster.
C The activation energy increases.
D The number of particles decreases.

28. Which condition gives the slowest reaction between zinc and hydrochloric acid?

A zinc powder, 2.0 mol/dm3 acid, 40°C
B zinc granules, 2.0 mol/dm3 acid, 40°C
C zinc granules, 0.5 mol/dm3 acid, 20°C
D zinc powder, 0.5 mol/dm3 acid, 40°C

29. In collision theory, which collision is successful?

A any collision between reacting particles
B a collision with energy at least equal to activation energy and correct orientation
C a collision between product particles only
D a collision at low temperature only

30. Which statement about increasing pressure in a gas reaction is correct?

A It lowers activation energy.
B It increases rate by forcing gas particles closer together.
C It decreases rate because particles move more slowly.
D It has the same effect as removing a catalyst.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

31. In a reaction between gases, pressure is doubled at constant temperature.

Which statement best explains the rate change?

A particles have twice the kinetic energy
B particles collide more frequently per second
C activation energy becomes zero
D every collision becomes successful

32. A student investigates the reaction between magnesium and hydrochloric acid. The acid is in excess.

Which variable should be kept constant to compare the effect of acid concentration?

A concentration of acid
B temperature of acid
C time taken for reaction
D volume of hydrogen produced at each second

33. In an experiment, the rate is measured by 1/time.

Which result gives the highest rate?

A time = 20 s
B time = 40 s
C time = 80 s
D time = 100 s

34. Sodium thiosulfate reacts with hydrochloric acid. The time for the cross to disappear is recorded.

experiment	concentration of sodium thiosulfate	time / s
A	high	22
B	medium	40
C	low	80
D	very low	160

Which conclusion is best?

A lower concentration increases rate
B rate is independent of concentration
C higher concentration increases rate
D concentration affects final amount of sulfur only

35. A catalyst is used in the decomposition of hydrogen peroxide.

Which catalyst is commonly used?

A manganese(IV) oxide
B sodium chloride
C copper(II) sulfate solution only
D calcium carbonate

36. Hydrogen peroxide decomposes to water and oxygen. A catalyst is added.

Which measurement can be used to follow the rate?

A volume of oxygen produced per unit time
B mass of water formed only after evaporation
C colour change from blue to colourless
D pH change from 14 to 1

37. Which statement about enzymes is correct?

A enzymes are biological catalysts
B enzymes increase activation energy
C enzymes are always used up
D enzymes only work at 100°C

38. Which statement about catalyst mass during a reaction is correct?

A it decreases to zero
B it increases because product sticks to it permanently
C it remains unchanged overall
D it becomes one of the products

39. A catalyst speeds up a reaction.

Which quantity definitely remains unchanged?

A activation energy
B overall energy change
C initial rate
D frequency of successful collisions

40. Which statement about surface area is correct?

A Powdered solid reacts faster because more particles are exposed for collisions.
B Large lumps react faster because they contain more total particles per gram.
C Surface area changes the activation energy.
D Surface area changes the final amount of product when the same mass is used with excess acid.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

41. A graph of gas volume against time has a steep slope at the start and then becomes horizontal.

Which statement is correct?

A The reaction is slowest at the start.
B The slope represents rate of reaction.
C The horizontal section shows the reaction is fastest.
D The final volume depends only on the catalyst.

42. A student draws two curves for the same reaction. Curve X reaches 100 cm3 of gas in 40 s. Curve Y reaches 100 cm3 of gas in 80 s.

Which statement is correct?

A X has a faster rate but same final amount of gas.
B Y has a faster rate but same final amount of gas.
C X produces more gas than Y.
D Y must have used a catalyst.

43. A student draws two curves. Curve X reaches 80 cm3. Curve Y reaches 120 cm3. Curve Y is also steeper at the start.

Which explanation could be correct?

A Y used more limiting reactant and higher temperature.
B Y used only a catalyst.
C Y used larger lumps of the same mass.
D Y used lower concentration acid and less reactant.

44. Which graph would show the reaction rate directly?

A volume of gas against time
B mass of flask against time
C gradient of gas-volume graph against time
D temperature against time only

45. In a gas-volume graph, why does the curve level off?

A gas particles stop moving
B the gas syringe becomes heavier
C the limiting reactant has been used up
D the catalyst has dissolved

46. A reaction is carried out in a closed gas syringe. Gas is produced.

Which statement is correct?

A gas volume increases and mass of apparatus decreases
B gas volume increases and mass of apparatus stays the same
C gas volume decreases and mass of apparatus decreases
D gas volume stays constant from the beginning

47. A reaction is carried out in an open flask on a balance. Gas escapes.

Which statement is correct?

A mass decreases because gas leaves the flask
B mass increases because gas has mass
C mass stays constant because atoms are conserved
D mass decreases because atoms are destroyed

48. Which experiment would best compare the effect of surface area on rate?

A equal masses of marble chips and powder with same volume and concentration of acid at same temperature
B different masses of marble chips with different acids
C same marble chips with acid at different temperatures
D marble powder with acid and marble chips with water

49. Which experiment would best compare the effect of concentration on rate?

A same mass and surface area of calcium carbonate with equal volumes of different concentrations of hydrochloric acid at same temperature
B different masses of calcium carbonate with same acid
C same acid but different temperatures
D different solids with the same acid

50. A reaction between sodium thiosulfate and hydrochloric acid is repeated with water added to dilute the sodium thiosulfate.

Which result is expected?

A the cross disappears faster because water is a catalyst
B the cross disappears slower because fewer thiosulfate particles collide each second
C the final sulfur amount must increase
D the activation energy becomes higher

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

1. B

A wrong: Larger chips have smaller surface area, so lower initial rate.
B right: Same mass of powdered marble chips has larger surface area, so rate increases, but total CaCO3 is unchanged, so final gas volume is unchanged.
C wrong: More marble chips can increase final gas volume if acid is excess.
D wrong: Less acid may reduce final gas volume if acid becomes limiting.

2. A

A right: Gas volume rises quickly at first, then becomes horizontal when magnesium is used up.
B wrong: Gas cannot increase forever.
C wrong: Reaction does produce gas.
D wrong: Rate is fastest at the start, not slowest.

3. B

Average rate = volume / time
= 120 / 60
= 2.0 cm3/s

A wrong: Divides wrongly.
B right: 2.0 cm3/s.
C wrong: Uses time as rate.
D wrong: Multiplies volume by time.

4. C

Rate from 0–20 s = 80 / 20 = 4.0 cm3/s
Rate from 20–60 s = (120 – 80) / 40 = 1.0 cm3/s

A wrong: Rate changes.
B wrong: Later rate is slower, not faster.
C right: Reaction slows down after 20 s.
D wrong: Gas still increases after 20 s.

5. A

A right: Shorter time means faster reaction.
B wrong: Slower than 15 s.
C wrong: Slower than 15 s and 30 s.
D wrong: Slowest listed.

6. B

A wrong: Sulfur dioxide is a gas, not the cloudy solid.
B right: Sulfur forms as a yellow solid suspension, making the mixture cloudy.
C wrong: Sodium chloride is soluble.
D wrong: Hydrogen gas is not the reason.

7. A

A right: Higher temperature increases particle speed, collision frequency, and energy of collisions.
B wrong: Lower concentration decreases rate.
C wrong: Lower surface area decreases rate.
D wrong: Removing catalyst decreases rate.

8. B

A wrong: Temperature increases both collision frequency and collision energy.
B right: Higher concentration gives more particles per volume, so collisions are more frequent.
C wrong: Catalyst lowers activation energy.
D wrong: Cooling decreases rate.

9. B

A wrong: Catalysts do not increase particle energy.
B right: Catalysts lower activation energy and are chemically unchanged overall.
C wrong: Catalysts do not increase final yield in a normal completed reaction.
D wrong: Catalysts are not used up.

10. A

A right: Catalyst increases rate, so curve is steeper, but same reactant amounts give same final gas volume.
B wrong: Catalyst does not increase final volume.
C wrong: Catalyst makes curve steeper, not less steep.
D wrong: Final volume does not increase.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

11. Faulty question

B, C and D all increase the initial rate compared with A: powder increases surface area, higher concentration increases collision frequency, and higher temperature increases collision energy and frequency.

A wrong: This is the control/slow baseline.
B possible: Powder reacts faster due to greater surface area.
C possible: Higher concentration reacts faster.
D possible: Higher temperature reacts faster.

There is not enough numerical information to decide which of B, C or D is fastest overall. Correct version should compare only one variable at a time.

12. B

A wrong: Atoms do not become lighter.
B right: Hydrogen gas escapes from the reaction mixture, so mass decreases.
C wrong: Acid does not completely evaporate.
D wrong: Zinc chloride is soluble.

13. A

A right: Mass falls quickly at first as CO2 escapes, then becomes constant when reaction stops.
B wrong: Gas escaping does not increase mass.
C wrong: Rate decreases and then stops.
D wrong: Gas is produced, so mass changes.

14. C

A wrong: Gas volume becoming constant does not mean gas dissolved.
B wrong: Particles never stop moving.
C right: The reaction stops because the limiting reactant has been used up.
D wrong: Catalysts are not used up.

15. B

A wrong: Neutralisation produces no gas.
B right: Calcium carbonate + hydrochloric acid produces CO2 gas.
C wrong: This is precipitation, not gas production.
D wrong: Copper(II) oxide + acid produces salt and water, no gas.

16. A

A right: Sodium thiosulfate + hydrochloric acid produces sulfur, making the solution cloudy and hiding the cross.
B wrong: Magnesium + acid is measured by gas volume or mass loss.
C wrong: Neutralisation does not make the cross disappear.
D wrong: Copper carbonate + acid produces gas, not cloudy sulfur.

17. A

A right: Magnesium + acid produces hydrogen gas that escapes, causing mass loss.
B wrong: Neutralisation produces no gas.
C wrong: Precipitation in a covered beaker is not best for mass-loss rate.
D wrong: Sealed tube prevents mass loss.

18. B

A wrong: High activation energy means fewer successful collisions at low temperature.
B right: More particles need enough energy to overcome the high activation energy.
C wrong: Activation energy does not decide exothermic/endothermic.
D wrong: Catalysts can lower activation energy.

19. B

A wrong: Catalyst does not give particles more energy.
B right: Lower activation energy means more collisions have enough energy to be successful.
C wrong: Particles do not move more slowly because of the catalyst.
D wrong: Catalyst does not increase number of reactant particles.

20. A

A right: As reactants are used up, concentration falls, so successful collisions become less frequent.
B wrong: Product concentration usually increases.
C wrong: Activation energy does not normally increase during reaction.
D wrong: Temperature does not always decrease.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

21. A

Rates by interval:

0–10 s: 35 / 10 = 3.5 cm3/s
10–20 s: 25 / 10 = 2.5 cm3/s
20–30 s: 15 / 10 = 1.5 cm3/s
40–50 s: 3 / 10 = 0.3 cm3/s

A right: Fastest interval is 0–10 s.
B wrong: Slower than 0–10 s.
C wrong: Slower.
D wrong: Much slower.

22. B

Average rate from 20–40 s
= (82 – 60) / (40 – 20)
= 22 / 20
= 1.1 cm3/s

A wrong: Half the correct value.
B right: 1.1 cm3/s.
C wrong: Too high.
D wrong: This is volume change, not rate.

23. A

A right: Powder has greater surface area, so faster rate/steeper curve; same mass gives same final gas volume.
B wrong: Final gas volume does not decrease.
C wrong: Powder does not make reaction slower.
D wrong: Final gas volume does not increase.

24. B

A wrong: Final gas volume depends on amount of CaCO3 because acid is excess.
B right: Higher acid concentration increases initial rate, but same CaCO3 mass gives same final CO2 volume.
C wrong: Initial rate increases.
D wrong: Initial rate does not decrease.

25. C

A wrong: Larger pieces affect rate, not final gas volume.
B wrong: Catalyst affects rate, not final amount.
C right: If acid is limiting, more acid gives more CO2.
D wrong: Temperature affects rate, not final amount.

26. A

A right: A and B use the same mass of Mg, so same final H2 volume; higher surface area in B gives faster initial rate.
B wrong: C has more Mg, so final gas volume is larger.
C wrong: C has more Mg, so final gas volume differs.
D wrong: C has more Mg, so final gas volume differs.

27. B

A wrong: Same amounts of reactants usually give same final product amount.
B right: Higher temperature increases rate, so completion is reached faster.
C wrong: Activation energy does not increase.
D wrong: Number of particles does not decrease just because temperature increases.

28. C

A wrong: Powder, high concentration and high temperature give fast rate.
B wrong: High concentration and high temperature still fast.
C right: Granules have lower surface area, acid is dilute, and temperature is low.
D wrong: Powder and higher temperature make it faster than C.

29. B

A wrong: Not every collision is successful.
B right: Successful collisions need enough energy and correct orientation.
C wrong: Product-particle collisions are not the reacting collision.
D wrong: Low temperature usually reduces successful collisions.

30. B

A wrong: Pressure does not lower activation energy.
B right: Higher pressure brings gas particles closer together, increasing collision frequency.
C wrong: Temperature constant means average speed is unchanged.
D wrong: Removing catalyst lowers rate by a different reason.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

31. B

A wrong: Constant temperature means average kinetic energy stays the same.
B right: Higher pressure increases particle concentration, so collisions occur more frequently.
C wrong: Activation energy does not become zero.
D wrong: Not every collision becomes successful.

32. B

A wrong: Concentration is the independent variable being changed.
B right: Temperature must be controlled so only concentration affects rate.
C wrong: Time taken is measured, not controlled.
D wrong: Volume of hydrogen is a dependent measurement.

33. A

Rate = 1/time, so shortest time gives highest rate.

A right: 1/20 is largest.
B wrong: 1/40 is smaller.
C wrong: 1/80 is smaller.
D wrong: 1/100 is smallest.

34. C

A wrong: Lower concentration increases time, so lowers rate.
B wrong: Rate clearly changes.
C right: Higher concentration gives shorter time, so faster rate.
D wrong: This experiment measures rate, not final sulfur amount only.

35. A

A right: Manganese(IV) oxide is the common catalyst for hydrogen peroxide decomposition.
B wrong: Sodium chloride is not the usual catalyst.
C wrong: Copper(II) sulfate is not the standard O Level answer here.
D wrong: Calcium carbonate is not used.

36. A

A right: Rate can be followed by measuring oxygen volume produced per unit time.
B wrong: Evaporating water is not suitable.
C wrong: No blue-to-colourless colour change here.
D wrong: pH 14 to 1 is irrelevant.

37. A

A right: Enzymes are biological catalysts.
B wrong: Enzymes lower activation energy.
C wrong: Enzymes are not used up overall.
D wrong: Most enzymes denature at high temperature.

38. C

A wrong: Catalyst is not consumed.
B wrong: It should not permanently become product-coated in the basic definition.
C right: Catalyst mass remains unchanged overall.
D wrong: Catalyst does not become a product.

39. B

A wrong: Catalyst lowers activation energy.
B right: Overall energy change remains unchanged.
C wrong: Initial rate increases.
D wrong: Successful collision frequency increases.

40. A

A right: Powder exposes more particles at the surface, increasing collision frequency.
B wrong: Same mass has same total particles, just different exposed surface.
C wrong: Surface area does not change activation energy.
D wrong: Same mass with excess acid gives same final amount.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

41. B

A wrong: Reaction is fastest at the start because slope is steepest.
B right: Slope/gradient of gas-volume graph represents rate.
C wrong: Horizontal means rate is zero.
D wrong: Final volume depends on limiting reactant amount, not only catalyst.

42. A

A right: X reaches the same final volume in less time, so faster rate but same gas amount.
B wrong: Y is slower.
C wrong: Both reach 100 cm3.
D wrong: Y is slower, so it is not showing catalyst effect.

43. A

A right: More limiting reactant gives more final gas; higher temperature gives steeper initial curve.
B wrong: Catalyst changes rate but not final gas volume.
C wrong: Larger lumps would reduce rate and same mass would give same final volume.
D wrong: Lower concentration and less reactant would likely reduce rate and final volume.

44. C

A wrong: Volume-time graph shows amount, with rate from slope.
B wrong: Mass-time graph shows mass change, with rate from slope.
C right: Gradient of gas-volume graph against time shows rate directly.
D wrong: Temperature alone does not directly show rate.

45. C

A wrong: Gas particles do not stop moving.
B wrong: Gas syringe mass is irrelevant.
C right: Curve levels off when limiting reactant is used up and no more gas forms.
D wrong: Catalyst does not dissolve/use up.

46. B

A wrong: In a closed system, mass does not decrease.
B right: Gas volume increases, but total mass of apparatus stays the same because gas is trapped.
C wrong: Gas volume increases, not decreases.
D wrong: Gas volume does not stay constant from the beginning.

47. A

A right: Gas escapes from open flask, so measured mass decreases.
B wrong: Gas has mass, but it leaves the flask.
C wrong: Atoms are conserved overall, but balance measures only flask contents.
D wrong: Atoms are not destroyed.

48. A

A right: Same mass, same acid volume/concentration and temperature; only surface area changes.
B wrong: Multiple variables change.
C wrong: Tests temperature, not surface area.
D wrong: Different liquids/reactants make it unfair.

49. A

A right: Same solid mass/surface area, same acid volume, same temperature; only concentration changes.
B wrong: Changes mass, not concentration only.
C wrong: Tests temperature.
D wrong: Different solids make it unfair.

50. B

A wrong: Water is not a catalyst.
B right: Dilution lowers thiosulfate concentration, so fewer successful collisions occur each second and the cross disappears more slowly.
C wrong: Final sulfur amount does not automatically increase.
D wrong: Dilution does not change activation energy.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.