States of Matter
6 marks · NormalTwo students investigate the diffusion and solubility of a substance.
Student 1 observes that when a cotton wool ball soaked in concentrated ammonia solution is placed at one end of a closed tube, and a cotton wool ball soaked in concentrated hydrochloric acid is placed at the other end, a white ring of ammonium chloride forms closer to the hydrochloric acid end.
Explain why the white ring does not form in the centre of the tube.
Student 2 measures how much of a solid, potassium nitrate, can dissolve in 100 g of water at different temperatures.
Describe how the student can use the results to draw a solubility curve.
State how a solubility curve can be used to determine the mass of potassium nitrate that will dissolve at a specific temperature.
Give one reason why diffusion of gases occurs faster than diffusion of solids dissolved in water.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) The white ring forms closer to the hydrochloric acid end because ammonia particles are lighter and diffuse faster than hydrogen chloride particles. Therefore, ammonia travels further in the same amount of time. · must show relative speed, mass difference
(b) The student plots a graph of solubility (grams of potassium nitrate per 100 g water) on the y-axis against temperature on the x-axis and joins the points to form a curve. · must show axes labelled, plotting points, curve
(c) By reading the value on the y-axis that corresponds to the desired temperature on the x-axis. · must show read from curve for temperature
(d) Particles in gases move faster and are further apart than in liquids. · must show particle speed or spacing
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States ammonia particles are lighter or diffuse faster
A1 (a) · 1 mark · Correctly relates position of ring to faster ammonia movement/further travel
M2 (b) · 1 mark · Axes correctly identified (solubility vs temperature)
A2 (b) · 1 mark · Describes plotting points and drawing a smooth curve
B1 (c) · 1 mark · States reading y-axis value at specified x-axis point (for temperature)
B2 (d) · 1 mark · Mentions gas particles move faster or are further apart
Worked solution
a) The ring forms closer to the hydrochloric acid end because ammonia particles are lighter and move faster, so they travel further before meeting hydrogen chloride to form ammonium chloride.
b) The student places points for each temperature and corresponding solubility on a graph (y-axis: grams/100g water; x-axis: temperature), joining the points to make a curve (the solubility curve).
c) By looking at the temperature on the x-axis and reading upwards to the curve, then across to the y-axis to find the mass that dissolves.
d) Diffusion is faster in gases because the particles move more quickly and are more spread out than in liquids.