States of Matter and Changes of State
States of matter and changes of state, at IGCSE, extends the simple solid-liquid-gas particle model used at GCSE with the quantitative link between temperature and the kinetic energy of particles, the difference between evaporation and boiling, and the use of relative molecular mass to compare how fast different gases diffuse (Graham's law). Edexcel 4CH1 and Cambridge 0620/0971 both expect students to explain diffusion experiments, such as the ammonia and hydrogen chloride 'white ring' tube, quantitatively rather than just describing particles moving from high to low concentration.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Learn the particle arrangement, movement and relative energy of particles in solids, liquids and gases, and remember that in all three states the average kinetic energy of the particles increases as temperature increases.
- Learn the six changes of state and their state symbol pairs: melting (s to l) and freezing (l to s), boiling/evaporating (l to g) and condensing (g to l), and sublimation (s to g directly, with no liquid stage). Energy is absorbed (endothermic) when melting, boiling or subliming, and released (exothermic) when freezing or condensing.
- Distinguish evaporation (occurs at any temperature, only at the liquid's surface, no bubbles form) from boiling (occurs only at the boiling point, throughout the liquid, with bubbles of vapour forming). List four factors that increase the rate of evaporation: higher temperature, larger surface area, greater air movement over the surface, and lower humidity/pressure of the surrounding air.
- For diffusion-rate comparisons, apply Graham's law: the rate of diffusion of a gas is inversely proportional to the square root of its relative molecular mass (Mr), so a lighter gas diffuses faster than a heavier one at the same temperature.
- Use the ratio rate(A) / rate(B) = square root of (Mr(B) / Mr(A)) to compare how far two gases travel from opposite ends of a tube in the same time, since distance travelled is proportional to rate.
- For a heating or cooling curve, identify the flat, horizontal sections as changes of state (temperature stays constant while energy changes the arrangement of particles, not their temperature) and the sloped sections as heating or cooling within a single state.
Worked example
Cotton wool soaked in concentrated ammonia solution is placed at one end of a horizontal glass tube of length 100 cm, and cotton wool soaked in concentrated hydrochloric acid is placed at the other end, at the same moment. Ammonia gas (NH3, Mr = 17) and hydrogen chloride gas (HCl, Mr = 36.5) diffuse along the tube and react to form a white ring of ammonium chloride where they meet. Calculate the distance from the hydrochloric acid end at which the white ring forms.
- Identify the Mr values: NH3 = 17 and HCl = 36.5.
- Apply Graham's law to find the ratio of distances travelled: distance(NH3) / distance(HCl) = square root of (Mr(HCl) / Mr(NH3)) = square root of (36.5/17) = square root of 2.147 = 1.465.
- Set up the total distance equation: distance(NH3) + distance(HCl) = 100 cm, with distance(NH3) = 1.465 x distance(HCl).
- Substitute: 1.465 x distance(HCl) + distance(HCl) = 100, so 2.465 x distance(HCl) = 100.
- Solve: distance(HCl) = 100 / 2.465 = 40.6 cm.
- Final answer: the white ring forms 40.6 cm from the hydrochloric acid end (and 59.4 cm from the ammonia end), because the lighter ammonia gas diffuses faster and travels further.
Practice questions
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Q1State the name given to the direct change of state from a solid to a gas, without passing through the liquid state.Show answer
Answer: Sublimation.
Q2State whether melting is an exothermic or endothermic process, and explain why in terms of particle energy.Show answer
Answer: Endothermic; energy is absorbed from the surroundings to weaken the forces of attraction holding particles in fixed positions in the solid lattice, giving the particles enough energy to move past each other as a liquid.
Q3Give two factors, other than temperature, that increase the rate of evaporation of a liquid.Show answer
Answer: Any two of: a larger surface area, increased air movement (a draught) over the surface, lower humidity/pressure of the surrounding air.
Q4State one difference between boiling and evaporation.Show answer
Answer: Boiling occurs throughout the liquid (with bubbles of vapour forming) at one fixed temperature, the boiling point; evaporation occurs only at the liquid's surface and can happen at any temperature below the boiling point.
Q5Gas X has a relative molecular mass of 64 and gas Y has a relative molecular mass of 4. State, with a reason, which gas diffuses faster.Show answer
Answer: Gas Y diffuses faster, because it has a lower relative molecular mass, and by Graham's law the rate of diffusion is inversely proportional to the square root of the relative molecular mass.
Q6Two gases, P (Mr = 16) and Q (Mr = 64), are released at the same moment from opposite ends of a 90 cm tube. Calculate the distance from gas Q's end at which the two gases meet.Show answer
Answer: 30 cm. Distance ratio: distance(P)/distance(Q) = square root of (64/16) = 2, so distance(P) = 2 x distance(Q). Since distance(P) + distance(Q) = 90, then 3 x distance(Q) = 90, so distance(Q) = 30 cm.
Q7Explain, in terms of particles, why the temperature of water remains constant at 100 degrees C while it is boiling, even though the water is still being heated.Show answer
Answer: The energy supplied is used to overcome (break) the forces of attraction between the water particles so they can escape as a gas, rather than to increase the average kinetic energy of the particles, so the temperature does not rise until all the liquid has turned to gas.
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
A liquid is heated and its temperature is recorded every minute, producing a graph. Describe and explain the shape of the graph between the liquid's boiling point being reached and all the liquid turning to gas.
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Gas A has Mr = 28 and gas B has Mr = 112. Cotton wool soaked in gas A and cotton wool soaked in gas B are placed at opposite ends of a glass tube of length 80 cm at the same time. Calculate the distance from gas B's end at which a visible reaction product first forms.
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Ammonia gas (NH3, Mr = 17) and hydrogen chloride gas (HCl, Mr = 36.5) react to form a white solid, ammonium chloride, where they meet: NH3(g) + HCl(g) -> NH4Cl(s). Cotton wool soaked in concentrated ammonia solution is placed at one end of a horizontal glass tube of length 96 cm, and cotton wool soaked in concentrated hydrochloric acid is placed at the other end, at the same time. (a) Explain, in terms of particles, why a white ring forms inside the tube rather than at either end. (b) Calculate the distance from the hydrochloric acid end at which the white ring forms.
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Free printable worksheet
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This topic is chapter 11 of IGCSE Science Workbook, the whole course as one free printable PDF.
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