States of Matter and Diffusion: Chemistry 0620 (Cambridge O Level / IGCSE)

An electronics repair technician heats a small blob of solder (a metal alloy) with a hot soldering iron until it turns into a liquid. The technician then removes the iron and watches the liquid solder cool down until it turns solid again on the circuit board.

Which change of state occurs as the liquid solder cools and turns solid?

A workshop technician finds four sealed, unlabelled containers left over from a materials demonstration. According to the technician's notes, at room temperature: container A holds a block of solid candle wax, container B holds a small volume of liquid vegetable oil, container C holds helium gas trapped inside a small sealed balloon, and container D holds a small chunk of solid carbon dioxide (dry ice).

(a) Describe the particle separation and particle motion in container A (solid wax) and in container C (helium gas). [2]

(b) The candle wax in container A is heated gently until all of it turns into a liquid. State the name of this change of state. [1]

(c) The oil in container B is placed in a freezer and cools until all of it turns into a solid. State the name of this change of state. [1]

(d) The dry ice in container D is left standing, unsealed, at room temperature. Over several minutes, it disappears completely, without ever forming a puddle of liquid.

(i) Name this change of state. [1]

(ii) Describe how the particle separation and particle motion of the carbon dioxide change as this happens. [2]

An engineering laboratory is testing a new low-melting-point metal alloy, alloy M, designed for use inside the fusible links of automatic fire-sprinkler systems. Alloy M must melt reliably at a known, fixed temperature so that a sprinkler activates as soon as a fire starts.

A technician places a solid sample of alloy M inside a small furnace that supplies heat energy at a constant rate, and records the alloy's temperature every minute:

(a) State the melting point of alloy M, and explain how you identified this value from the data. [2]

(b) Explain, using kinetic particle theory, why the temperature of alloy M does not increase between $t = 4$ min and $t = 7$ min, even though the furnace continues to supply energy at a constant rate throughout the experiment. [2]

(c) Calculate the average rate of temperature increase, in $^{\circ}\text{C}$ per minute, of the solid alloy M between $t = 0$ and $t = 4$ minutes. [2]

(d) A second experiment is carried out using a much larger mass of alloy M in the same furnace, which supplies heat energy at the same constant rate as before. State and explain how the length of the constant-temperature (plateau) section of the new temperature–time graph would compare with the one recorded above. [2]

A meteorologist fills the rubber envelope of a weather balloon with a fixed mass of helium gas at ground level, then seals the neck of the envelope so that no gas can escape or enter afterwards. The envelope is flexible: it stretches to a larger size whenever the trapped gas pushes outward harder than the surrounding air pushes inward, and it stops stretching as soon as the two pressures are equal again. So, once sealed, the pressure of the trapped helium always adjusts until it matches the pressure of the atmosphere immediately around the balloon.

(a) Before release, the sealed balloon is left standing in direct sunlight at ground level for several minutes, which warms the trapped helium. The atmospheric pressure at ground level stays constant throughout. State and explain, using the kinetic particle model, what happens to the volume of the helium gas as it warms in the sunlight. [3]

(b) The balloon is then released and rises into the atmosphere, where the atmospheric pressure is considerably lower than at ground level. Assume the temperature of the trapped helium stays approximately the same as the balloon climbs. State and explain, using the kinetic particle model, what happens to the volume of this fixed mass of trapped gas as the surrounding pressure falls. [3]

(c) As the balloon continues to rise, the surrounding pressure keeps falling, and at the same time sunlight continues to warm the trapped helium, raising its temperature further. State, with a reason, what happens to the volume of the trapped helium when both of these changes happen at the same time. [2]

An air-freshener company tests two automatic diffuser refills, J and K, fitted at opposite ends of a sealed corridor of equal length. Both refills begin releasing their scent vapours into the still air at exactly the same moment and at the same room temperature. A sensor fixed at the midpoint of the corridor detects each scent as soon as its vapour arrives there.

The relative molecular mass, $M_r$, of the scent compound in refill J is $34$. The relative molecular mass of the scent compound in refill K is $136$.

Which statement about the two scent vapours is correct?

A jeweller heats a small blob of solid gold in a crucible with a blowtorch until it turns into a shiny, flowing liquid, ready to be poured into a mould. Just before pouring, the jeweller examines the liquid gold inside the crucible.

Which statement correctly describes the particle separation, arrangement and motion of the liquid gold at this moment?

A school laboratory technician places a beaker of crushed ice, initially at $-10\,^{\circ}\text{C}$, onto a hot plate that supplies heat energy to the beaker's contents at a constant rate. The technician records the temperature of the contents every minute as the ice warms, melts, and the resulting water is then heated until it boils:

(a) State the melting point and the boiling point of the substance in the beaker, and explain how you identified each value from the table. [3]

(b) State the state(s) of matter present in the beaker at $t = 3$ min and at $t = 12$ min. [2]

(c) Using the particle model, explain why the plateau at $100\,^{\circ}\text{C}$ lasts longer than the plateau at $0\,^{\circ}\text{C}$, even though the hot plate supplies heat energy to the beaker's contents at the same constant rate throughout the experiment. [3]

A meteorology student records three everyday observations of water changing state during a warm, humid day.

Observation 1: A shallow puddle of rainwater on a pavement slowly shrinks and disappears over several hours in the sun, even though the air temperature never rises above about $28\,^{\circ}\text{C}$.

Observation 2: In the school kitchen, a kettle of water is left switched on until the water bubbles vigorously throughout the whole kettle, with the water's temperature holding steady at $100\,^{\circ}\text{C}$.

Observation 3: A tall glass of iced lemonade, taken outside on the same humid day, quickly becomes covered in small droplets of liquid water on its cold outer surface.

(a) Name the change of state occurring in Observation 1, and state whether this change happens only at the surface of the liquid or throughout the whole liquid. [2]

(b) Name the change of state occurring in Observation 2, and state one key difference between this change of state and the one in Observation 1, in terms of the temperature at which each occurs. [2]

(c) Name the change of state occurring on the outside of the glass in Observation 3, and explain, in terms of particle energy, why this change occurs on the cold glass surface rather than in the surrounding warm air. [3]

A school technician sets up two identical, tall glass cylinders, P and Q, each filled with the same volume of still water and left undisturbed on a bench. Cylinder P contains water at $20\,^{\circ}\text{C}$ (room temperature) and cylinder Q contains water at $60\,^{\circ}\text{C}$. At exactly the same moment, the technician gently lowers one small crystal of potassium manganate(VII), a deep purple solid, into the bottom of each cylinder, without stirring or otherwise disturbing the water in either cylinder. Over the next hour, the technician watches a purple colour spread upward from each crystal.

(a) Define diffusion. [1]

(b) State, with a reason based on the kinetic particle model, in which cylinder, P or Q, the purple colour spreads through the water faster. [3]

(c) Even in the faster of the two cylinders, the purple colour takes many minutes to spread through the whole depth of water, rather than mixing instantly. Suggest why this is, in terms of the particles present. [2]

A school science technician sets up two identical, tall gas jars, R and S, each standing upright on a bench and filled with still air. A horizontal line is marked in permanent ink around the outside of each jar, exactly $20\,\text{cm}$ above its base. At the same moment, using a syringe pushed through a rubber seal in the base of each jar, the technician releases a small, fixed volume of coloured gas into the bottom of each jar: reddish-brown bromine gas, $\text{Br}_2$ ($M_r = 160$), into jar R, and brown nitrogen dioxide gas, $\text{NO}_2$ ($M_r = 46$), into jar S. Neither gas reacts with air, and both jars are kept at the same room temperature throughout.

A stopwatch is started at the moment of release, and the technician records the time taken for each colour to be first seen at the marked line:

(a) Define diffusion. [1]

(b) State and explain, in terms of relative molecular mass and the kinetic particle model, which of the two gases, bromine or nitrogen dioxide, has the faster average particle speed at the same temperature. [3]

(c) Explain why the times recorded in the table are consistent with your answer to part (b). [2]

(d) The technician repeats the experiment using jar S, but this time replaces the nitrogen dioxide with the same volume of chlorine gas, $\text{Cl}_2$ ($M_r = 71$), released under the same conditions. Predict, with a reason, how the time taken for the colour to reach the $20\,\text{cm}$ mark in this new experiment would compare with the original time of $23\,\text{s}$ recorded for nitrogen dioxide. [2]