HCI Assignment 08 Answer Kinetic particle model of matter

Kinetic Model of Matter 1 Name: Index No: Date: Class: Assignment 08: Kinetic particle model of matter 1. The set of statements below c ompare the energy or the motion of substances in different states of matter. State if each statement is true (T) or false (F). [6] (a) Comparing a piece of ice at -28 ºC and a piece of ice at -1 ºC, (i) the molecules in both substances have very low potential energy. ( T ) (ii) the molecules in ice at -28 ºC move faster than molecules in ice at -1 ºC. ( F ) (b) Comparing a bottle of water vapour at 25 ºC and a bottle of liquid water at 25 ºC, (i) the molecules in the bo ttle of water vapour are further apart than the molecules in the bottle of liquid water. ( T ) (ii) the molecules in both bottles have different amounts of average kinetic energy ( F ) (c) Comparing ammonia gas at 15 ºC and ammonia liquid at -15 ºC, (i) the molecules in ammonia gas have higher potential energy than the molecules in ammonia liquid. ( T ) (ii) the molecules in ammonia liquid have a lower average kinetic energy than the molecules in ammonia gas. ( T ) 2. A student conducts an experiment to observe the behaviour of dust particles when placed in an illuminated glass cell. Under the microscope, the following motion was observed. (a) Describe the motion of this particle. [1] Particle moves in a continuous and random motion. (accept: haphazard, erratic) [1] (b) Explain what could have caused the motion observed. [2] Air molecules collide (or bombard) unevenly on dust particle [1] and exerts an unbalanced force on dust particle [1] which results in this motion. [Reject: dust particles collide with other dust particles] HWA CHONG INSTITUTION Sec 3 Physics

Kinetic Model of Matter 2 (c) Explain the following: [3] (i) The dust particles appear as bright sparks of light when seen through the microscope. Dust particles reflect light from light source. [1] [Accept: scatter light] (ii) The smaller dust particles move faster while the larger dust particles move slower. For same net force acting on the dust particles, smaller particles have larger acceleration since they have smaller mass (or smaller inertia), thus they move faster. [1] [Accept: For same average kinetic energy of the dust particles, smaller particles move at higher speeds since they have smaller mass.] (iii) When temperature increases, the dust particles move faster. Air molecules move faster /gain kinetic energy/kinetic energy of air molecules increases , which ca use them to collide more frequen tly/forcefully with dust particles and exert a larger net force to make dust particles move faster. [1] [Reject: Average kinetic energy of dust particles increase and they move faster.] 3. Fig. 3.1 shows a gas jar full of air at atmospheric pressure. Fig. 3.2 shows an identical jar of which air has been pumped out. Each jar is connected to a reservoir containing a dark brown liquid which vaporises very quickly into a brown gas. The taps connecting the jar to the reservoirs are open at the same time. (a) What is the likely observation of the brown gas in the two gas jars? [1] Brown gas filled up jar in Fig. 3.2 (or evacuated gas jar) more quickly. [1] (b) Explain your observations in (a) using ideas about molecules. [3] All particles are in a continuous and random motion. [1] Brown gas particles spread out from a region of higher concentration to a region of lower concentration. [1] Rate of diffusion of brown gas is increased in evacuated jar because there are no molecules in jar to collide into gas molecules when they enter. [1] [Reject: Gas molecules face less resistance when they enter evacuated jar.] 4. The diagram shows some gas trapped inside a cylinder by a light frictionless piston in a syringe. air Fig. 3.1: gas jar full of air Fig. 3.2: evacuated gas jar

Kinetic Model of Matter 3 0 50 100 150 200 250 300 350 400 450 0 50 100 150 200 250 300 350 400 450 P / kPa State how the value of each quantity below compares before and after the piston is gradually pushed in by 20 cm 3. For each quantity, write either “greater than”, “the same as” or “smaller than ” in the space below. [3] quantity comparison the average distance between the gas molecules smaller than the pressure of the gas after the piston stops moving greater than the average speed of the gas molecules the same as the frequency of collisions between the gas molecules and the piston and cylinder walls greater than the total force exerted by the gas molecules per unit area on the piston and cylinder walls greater than the temperature of the gas after the piston stops moving the same as 5. A student pulls the piston of a large calibrated gas syringe to draw in a fixed mass of air. Then he connects the nozzle of the syringe to a pressure gauge. The syringe is not thermally insulated. The piston is gradually pulled out within the syringe and the student measures the volume V of the trapped air and its pressure P at several different positions of the piston. A graph of P against V is plotted as shown below. V / cm3 piston trapped air pressure gauge gas syringe

Kinetic Model of Matter 4 (a) The student was told that P and V are related by the equation: PV = constant (i) If this relationship is correct, state in words, how P is related to V, [1] P is inversely proportional to V at a constant temperature. [Reject: P is inversely related to V.][Accept: P varies inversely with V at a constant temp.] (ii) By using an exact point from the graph, calculate the value of the constant in the equation and state its unit. [2] Value of constant is 20 000 kPa cm3. [1]: correct value [1]: correct unit [Accept: 20 J] [Reject: 20 Nm as ‘Nm’ should only be used as unit for torque.] Possible exact points: (100, 200), (200, 100), (400, 50), (50, 400). (b) Given that the volume of the syringe is increased from 150 cm 3 to 300 cm3, describe the change in pressure of the trapped air in the syringe. Support your answer with suitable calculations. [2] P1 V1 = 20 000 P1 (150) = 20 000 P1 = 133.3 kPa (to 2 s.f.) P2 V2 = 20 000 P2 (300) = 20 000 P2 = 66.67 kPa (to 2 s.f.) Change in pressure = 66.67 – 133.3 = - 67 kPa (to 0 d.p.) [1: with correct working shown] When volume of syringe is increased from 150 cm 3 to 300 cm 3, pressure of trapped air decreased [1] by 67 kPa. [Accept: Pressure halved, with appropriate substitution shown as working.] (c) A warning is printed on the side of the syringe stating that the gas inside must not exceed a pressure of 500 kPa. State the numerical limit that this pressure constraint has on the volume of the trapped air. [1] Using PV = constant, when P = 500 kPa, V = 20 000 / 500 = 40 cm3 [1] (d) State one step taken during the experiment to keep the temperature of the ai r in the syringe approximately constant. [1] Either: The piston is gradually pulled out within the syringe. Or: The syringe is not thermally insulated. [1] (e) Explain, using ideas of molecules , why the pressure of the trapped air decreases as its volume increases when its temperature is kept constant. [3] (Trigger) When its volume is increased, number of air molecules per unit volume decreased. [1] Frequency of c