Temperature, Heat Transfer, Calorimetry

Temperature

Temperature is the degree of hotness or coldness of an object. It is measured using a thermometer. A thermometer employs a substance with a property referred to as a thermometric property that varies with temperature. 

Thermometers are grouped according to the thermometric substance they use, for example liquid-in-glass thermometers in which the thermometric property is the length of a liquid (such as mercury and alcohol) column; resistance thermometers where the thermometric property is the resistance for instance of a nichrome wire; constant-volume gas thermometer where the thermometric property is the pressure of a gas for example air. All thermometers are calibrated by relating the change in the thermometric property to a specific temperature.

To construct a liquid-in-glass alcohol thermometer for instance, the alcohol is placed in a hollow glass bulb fitted with a thin capillary tube. The arrangement is then dipped in melting ice. This causes the alcohol to contract and fall down the capillary tube. The level the alcohol attains is assigned 00C. The arrangement is then dipped in boiling water. This causes the alcohol to expand and rise up the capillary tube. The level it attains is assigned 1000C.

The space between the 00C and 1000C marks is then divided into 100 equal divisions.

Liquid-in-glass thermometers for the most part use alcohol or mercury as thermometric substances. Advantages of alcohol as a thermometric liquid over mercury include:

  • it has a lower freezing point and therefore can measure lower temperatures
  • Large coefficient of expansion hence very sensitive to temperature change.
  • less hazardous
  • display easy to see

Disadvantages of alcohol as a thermometric liquid: 

  • less durable as it evaporates
  • can polymerize – combine to form larger molecules.

Advantages of mercury over alcohol as a thermometric liquid

  • does not evaporate easily hence more durable
  • good conductor of heat hence more accurate as heat is distributed more evenly leading to even expansion
  • has fast response time

Disadvantages

  • cannot measure very low temperature
  • low thermal coefficient – less sensitive to small changes in temperature
  • mercury is very toxic 

Heat transfer

All states of matter transmit heat through one or more of the three modes of heat transfer, namely;

  • Convection
  • Conduction
  • Thermal radiation

Convection:

Convection occurs in liquids and gases (fluids) and involves actual movement of molecules by virtual of density difference between hot and cold molecules. As an example, say a sufuria containing water is placed over a fire. The base of the sufuria heats up and then heats the layer of water molecules it is in contact with. Hot molecules being relatively less dense rise up while the denser cold molecules sink down to take their place. The cold molecules are in turn heated, move up, while colder molecules higher up sink.

The process continues until the water attains a uniform temperature. 

Convection can be demonstrated by placing a grain of potassium permanganate at a corner of a beaker containing water and heating that corner.

It is observed that the hotter coloured water from the corner shoots upwards before curling downwards in what is referred to as a convectional current. 

NOTE 1: If an ice cube is placed at the bottom of a long tube containing water, and the water above the ice cube heated, the ice cube does not melt. This is because the hotter less denser water molecules rise and therefore do not interact with the ice cube. 

Convection is also observed in air, for example, sea breeze experienced during the day where cold currents from the sea move to the land while warm currents from land move to the sea. Sea breeze occurs because land and hence the air above it heats faster than water. The beach or sea side is therefore always cool even on very hot days. At night, land breeze occurs as warm air from the sea moves to the land and cool air from the land move to the sea. This occurs because land and consequently air above it cools faster than water. The seaside or beach is therefore warm even on cold nights.

NOTE 2: To reduce heat loss through convection for instance in a thermos flask, the flask is made of two layers of glass with vacuum in the middle. Convection occurs in the presence of a medium which means that it cannot occur in vacuum. 

Conduction:

Conduction is the process by which heats flows from a region of high temperature to a region of low temperature without there being net movement of the material. It is in most cases observed in solids. Absorption of heat causes the solid molecules to vibrate faster about their fixed positions and as a result heat is transferred to cooler molecules. Unlike insulators, metals have free electrons and therefore in addition to the vibratory motion of the molecules, the free electrons take part in the transfer of heat. When the electrons gain heat energy, they become more agitated and transfer heat energy to other electrons through collision. Transfer of heat by means of free electrons is more effective than in the vibration of molecules. This makes metals good conductors of heat. 

Factors affecting conduction of heat

Consider a thin metal disc of uniform cross-section area A and thickness Δx. Suppose that the hotter face is at a temperature θ and the cooler face at a temperature θ-  Δθ. Suppose too that the rate of heat flow is ΔQ/Δt 

If no heat is lost to the surrounding, it follows that;

(i) the rate of heat flow is directly proportional to the temperature gradient ΔQ/Δx, that is; 

                                                                              (i)

(ii) the rate of heat flow is directly proportional to the cross-sectional area A.

                                                                                (ii)

Combining equations (i) and (ii) leads to;

                                                                                      (iii)

                                                                                (iv)

where k is a constant of proportionality referred to as coefficient of thermal conductivity. It is negative to make the RHS of equation (iv) positive. Coefficient of thermal conductivity is dependent on the nature of the material. Good conductors like most metals have a large coefficient of thermal conductivity while poor conductors such as glass and wood have a smaller coefficient of thermal conductivity. Equation (iv) represents Fourier’s law of thermal conduction. It states that the rate of heat transfer through a homogeneous solid is directly proportional to the cross-sectional area and the temperature gradient.

It is important to note that:

(i) Metals at room temperature feel cold to the touch because they conduct the heat away. Insulators such as wood and plastic at room temperature on the other hand are warm to the touch because they are poor conductors of heat. 

(ii) Heat loss through conduction for instance in a thermos flask is reduced by the presence of vacuum and the plastic stopper, both of which are poor conductors of heat. Additionally, the walls of the flask are usually made of glass, also a poor conductor of heat. 

Thermal radiation:

Thermal radiation involves transfer of heat in the form of electromagnetic radiation, and unlike convection and conduction, does not require a medium. Any object with a temperature above absolute zero (0 K, the lowest temperature possible) emits electromagnetic radiation made up of a continuous range of wavelengths/energy referred to as an electromagnetic spectrum. The peak energy of the emitted radiation depends on the temperature of the object, with relatively less-hot objects such as hot embers emitting energy in the infrared region of the electromagnetic spectrum, hotter objects for example the sun emitting energy in the white light region, and very hot objects like some stars emitting energy in the blue and UV region.

It is important to note that;

(i) Dull and matt surfaces absorb (and emit) heat in the form of electromagnetic radiation faster than smooth and shiny surfaces. A person wearing a black fluffy sweater thus feels warmer than someone wearing a white smooth one. 

(ii) While all components of the electromagnetic spectrum aid in heat transfer, most heat is transferred in the form of infrared radiation. 

(iii) To reduce heat loss through thermal radiation in a thermos flask, the inner walls are silvered to reduce the absorption of infrared radiation.

Expansivity

All three states of matter (solids, liquids and gases) increase in size without change in the quantity of matter (mass) when heated. Matter is made up of atoms/molecules which are in continuous motion, translatory in fluids and vibratory about fixed positions in solids. When a solid is heated for instance, the vibratory motion of the molecules increases leading to an increase in the intermolecular spacing hence an increase in size (length, area, volume).

The degree of expansion is different for different solids due to differences in intermolecular forces. For some solids, the intermolecular forces are weak and therefore the molecules can easily move apart (think of a weak spring which stretches easily). For other solids, the forces are much stronger making it difficult for the molecules to move apart. Solids with weaker intermolecular forces expand more than those with stronger intermolecular forces. Solids have definite shapes and so when heated, each of their linear dimensions increases as the temperature increases. Suppose for example a wire of length L is heated such that its temperature increases by ∆T. Suppose too that the increase in temperature causes the length of the wire to increase by ∆L. It can be shown that the change in length is directly proportional to the initial length L and the change in temperature ∆T, that is:

                                                                                                            (i)

                                                                                                         (ii)

Combining equations (i) and (ii) leads to:

                                                                                                       (iii)

                                                                                                  (iv)

Where α is a constant of proportionality called coefficient of linear expansion. Different materials have different coefficients of linear expansion. Materials that expand easily have a relatively higher coefficient of expansion compared to those that do not expand as easily.

Calorimetry

In physics, calorimetry involves the measurement of the amount of heat transferred between objects. Heat (net heat) flows from hot body to cold body it is in thermal contact with until the two bodies acquire the same temperature. The bodies are then said to be in thermal equilibrium. If the two systems are isolated (no heat lost to the surrounding), then at thermal equilibrium, the heat lost by the hot body equals the heat gained by the cold body. There are various terms associated with calorimetry:

Heat/thermal capacity (C): This refers to the amount of heat required to produce a unit rise in temperature in a substance. This means that if an amount of heat equal to Q is added to a substance such that its temperature rises by Δθ, then;

                                                                                    (i)

                                                                                 (ii)

Specific heat capacity (c)This refers to the heat required to change the temperature of a unit mass (1 kg) of a substance by 1 unit (10C or 1 K). If for example the temperature of an object of mass m changes by ∆θ when heat equal to Q is added, the specific heat capacity c  is given by:

                                                                                  (iii)

Equation (iii) is often expressed as;

                                                                            (iv)

Water has a high specific heat capacity and as a result can absorb a relatively large amount of heat before reaching the boiling point. It is for this reason that water is used as a coolant.

Latent heat; This refers to the amount of heat required to change the state of matter. Latent heat is also referred to as hidden heat because it does not lead to an increase in temperature. The amount of heat required to change the state of a unit mass of a solid to a liquid (or liquid to solid) without a change in temperature is referred to as the latent heat of fusion. For example, if ice at -100C is heated, its temperature increases until it attains a temperature of 00C. The temperature then remains constant even with the addition of more heat. During this time however, the ice melts and turns to water at 00C. If heating stopped when the temperature of ice was at 00C, the ice would not have melted. This means that heat is required to melt the ice, and that heat is the latent heat of fusion. The heat is used to weaken the cohesive force between the ice molecules as opposed to raising the temperature. The temperature at which substances melt (melting point) is different for different substances. It is important to note that;

  • Presence of impurities lower the melting point. 
  • Increase in pressure lowers the melting point
  • The higher the cohesive force the higher the melting point. For example, ice aside, the melting point of wax is lower than that of metal.

Continued application of heat to water at 00C causes its temperature to rise up to 1000C at normal atmospheric pressure. The increase in temperature remains constant as the water changes to vapour. The heat required to change the state of a unit mass of a liquid to gas (or gas to liquid) without a change in temperature is called latent heat of vaporization. Note that various factors affect the boiling point. These include;

  • Pressure on the liquid. Water for instance has a higher boiling point at sea level where the atmospheric pressure is high compared to the top of a mountain where the pressure is low.
  • Impurities in the liquid raise the boiling point 
  • The lower the cohesive force, the lower the boiling point. 

Thus, if Q be the heat required to change the state of a substance of mass m without there being a change in temperature, the latent heat L is given by;

                                                                                      (v)

                                                                                 (vi)

Vaporization is associated with a cooling effect. If for example methylated spirit is poured on the palm of the hand, the palm feels cold. Methylated spirit has a low latent heat of vaporization and therefore vaporizes easily.  Heat drawn from the palm is used to vaporize the methylated spirit hence the palm feels cold. In nature, sweating causes cooling effect when the sweat evaporates as it draws heat of vaporization from the body. This means that if the sweat does not evaporate (for example in very humid and non-windy areas), the cooling effect is minimal. It is for this reason that sportswear is made of materials that allow sweat to pass through and consequently evaporate (as opposed to an absorbent material which keeps the skin dry instead of allowing the sweat to evaporate).

Phase transition

Phase transition from solid to vapour can be represented by a temperature-heat graph.

Anomalous behaviour of water 

Water behaves abnormally when heated from 0 0C to 4 0C in that it contracts as opposed to expand. Above this threshold temperature, the water starts expanding as expected.  

From the definition of density, a reduction in volume leads to an increase in density, mass constant.  The density of water therefore increases between 0 and 4 0C after which the density starts to reduce. The mass remains constant.

Experiment to determine specific heat capacity of a solid by the method of mixtures.

  • The solid is weighed and its mass mnoted
  • To heat the solid, the solid is suspended with a thread and submerged in a beaker containing boiling water.
  • Meanwhile, an empty dry calorimeter is weighed and the mass, say mce noted.
  • The calorimeter is then half-filled with cold water, weighed and mass mcw and temperature Twinoted.
  • Mass of water in the calorimeter is then obtained as: mw=mcw -mce
  • When the solid and the boiling water attain a steady temperature, the steady temperature, say Tsi is measured.
  • The solid is then removed from the boiling water, gently dipped in the calorimeter, and the contents well stirred. The final highest temperature reached, say Tf , is noted
  • Assuming that no heat is lost to the surrounding, the heat lost by the solid should be equal to the heat gained by the water and calorimeter, i.e.,

Where cs, cw and cc represent the specific heat capacities of the solid, water and calorimeter respectively. Thus, 

Experiment to determine latent heat of vaporization

Method 1

An electric heater converts electrical energy to heat. If the heater is connected across a voltage V such that a current, I, flows through the heater for a time t, the heat energy E produced is equal to; 

Experimental set-up for the experiment is as shown

  • The power supply is switched on and the voltage V and current recorded.
  • As the water heats up, an empty beaker for collecting condensed steam is weighed and the mass m1 recorded.
  • The beaker is then placed in a container lined with crashed ice.
  • As soon as the water being heated starts to boil, the clock is started and the beaker in crashed ice placed under the condensed steam outlet.
  • Once an appreciable amount of condensed steam (steam that has turned into water) is collected, the time is noted, say t, and the beaker together with the condensed steam weighed and the mass, say m2 recorded.
  • The mass m of the condensed steam is obtained as;
  • Assuming no heat is lost, the heat supplied by the heater (Vit) from when the water started boiling was used to generate the steam of mass m. If L be the specific latent heat of vaporization, then;

Method 2:

A beaker is filled with water, placed on a weighing balance and heated with an immersion heater.

The voltage (v) across, and current (I) through, the heater is noted. Immediately the water starts boiling, the reading on the balance m1 is noted and the stop watch started. The final reading on the balance m2 is then noted after a time t seconds.

As soon as the water starts boiling, temperature becomes constant and the heat supplied by the heater (electrical energy, E) is used to turn water into steam. If no heat is lost, then;

Now,

 

m is the mass of the steam generated given by

 

Hence

Equation (*) may therefore be expressed as;


Examples


KCSE 2021

(1) On the axes provided, sketch a graph of density against temperature for water between 0℃ and 10℃

(2) Figure 6 shows a graph of temperature against time for a pure molten substance undergoing cooling

Explain what happens to the substance in region BC (2 marks)

Region BC: The substance undergoes change of state from molten state to solid as no change in temperature occurs. 

2 a) Define the term “specific latent heat of fusion” (1 mark)

Quantity of heat required to change a unit mass of the material from solid state to liquid without change in temperature.

b) Ice of mass 5g at a temperature of −10℃ is immersed into 10.5g of hot water at 100℃ in a container of negligible heat capacity. All the ice melts and the final temperature of the mixture is 40℃. Assuming there are no heat losses to the surrounding and taking specific latent heat of fusion for ice as Lf. (Cwater = 4200 Jkg-1 K-1 and Cice = 2100 Jkg-1 K-1). Determine the:

(i) heat lost by the water. (3 marks)

(ii) heat gained by ice from −10℃ to 0℃ (3 marks)

iii) heat required to melt the ice in terms of Lf (1 mark)

(iv) heat gained by the melted ice. (2 marks)

(v) specific latent heat of fusion. (3 marks)

(3) Figure 8 shows two pieces of ice A and B trapped using wire gauze in a larger beaker containing water.

Heat is supplied at the center of the base of the beaker as shown. State the reason why B melted earlier than A. (1 mark)

Heated water at the bottom becomes less dense which rises to the 

top. Hence ice B melts earlier than A.


KCSE 2020

(1) Two similar containers A and B are filled with equal masses of water at the same temperature. Container A is made of copper while container B is made of glass. Heat is then supplied to the containers at the same rate. State with a reason, the container in which water boils first. (2 marks)

 

where k is a constant of proportionality referred to as coefficient of thermal conductivity. Good conductors like most metals have a large coefficient of thermal conductivity while poor conductors such as glass and wood have a smaller coefficient of thermal conductivity.  Copper is a good conductor of heat relative to glass hence the water in A boil first

(2) (a) Figure 14 shows a setup that can be used to determine the specific latent heat of vaporisation of water. A beaker containing some water was placed on a weighing balance and an immersion heater rated 500 W immersed in the water.

The water was then heated until it boiled. When the water started boiling, the initial reading on balance was noted and the stop watch started immediately. The final reading on the balance was then noted after a time t seconds.

(a) State how the mass of steam can be measured using this setup. (1 mark)

Mass of steam (m) produced equals the difference between final (m2) and initial (m1) balance readings. That is;

(b) Write down an expression for the heat supplied by the heater. (1 mark)

(c ) Determine the specific latent heat of vaporisation of water. (3 marks)

the heat supplied by the heater (electrical energy, E) is used to turn water into steam. If no heat is lost, then;

(3) It is observed that when methylated spirit is poured on the palm, the palm feels colder as it dries up. Explain this observation. (2 marks)

Methylated spirit has a relatively low specific heat capacity and latent heat of vaporization. Heat drawn from the palm is used to vaporize the methylated spirit hence the palm feels cold.


KCSE 2019

(1) Figure 2 shows a round bottomed flask containing a coloured liquid.

The flask is fitted with a capillary tube. It is observed that on holding the flask with bare hands, the level of the liquid in the capillary tube initially drops slightly and then rises. Explain this observation. (3 marks)

When a liquid (say water) and a solid (say glass) are subjected to the same temperature, the solid absorbs heat at a higher rate compared to the liquid. The glass expands at a higher rate compared to a liquid hence the initial reduction of liquid level. As the liquid gains more heat, it expands more and the level goes up.

(2) Figure, 3 shows two metal rods A and B of equal length made of the same material but different diameters

Wax is attached at one end of each rod. A source of heat is placed between the two metal rods. State with a reason, what is observed on the wax. (2 marks)

Wax on rod B drops off first. The larger the cross-sectional area the higher the rate of heat flow. 

(3) State the meaning of the term “heat capacity.” (1 mark)

Heat capacity is the quantity of heat energy required to raise the temperature of a substance by 1 K.

(4) State how pressure affects the melting point of a substance. (3 marks)

Increase in pressure lowers the melting point while decrease in pressure raises the melting point.

(5) Figure 7 Shows a setup of apparatus that may be used to measure the specific latent heat of vaporisation of steam.

(a) Describe how the mass of condensed steam is determined. (3 marks)

Measure the mass of the empty beaker m1

Measure the mass of the beaker plus the condensed steam m2

Mass of condensed steam

 

(b) Other than mass and time, state two other measurements that should be taken during the experiment. (2 marks)

  • Voltage
  • current

c) Show how the measurements in (c)(ii) can be used to determine the specific latent heat of vaporisation of water. (2 marks)

Assuming no heat is lost,

Heat generated by electric heater = Heat used to produce steam

(d) State the precautions that should be taken so that the mass of the condensed steamed measured corresponds to the actual mass of steam collected during the time recorded in the experiment (1 mark)

  • Covering the beaker to reduce loss of vapour
  • Placing the beaker collecting the condensed steam in crashed ice to lower the temperature of the condensed vapour thereby reducing evaporation
  • The flask for boiling water should be tightly sealed so that steam generated does not escape.

(e) State why it is not necessary to measure temperature in this set up. (1 mark)

The temperature of a boiling liquid is constant


KCSE 2018

(1)  Figure 3 shows the shape of a bimetallic strip after it was cooled below room temperature.

Explain why the strip curved is as shown. (2 marks)

Strip is curved because invar has a higher coefficient of linear expansion than brass. This means that invar expands more than brass.

(2) State two ways in which a mercury-based thermometer can be modified to read very small temperature changes. (2 marks) 

  • Make bulb thinner so that heat reaches the bulb easily
  • Make capillary tube narrower so that a small expansion produces a longer mercury column.

(3) State two differences between boiling and evaporation. (2 marks)

  • Boiling occurs at all temperature while boiling occurs at a specific fixed temperature
  • Evaporation occurs at the surface while boiling involves the entire liquid

(4) State three ways in which loss of heat by conduction is minimised in a vacuum flask. (3 marks)

  • Glass walls
  • Vacuum
  • Plastic stopper

(5) In a certain experiment, 50 g of dry steam at 100 c was directed into some crashed ice at o C.  Given that latent heat of vaporization of water is 2.26x106 J/kg, latent heat of fusion of ice is 3.34x105 and specific heat capacity of water of 4.2x103 J/kg0C. Determine the:

(i) quantity of heat lost by steam to change to water at 100 °C. (2 marks)

quantity of heat lost by 0.05 kg steam to change to water at 100 °C heat if specific heat of vaporization of water 2,260 kJ/kg

(ii) quantity of heat lost by the water to cool to 0°C.(2 marks)

Water cools hence temp not constant

(iii) mass of ice melted at 0 °C.(2 marks)

If mass of ice be m;


KCSE 2017

(1) When a liquid is heated in a glass flask, it is observed that the level at first goes down and then rises. Explain this observation. (2 marks)

When a liquid (say water) and a solid (say glass) are subjected to the same temperature, the solid absorbs heat at a higher rate compared to the liquid. Objects expand when heated and as such glass will expand at a higher rate compared to a liquid hence the initial reduction of liquid level. As the liquid gains more heat, it expands more and the level goes up.

(3) A student who wanted to take a bath mixed 4 kg of water at 80 °C with 6 kg of water at 20 °C. Determine the final temperature of the water. (3 marks)

suppose after mixing the hot and cold water, the temperature of the mixture becomes θ. If  c be the specific heat capacity of the water;

    


Practice Questions


 KCSE 2016

(1) Figure 3 shows a hot water bath with metal rods inserted through one of its ends. Some candle wax is fixed at the end of each rod. Use this information to answer questions 5(a) and 5(b).

(a) What property of metals could be tested using this set-up? (1 mark)

(b) Besides the length of the rods that is kept constant, what else should be kept constant when comparing the property for the different metal rods? (1 mark )

(2) Figure 4 shows a uniform light bar resting horizontally on corks floating on water in two beakers A and B.

Explain why the bar tilts towards side A when equal amount of heat is supplied to each beaker (2 mark)

(3) Figure 5 shows an aluminium tube tightly stuck in a steel tube.

Explain how the two tubes can be separated by applying a temperature change at the junction given that aluminium expands more than steel for the same temperature rise. (2 marks)

(4)  Water of mass 3.0 Kg at 20°C is heated in an electric kettle rated 3.0KW. The water is heated until it boils at 100°C. Given that the specific heat capacity of water = 4200J Kg-1 K-1, heat capacity of the kettle = 450.1K-1, specific latent heat of vaporisation of water = 2260 kJ Kg-1.

Determine:

(i) the heat absorbed by the water. (3 marks)

(ii) heat absorbed by the electric kettle. (2 marks)

(iii) the time taken for the water to boil. (3 marks)

(iv) how much longer it will take to boil away all the water. (3 marks)


KCSE 2015

(1) Two containers A and B of equal dimensions but different metals are fitted with identical glass casings. The two containers initially at the same temperature are simultaneously filled with boiling water. It is observed that the glass casing on A breaks earlier than the one on B. Explain this observation. (2 marks)

(2) Figure 6 shows a glass with water fitted with two identical thermometers A and B. It is heated as shown

State with a reason which one of the two thermometers shows a higher temperature (2 marks)

(3)  Figure 10 shows an incomplete set up that can be used in an experiment to determine the specific heat capacity of a solid of mass m by electrical method.

(i) Complete the diagram by inserting the missing components for the experiment (2 marks)

(ii) Other than temperature, state three measurements that should be taken. (3 marks)

(iii) The final temperature was recorded as 0. Write an expression that can he used to determine the specific heat capacity of the solid. (2 marks)

(4) State three ways of increasing the sensitivity of a liquid-in-glass thermometer. (3 marks)


KCSE 2014

(1) Figure 4 shows a source of heat placed at equal distances from two identical flasks X and Y containing air. The surface of X is painted black while Y 1S clear.

X and Y are linked by a U-tube filled with water whose levels S and T are initially the same. It is later observed that S falls while T rises. Explain this observation. (2 mark)

(2) On the axis provided, sketch the graph which shows the relationship between volume and temperature of a fixed mass of water in the temperature range 0°C to 10°C. (1 mark)

(3) Figure 8 shows a graph of the variation of temperature with time for a pure substance heated at a constant rate.

Assuming that heat transfer to the surroundings is negligible, state the changes observed on the substance in region:

(a) BC; (1 mark)

(b) DE. (1 marks)

(4) In an experiment to determine the specific latent heat of vapourization of water, steam of mass 10 g at 100°C is passed into 100 g of Water initially at 20°C in a container of negligible heat capacity. The temperature of the Water rises to 70°C.(Take the specific heat capacity of water as 4.2 x 10“ J kg" K" and the boiling point of water as 100°C)

(i) Determine the specific latent heat of vapourization of water. (4 marks)

(ii) State two sources of error in this experiment. (2 mark)


KCSE 2013

(1) Figure 3 shows a piece of wood fitted into a copper pipe and a piece of paper wrapped tightly around the junction.

It is observed that when a flame is applied around the paper at the junction, the side of the paper around the wood burns first. Explain this observation. (2 marks)

(2) Figure 5 shows a Bunsen burner.

Explain how air is drawn into the burner when the gas tap is open. (3 marks)

(3) State the meaning of the term “specific latent heat of fusion”. (1 mark)

(4) Figure 8 shows a set up of apparatus used in an experiment to determine the specific latent heat of fusion of ice.

The following readings were noted after the heater was switched on for 5 minutes:

- mass of beaker = 130 g

- mass of beaker + melted ice = 190 g

(a) Determine the:

(i) energy supplied by the 60 W heater in the 5 minutes. (3 marks)

(ii) specific latent heat of fusion of ice. (4 marks)

(b) It was observed that some of the crushed ice melted even before the heater was switched on. State a reason for this observation. (1 mark)


KCSE 2012

(1) Figure 2 shows a flat bottomed flask containing some water. It is heated directly with a very hot flame.

Explain why the flask is likely to crack. (2 marks)
(2) Figure 4 shows a graph of temperature against time when melting pure ice at 00C is heated uniformly. 


Explain what happens between parts; 

(a) OA        (1 mark)

(b) AB     (1 mark)

(3) Figure 11 shows how an underground room was ventilated. It had two vents, one at A and the other at B. A fire was lit at point C. 

Explain what happened to the ventilation when the fire was lit. (3 marks)

(4) Explain how a vaccum flask minimizes loss of heat through radiation. (1 mark)

(5) In an experiment to determine the unusual expansion of water, a fixed mass of water at 00C was heated until its temperature reached 200C. On the axis provided, sketch a graph of density against temperature of the water. From 00C to 200C. (2 marks)

(6) An immersion heater rated 2.5 kW is immersed into a plastic jug containing 2 kg of water and switched on for four minutes. Determine:
(a) The quantity of heat gained by the water; (2 marks)

(b) The temperature change of the water. (3 marks) (take specific heat capacity of water as 4.2 × 103 Jkg-1 K-1)


KCSE 2011

(1) Figure 3 shows an aluminium tube tightly stuck in a steel tube.

Explain how the two tubes can be separated by applying a temperature change at the junction given that aluminium expands more than steel for the same temperature rise.  (2 marks)

(2)  Figure 4 shows two identical beakers P and Q full of water at 90°C. Two similar cold wet clothes are wrapped, one around the top of P and the other around the bottom of Q.

State with a reason, the beaker in which the water cools faster. (2 marks)

(7) ) In an experiment to determine the specific heat capacity of a metal, a 100g of the metal was transferred from boiling water to a lagged copper calorimeter containing cold water. The water was stirred and a final steady temperature was realized. The following data was recorded.

  • Initial temperature of cold water and calorimeter = 20°C.
  • Temperature of boiling water = 99°C.
  • Final temperature of water, calorimeter and the metal = 27 .7"C.
  • Mass of cold water and calorimeter = 130g.
  • Mass of calorimeter = 50g.

(Take specific heat capacity of water as 42O0Jkg"K"‘)

(Specific heat capacity of copper as 40OJkg“K"').

(a) Use the data to determine:

(i) the heat gained by the water and the calorimeter; (3 marks)

(ii) the specific heat capacity of the metal. (3 marks)

(b) State one possible source of error in the value of the specific heat capacity obtained in the experiment. (1 mark)


KCSE 2010

(1) When a liquid is heated in a glass flask, its level at first falls, then rises. Explain this observation.             (2 marks)

(3) Water of mass 3.0kg initially at 20°C is heated in an electric kettle rated 3.0KW. The water is heated until it boils at 100°C.

(Take specific heat capacity of water 4200Jkg-1K-1,

Heat capacity of the kettle =450JKg-1, K-1

Specific latent heat of vaporization of water = 2.3mJkg-1).

(a) Determine:

(i) the heat absorbed by the water;       (2 marks)
(ii) heat absorbed by the electric kettle;     (2 marks)
(iii) the time taken for the water to boil;      (2 marks)

(b) how much longer it will take to boil away all the water.       (3 marks)

 


Dr. Margaret W. Chege

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