Studio Session 10

Changes of phase

Substances can exist in solid, liquid or gas phases.  At a constant pressure changes of phase always occur at the same temperatures for any pure substance.

The temperature is a measure of the internal, disordered energy of a substance.  The absolute temperature of any substance is proportional to the average translational kinetic energy associated with the random motion of the molecules of the substance.

m<v2> = (3/2)kBT

In SI units the scale of absolute temperature is Kelvin (K).  But we use different temperature scales in everyday situations.  The Kelvin scale is identical to the Celsius (oC) scale, except it is shifted so that 0 oC equals 273.15 K.

If an equation in physics contains the temperature T, this temperature is always the absolute temperature.  In SI units it is always measured in K.  If an equation contains a temperature difference ΔT, this temperature difference can be measured in K or oC in SI units, since both scales give the same ΔT.

In this session you will experimentally investigate a change of phase of water.

Equipment needed:

Open a Microsoft Word document to keep a log of your experimental procedures, results and discussions.  Address the points highlighted in blue.  Answer all questions.


Discussion:

Specific heat capacity

Consider the experiment described below.

A 1.0 kg mass and a 3.0 kg mass with different initial temperatures are placed together inside a well-insulated container and allowed to come to thermal equilibrium.  The insulated container prevents any transfer of energy to or from the environment (including the container itself).

Mass 1.0 kg 3.0 kg
Initial temperature 100 C 160 C
Specific heat capacity 440 kJ/(kg C) 73.33 kJ/(kg C)
Final temperature 120 C 120 C
ΔT +20 C -40 C

Do you agree or disagree with the following statements?  Explain why you agree or disagree.


Experiment

Make sure the Pasco 850 interface is turned on.  Open the Capstone program.

Start with a known quantity of hot water in a large Styrofoam cup.  Measure its temperature, and then add ice at its melting temperature (0 oC) until the temperature of the water drops to about 10 oC.  Measure the final temperature and determine the amount of ice added.  Determine the latent heat of fusion of ice/water using the measured masses and temperatures and value of the specific heat capacity of water.

Procedure:

mass of cup (kg) mcup  
mass of cup + water (kg) mcup + mw  
mass of cup + water + ice (kg) mcup + mw + mice  
mass of water (kg) mw  
mass of ice (kg) mice  
initial temperature of water (oC) Ti  
final temperature of water (oC) Tf  
latent heat of fusion (kcal/kg) Lf (measured)  
latent heat of fusion (kcal/kg) Lf (accepted)  

Data Analysis:

Discuss:


Discussion:

Regulating the temperature of the human body

The human body has the ability to regulate its temperature so that it remains very close to 37 C.  If the body is overheating as a result of strenuous exercise, the surface blood vessels dilate to increase the blood flow to the surface areas.  Heat is carried by the blood to the surface where it causes the skin temperature to increase.  The body also begins to produce sweat.  The rate of sweating increases strongly with body temperature above 37C. 


image 

Conduction, evaporation, convection, and radiation can now transfer heat from the skin to the environment.  The chart below shows the relative importance of these heat transfer mechanisms at different environmental temperatures.


image

Questions:

Evaporation

The body secrets water on to the skin from sweat glands.  As this water evaporates, the latent heat of vaporization is removed from the body.  The rate of evaporation increases as the degree of saturation of the surrounding air decreases.  In warm humid weather, the rate may be so low that a layer of water accumulates on the skin.  In still air, the air near the skin will become saturated and evaporation will stop.  Evaporation will increase if this saturated air is continually removed by wind or an artificially produced air stream.

Questions:


Convert your log into a session report, certify with you signature that you have actively participated, and hand it to your instructor.