# Boiling Water Demonstration: Heating Curve

Water is placed in a beaker.  A digital temperature probe measures the temperature of water as it is heated on a heating plate.  The temperature increases from room temperature, 25°C to the boiling point 100°C.  As the liquid boils, the temperature of the liquid remains at 100°C.  The temperature probe is connected to a Vernier computer interface and a graph of temperature versus time is displayed on a computer monitor or PC. This graph is part of a heating curve for water. The amount of heat required to change 1.00 mole of water at 25°C to 1.00 mole of steam at 100°C is calculated.

An optional Computer Animation depicting at the particulate level of representation (particulate nature of matter) shows water molecules in the liquid phase gaining sufficient energy to break free of the intermolecular forces holding the H2O molecules in the liquid phase and enter the gas phase is available.  This computer animation is helpful in helping students to visualize the dynamic aspects of what molecules of water are doing during boiling of a liquid.

Curriculum Notes

This demonstration works well when discussing heating curves, energy required to heat a substance and the relationship among intermolecular forces and phase changes.

Student Difficulties (Misconceptions)

1.  A significant percentage of students think that when water boils hydrogen (H2) and oxygen gas O2(g) are produced.

2H2O(l) --> 2H2(g) + O2(g)  ***An incorrect equation representing the boiling of water.

The scientific view is the H2O molecules in the liquid phase gain enough energy to break the intermolecular forces between water molecules and enter the gas phase as H2O molecules.

H2O(l) --> H2O(g)

The energy added during the phase change of a liquid to a gas goes to breaking the intermolecular forces (hydrogen bonding, dipole-dipole, and London Dispersion Forces) of attraction.  Water molecules gain energy and increase the distance among its neighboring water molecules, an increase in potential energy.  When the distance increase the electrostatic force of attraction decreases according to Coulomb's Force Law.

F = k [qi . q2)/r2]

2.  A significant percentage of students think that when water boils the temperature increases and the average kinetic energy of the H2O increases.  In fact, when a pure liquid boils the temperature of the liquid does not change, therefore the average kinetic energy of the liquid does not change.  The Kinetic Molecular Theory explains that temperature of a system is proportional to the average kinetic energy.

average KE = average 1/2 mv2   = 3/2[R/NA]T

R is the gas constant 8.314 J/mol K

NA is Avogadro's Number   T is temperature in units of Kelvin

Other Resources

URL PHET

Learning Objectives

1.  Describe, at the particle-level of representation, what the molecules are doing during a phase change.

2.  Apply the terms Kinetic Energy and Potential Energy to describe what occurs to the system and to the molecules when heating liquid water and boiling the water. (states of matter).

3.  Connect particle-level descriptions of matter, including different phases, with macroscopic observations and properties of matter.

One day of lead time is required for this project.
Discussion

Boiling represents the transition from liquid to gas, a phase change.  The boiling point of a liquid is the temperature at which the vapor pressure equals the external pressure. For a pure substance, the temperature of the liquid does not change when it is boiling.  The average kinetic energy of the liquid does not change when a liquid boils.  During boiling, heat is being added to the liquid.  The added energy goes to overcoming the intermolecular forces of attraction holding the water molecules together as a liquid, increasing the distance between molecules - the potential energy of the system increases.   The length of the line of the heating curve for water at 100 °C is the amount of heat required to change all of the liquid into a gas.

This heat is called the change in enthalpy of vaporization, ΔHvap. Vaporization is a specific scientific term for changing a pure liquid into the gas phase.  The  amount of heat required to change 1.00 mole of a liquid to a gas at the boiling point is

q = nΔH vaporization

Materials

water, beaker, hot plate, ring stand and clamp, temperature probe, computer interface, computer display

Footnotes

References

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© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group