IB Objectives:
Describe the kinetic molecular theory as a model to explain physical properties of matter (solids, liquids and gases) and changes of state.
Interpret observable changes in physical properties and temperature during changes of state.
Convert temperatures between Celsius and Kelvin scales.
In my initial state, I existed as part of a highly ordered, crystalline solid structure. My kinetic energy was at a minimum, primarily manifesting as constant vibrations or oscillations about a fixed equilibrium point within the lattice. Strong interatomic forces held me firmly in place, giving the substance its definite shape and volume. As external thermal energy was steadily transferred to our system, my vibrational kinetic energy increased, and the overall temperature of the solid began to rise predictably. This period of increasing temperature continued until we reached a critical point: the melting point. At this precise temperature, a significant change occurred; despite continuous energy input, the temperature of the system remained constant. This absorbed energy, known as the latent heat of fusion, was not used to increase my kinetic energy, but rather to overcome the strong interatomic forces and break down the rigid lattice structure, initiating my transition to a new phase.
Once the lattice was sufficiently disrupted by the absorbed latent heat, I transitioned into the liquid state. My kinetic energy had increased enough to allow for new forms of motion: translation (sliding past other atoms) and rotation, in addition to continued vibration. While still in close proximity with my fellow atoms, the interatomic forces were now partially overcome, giving the liquid a definite volume but an amorphous shape, conforming to the container. As heating continued, the additional thermal energy was once again converted into my kinetic energy, causing the temperature of the liquid to steadily rise. This period of increasing temperature continued until we reached another critical threshold: the boiling point, also known as the vaporization temperature. Here, another plateau would be observed on a temperature-time graph.
At the boiling point, the absorbed energy, the latent heat of vaporization, was entirely dedicated to providing the necessary energy to completely overcome the remaining interatomic forces holding me within the liquid phase. During this process, my kinetic energy remained constant, hence the stable temperature, but my potential energy increased significantly as I gained complete freedom from my neighbors. Once enough energy was absorbed, I became a gas. In this state, my kinetic energy was very high, and my motion was predominantly rapid, random translational movement, allowing me to expand to fill any volume. Interatomic forces were now negligible, and my interactions with other gas particles were limited to fleeting, elastic collisions. As more thermal energy continued to be supplied to the gaseous system, my kinetic energy further increased, resulting in a sharp and continuous rise in the temperature of the gas.
This relentless absorption of thermal energy eventually propelled me into a state of extreme energy. My kinetic energy became so immense that during collisions with other particles, the impacts were powerful enough to cause ionization. This meant that my outermost electron gained sufficient energy to escape the electrostatic attraction of my nucleus, becoming a "free electron" while I remained a positively charged ion. I had entered the plasma state, a superheated, ionized gas where the system consists of a mixture of ions and free electrons. In this phase, the temperature continued to climb to extraordinarily high levels as the incoming energy contributed to further ionization and the continued acceleration of these charged particles, driven by strong electromagnetic forces. This represents the ultimate manifestation of thermal energy, where the very structure of atoms is overcome.
Task 1: Draw the heating curve. Sketch the approximate shape of the heating curve based on the descriptions in the text.
Task 2: Annotate the Graph with Vocabulary. Add labels and annotations using the bolded IB-appropriate vocabulary from the essay:
Sections of the curve: Plasma, Gas, Liquid, Solid.
Plateaus: boiling, melting.
Energy changes: Latent heat of recombination (optional, but good for HL), Latent heat of vaporization (released), Latent heat of fusion (released).
Particle behavior descriptions: High-velocity translational motion, electrons stripped away (plasma); chaotic, random motion (gas); constrained, fluid movement (liquid); fixed lattice position, oscillations (solid).
Key points: boiling point temperature, melting point temperature.
Given a beaker of ice and water, predict the cooling curve as it sits open in a room with at temperature of 22 C.
Regardless of state of matter, all particles have some quantity of kinetic energy; thus, matter and energy are closely linked.
The kinetic molecular theory of matter uses this link to explain physical properties and behaviors of solids, liquids, and gases, as well as change of state.
Matter can change state, reversibly, depending on its kinetic energy.
A substance’s state of matter is shown by letters in brackets after the formula: (s) for solid, (ℓ) for liquid, and (g) for gas. Ex. H2O (ℓ)
Complete the diagram at the right, adding names of each state change process. Vaporization is given; there are 5 more.
Freezing
Melting
Sublimation
Deposition
Condensation
Temperature is a measure of the average kinetic energy of a sample.
In a hotter substance, particles are moving around more.
A heating curve or cooling curve is a line graph that represents the changes in state of matter that occur as heat is added to (or removed from) a substance.
These curves show states of matter, as well as melting point and boiling point, which are represented by the flat portions of the graph.
Refer to the letters on the graph to the right:
a → b As the solid is heated, the kinetic energy of particles increases, increasing temperature.
b → c Melting point. All energy added is used to start to break the forces between particles, so temperature stays constant. Particles are no longer locked in their position as solids. This temperature is specific to the substance that is melting/freezing.
c → d Liquid is heated, and kinetic energy of the particles increases, increasing temperature.
d → e Boiling point. Now there is enough energy to completely break the forces between particles to form a gas. Temperature remains constant. Notice: more energy is required here (longer flat line), because ALL forces between particles are broken, rather than just some. This temperature is specific to the substance that is boiling/condensing.
e → f Gas is heated, and kinetic energy of particles increases, increasing temperature.
This should be similar to your data from the CLOSE passage above.
You should be able to explain the relationship between the two curves shown (heating and cooling).
You have a liquid substance at 80°C, and its melting point is 35°C. Sketch and label this substance’s cooling curve, going down to 25°C.
Why would a burn caused by steam be more dangerous than a burn caused by boiling water, when both are at the same temperature?
Temperature Scales
We know that temperature is dependent upon the average kinetic energy of the particles in a substance.
So, how did temperature scales come about, to actually quantify this kinetic energy?
Many attempts were made to measure relative temperature, but the first widely accepted scale that of physicist Daniel Fahrenheit.
This scale was based on the freezing point of brine, and human body temperature.
Now of course we use the Celsius scale in science, which is based on the freezing and boiling points of water (0°C and 100°C).
Celsius, while useful for temperatures encountered in everyday human experience, it is only a relative scale—its zero value has been arbitrarily assigned.
The Kelvin scale, however, is an absolute scale, where values of Kelvin are directly proportional to the average kinetic energy of particles in the substance.
Absolute zero (0K) implies zero kinetic energy at this temperature.
One Kelvin unit has the same incremental value as one Celsius degree (°C), so an increase of 1 Kelvin is the equivalent of an increase 1°C. (The kelvin is the SI base unit of temperature.)
Since values on the Celsius and Kelvin scales are just shifted by 273.15 degrees, with the same size unit of temperature, it is simple to convert between the two:
K = °C + 273.15
In Data Booklet #4…