[Basic Chemistry] The Three States of Matter and Phase Changes [Class Encyclopedia]
In this article, we will summarize the key points to cover in class regarding the three states of matter and phase changes, which are studied in high school chemistry basics.
The three states of matter and phase changes serve as a bridge from junior high school science review to high school chemistry.
By incorporating the perspectives of thermal motion of particles and the attractive forces between particles, this topic also leads to an understanding of pure substances and mixtures, separation operations such as distillation and recrystallization, and the structure and properties of matter studied later.
The Three States of Matter and Phase Changes
Particles that make up matter include atoms, molecules, and ions.
The substances we see are made up of large numbers of these particles.
Substances around us usually take one of three states: solid, liquid, or gas.
These three states are collectively called the "three states of matter."
When temperature or pressure changes, matter may transition to another state without changing the type of substance itself.
This change is a "phase change."
There are six typical phase changes as follows.
Solid → Liquid: Melting
Liquid → Solid: Freezing
Liquid → Gas: Vaporization (Evaporation)
Gas → Liquid: Condensation
Solid → Gas: Sublimation
Gas → Solid: Deposition
The change from liquid to gas is collectively called "vaporization."
Vaporization that occurs at the surface of a liquid is "evaporation," and vaporization that also occurs from within the liquid is "boiling."
Also, while the change from gas to solid is referred to as "deposition," some textbooks may use "sublimation" to describe both directions: solid to gas and gas to solid.
It is a good idea to check the terminology used in your textbook for class.
States of Matter and Particle Behavior
So, why does the same substance have different states such as solid, liquid, and gas?
When considering the state of matter from the perspective of particles, the "thermal motion" of particles and the "attractive forces between particles" are important.
Particles are constantly in thermal motion. On the other hand, there is an attractive force acting between the particles.
The state of matter can be understood by the balance of the influence of these two factors.
Solid
In a solid, particles are arranged regularly and vibrate while maintaining a nearly constant position.
Because the influence of the attractive force between particles is greater than that of thermal motion, the particles cannot move significantly from their fixed positions.
Therefore, a solid maintains a constant shape and volume.
Liquid
In a liquid, particles are close to each other, but the regular arrangement like that of a solid has broken down.
Because particles can move while changing their positions relative to each other, the shape of a liquid changes to fit the container.
On the other hand, because the distance between particles does not change significantly, the volume is kept almost constant.
Gas
In a gas, particles are separated from each other and move freely through space.
While the attractive forces between particles do not disappear entirely in a gas, in a typical dilute gas, their influence is extremely small compared to thermal motion.
Therefore, a gas spreads throughout the entire container and does not have a fixed shape or volume.
Also, it has the characteristic of being more easily compressed compared to solids and liquids.
If students organize the concept that as matter changes from 'solid to liquid to gas,' thermal motion becomes more active and the influence of constraints due to attractive forces between particles becomes relatively smaller, it becomes easier for them to understand the differences between the three states by linking them to a particle model.
Phase Changes and Temperature Changes
As an example, consider the case of heating 18 g (1 mol) of ice at a constant rate under 1 atm of pressure.
When ice is heated, the temperature of the ice rises first.
When it reaches 0 °C, melting begins, and ice and water coexist.
Once all the ice has turned into water, the temperature of the water rises again.
When it reaches 100 °C, boiling begins, and water and water vapor coexist.
After all the water has turned into water vapor, the temperature of the water vapor rises.
When a pure substance undergoes a phase change under constant pressure, the temperature is kept constant while melting or boiling continues.
This is because the added heat is used not for raising the temperature, but for breaking down the arrangement of particles or weakening the constraints caused by attractive forces between particles.
At this time, if you organize each stage of heating from the perspective of particles, it looks like the following:
1. Heating a solid
Due to the added heat, the vibration of particles in fixed positions becomes more intense, and the temperature of the solid rises.
2. Melting of a solid
The added heat is used to break down the regular particle arrangement of the solid.
The heat absorbed at this time is called the 'heat of fusion.'
For a pure substance, the temperature remains constant until it becomes completely liquid.
3. Heating a liquid
Due to the added heat, the thermal motion of the particles becomes more active, and the temperature of the liquid rises.
4. Boiling of a liquid
The added heat is used to weaken the constraints due to attractive forces between particles and to pull the particles apart as a gas.
The heat absorbed at this time is called the 'heat of vaporization.'
For a pure substance, the temperature remains constant until it becomes completely gas.
5. Heating a gas
Due to the added heat, the thermal motion of the particles becomes even more active, and the temperature of the gas rises.
Note that melting points and boiling points change depending on pressure.
Also, in mixtures, the temperature may not remain constant during a phase change.
Therefore, adding conditions such as 'pure substance' and '1 atm' to the graph on the blackboard will be useful later when considering the differences between pure substances and mixtures.
Tips for Inquiry
In learning about phase changes, the question of 'where does the added heat go when the temperature remains constant' serves as a starting point for eliciting explanations using the particle model.
For example, an inquiry-based activity could involve slowly heating ice, recording the temperature and the state of the substance at regular intervals, and creating a heating curve.
Students can be asked to identify where the slope of the graph changes or where the temperature remains nearly constant, and to explain how the particles are behaving in each section using diagrams and words.
Furthermore, this can be developed into questions such as the following:
- How does changing the mass of the ice affect the time it takes to melt?
- How do changes in the intensity of heating affect the slope of the graph or the duration of the plateau?
- What differences appear in the heating curves of pure water versus saltwater?
- How does the boiling point change when atmospheric pressure changes?
- How can the use of energy during a phase change be represented using the particle model?
When conducting experiments, please be very careful about burns from heating equipment, boiling water, and steam, and replace them with demonstration experiments or analysis of recorded videos if necessary.
Consultations and Inquiries Regarding Lesson Planning
Phase changes is a unit that goes beyond a review of junior high school science, connecting learning to particle concepts, energy, separation operations, and the structure and properties of matter.
On the other hand, the depth of student understanding can change significantly depending on the order in which the particle model and heating curves are presented and what kind of questions are posed.
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