Changes of state

If you've ever taken a run or bike ride in freezing conditions before, you may have experienced that the water in your water bottle started to have small chunks of ice in it. What happened there was a change of state of the water in your bottle! Parts of your water went from being liquid to solid because it was so cold. In this article, we will explain what changes of state there are and how they occur.

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Table of contents

    Meaning of a change of state

    Let's start by definining a state!

    A state of matter is the configuration a certain material is in: this can be solid, liquid, or gas.

    Now that we know what is a state, we can study the meaning of change of state.

    A change of state is the process of turning from a solid, liquid, or gas into another one of those states.

    Materials will change state depending on how much energy they receive or lose. With an increase of energy in a material, the average kinetic energy of the atoms begins to increase, causing the atoms to vibrate more, pushing them apart to the point that they change their state. The fact that kinetic energy changes the state of materials makes this a physical process, rather than a chemical one, and no matter how much kinetic energy is put into or taken away from the material, its mass will always be conserved and the material will always stay the same.

    Changes of state and thermodynamics

    So we know what happens when Materials change their state, but why does this actually happen? Let's look into the thermodynamic aspects of changing states, and how energy plays a part in this.

    More energy put into material will result in it turning into a liquid or a gas, and energy being taken out of material will result in it turning into a liquid or a solid. This of course depends on whether the material is starting as a solid, liquid, or gas, and what the exact environmental conditions are. For example, if a gas loses energy, it can turn into a liquid, and if a solid gains energy, it can turn into a liquid as well. This energy is typically introduced into a material via an increase in temperature or an increase in Pressure, and both of these variables can cause different changes of state.

    Changes of State examples of the molecular structure of a solid, liquid, and gas StudySmarterFig. 1: An example of the molecular structure of a solid, liquid, gas.

    A change of state occurs through a loss or increase of energy within the molecules of the material, usually through a change in temperature or pressure.

    Examples of changes of state

    Below is a list of all changes of state we need to know about, and a short explanation describing what each one is.

    Freezing

    Freezing is the change of state that occurs when a liquid turns into a solid.

    A good example of this is when water turns into ice. As the temperature decreases, the water will begin to lose energy until each water molecule no longer has the energy to move around other water molecules. Once this occurs, the molecules form a rigid structure that is kept rigid by the attraction that occurs between each molecule: we now have ice. The point at which freezing occurs is known as the freezing point.

    Melting

    Melting is the change of state that occurs when a solid turns into a liquid.

    Melting is the opposite of freezing. Using our previous example, if the ice was subjected to higher temperatures, it would begin to absorb the energy from its warmer surroundings, which would, in turn, excite the molecules within the ice and give them the energy to move around each other again: we now have a liquid again. The temperature at which a material melts is known as the melting point.

    When the temperature scale for Celsius was first made, the freezing point of water (at atmospheric pressure) was taken as the 0-point and the melting point of water was taken as the 100-point.

    Evaporation

    Evaporation is the change of state that occurs when a liquid turns into a gas.

    When a material is a liquid, it is not entirely bound by the force of attraction between molecules, but the force still has some hold over them. Once a material has absorbed enough energy, molecules are now capable of freeing themselves from the force of attraction entirely and the material turns into a gaseous state: molecules fly around freely and are not affected by each other as much anymore. The point at which a material evaporates is known as its boiling point.

    Condensation

    Condensation is the change of state that occurs when a gas turns into a liquid.

    Condensation is the opposite of evaporation. When a gas enters an environment of a lower temperature or encounters something of a lower temperature, the energy within the gas molecules begins to be sapped by the cooler environment, causing the molecules to become less excited as a result. Once this happens, they begin to be bound by the forces of attraction between each molecule, but not entirely, so the gas then becomes a liquid. A good example of this is when a piece of glass or a mirror fogs up in a hot room. The vapour or steam in a room is a gas, and the glass or mirror is a colder material in comparison. Once the vapour hits the cold material, the energy within the vapour molecules is sapped out and into the mirror, warming it slightly. As a result, the vapour turns into liquid water that ends up directly on the cold mirror surface.

    Changes of State window with condensation StudySmarterFig. 2: An example of condensation. The warm air in the room hits the cold window, turning the water vapour into liquid water.

    Sublimation

    Sublimation is different from the other changes of state that we have previously gone over. Usually, a material needs to change state 'one state at a time': solid to liquid to gas, or gas to liquid to solid. However, sublimation forgoes this and has a solid turn into a gas without having to turn into a liquid!

    Sublimation is the change of state that occurs when a solid turns into a gas.

    This occurs through the increase of energy within the material to the point where the forces of attraction between the molecules are entirely broken, with no in-between phase of having to be a liquid. In general, the temperature and pressure of the material would have to be very low for this to occur.

    Changes of State process of sublimation StudySmarterFig. 3: The process of sublimation. The white fog is the consequence of condensation of water vapour on the cold, sublimated carbon dioxide gas.

    Deposition

    Deposition is the opposite of sublimation.

    Deposition is the change of state that occurs when a gas turns into a solid.

    An example of this is when frost is formed, as the water vapour in the air on a very cold day will encounter a cold surface, lose all its energy quickly, and change its state to solid as frost on that surface, never having turned into water.

    Changes of state and the particle model

    The particle model of matter describes how molecules within a material will arrange themselves, and the movement in which to arrange themselves. Each state of matter will have a way in which they are formed.

    Solids have their molecules lined up against one another, the bond between them strong. The molecules in liquids have a looser bond between each other but are still bound, just not as rigidly, allowing for a wider degree of movement: they slide over each other. In gases, this bond is entirely broken, and individual molecules are able to move completely independently of each other.

    Diagram of changes of state

    The figure below shows the entire process of how all the changes of state relate to one another, from solid to liquid to gas and back.

    Changes of State, matter changes, StudySmarterFig. 4: The states of matter and the changes they go through.

    Plasma

    Plasma is an often overlooked state of matter, also known as the fourth state of matter. When enough energy is added to a gas, it will ionise the gas, forming a soup of the nuclei and electrons that once were paired up in the gaseous state. Deionisation is the reverse of this effect: it is the change of state that occurs when a plasma turns into a gas.

    It is possible for water to be put into the three states of matter at the same time, in specific circumstances. Look at it here!

    Changes of State - Key takeaways

    • A change of state is the process of turning from a solid, liquid, or gas into another one of those states.

    • Solids have their molecules tightly bound.

    • Liquids have their molecules loosely bound and tend to slide over each other.

    • Gases have their molecules not bound at all.

    • A change of state occurs through a loss or increase of energy within the molecules of the material, usually through a change in temperature or pressure.

    • The six different changes of state are:

      • Freezing: liquid to solid;
      • Melting: solid to liquid;
      • Evaporation: liquid to gas;
      • Condensation: gas to liquid;
      • Sublimation: solid to gas;
      • Deposition: gas to solid.

    References

    1. Fig. 1- States of matter (https://commons.wikimedia.org/wiki/File:Solid_liquid_gas.svg) by Luis Javier Rodriguez Lopes (https://www.coroflot.com/yupi666) licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
    2. Fig. 4- State transition (https://commons.wikimedia.org/wiki/File:Physics_matter_state_transition_1_en.svg) by EkfQrin is licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/)
    Frequently Asked Questions about Changes of state

    What are the changes of state in solid, liquid, and gas?

    The changes of state are freezing, melting, evaporation, condensation, sublimation, and deposition.

    What is a change of state?

    A change of state is what happens when a material goes from being in one state of matter to another state.

    What are the energy changes associated with changes of state?

    The more energy that is added to a material, the more the material will turn from a solid to a liquid to a gas. The more energy that is taken away from the material, the more it will turn from a gas to a liquid to a solid.

    What causes a change of state?

    A change of state is caused by a change in temperature or a change in pressure.

    What are examples of changes of state?

    An example of a change of state is when ice encounters an increase in temperature and becomes liquid water. A further increase in temperature boils the water and turns it into vapour. Water vapour can cool down and become liquid water again during condensation. Further cooling will result in the water freezing and becoming ice once again.

    Test your knowledge with multiple choice flashcards

    An ice cube is left out of the freezer and slowly becomes liquid water as a result. What is the name of this change of state?

    A kettle is turned on and the water in it comes out of the top of it as steam. What is the name of the change of state that is occurring?

    Mercury is kept in a fridge, and then taken out. It slowly becomes a liquid. What is the name of this change of state?

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