Think about ice, water, and steam. These are all forms of the molecule H2O, but in different phases, or states of matter. Ice is a solid, water is a liquid and steam is a gas! But, what does this mean? Let's talk about liquids, solids and gases!
- First, we will talk about where solids, liquids, and gases are found in the periodic table.
- Then, we will look at facts about solids, liquids, and gases.
- We will also talk about kinetic energy and chemical activity.
- Lastly, we will compare solids, liquids, and gases.
Solids, Liquids, and Gases on the Periodic Table
Before diving into the periodic table, you need to remember that matter is anything that has mass and occupies space. The functional unit of matter is an atom, at least for chemistry. The simplest type of matter in chemistry is called an element, and an element is composed of only one type of atom!
Now, let's look at the periodic table, showing the state of the elements in nature. At room temperature (25 °C) and under standard pressure (1 atm) conditions, most elements are found in nature in the solid state. Some nonmetals such as nitrogen, oxygen, fluorine, chlorine, and hydrogen are found as gases, whereas bromine and mercury are found in the liquid state.
Now, let's dive into what solids, liquids, and gases are.
Facts about Solids, Liquids, and Gases
Elements can exist in three states of matter: solids, liquids, and gases. Molecules in these states of matter differ in their physical properties.
Physical properties are measurable properties that are used to describe compounds.
The state of matter of an element and the energy required to change from one state of matter to another is directly related to the strength of its intermolecular forces.
Intermolecular forces are referred to as attractive forces that exist between molecules.
Intermolecular forces are responsible for influencing the physical properties of a chemical compound. Some of the physical properties that can be affected by these forces are states of matter, mass, density, volume, hardness, boiling point (BP), and melting point (MP).
If you want to learn more about the different types of intermolecular forces, check out "Types of Intermolecular Forces"
Let's start by looking at solids.
Solids
Solids have a fixed shape and volume. They are not compressible because their particles are tightly arranged together. They also have a fixed position, and can only vibrate in place. Solids usually have strong intermolecular forces, and they can be classified as either Crystalline or Amorphous solids.
Crystalline solids are atoms, ions and molecules with a well-organized pattern and shape, such as a 3D structure.
Amorphous solids are particles that have random arrangements, so they lack an organized shape and/or pattern.
There are 4 different types of crystalline solids:
- Ionic solids
- Molecular solids
- Covalent network solids
- Metallic solids
Ionic Solids
Ionic solids have ionic bonds as their attractive forces. Their smallest units are ions. Solids in this category are brittle, hard, and have high melting and boiling points. Ionic solids can only conduct electricity in a water solution or molten state.
Sodium chloride (NaCl) is a type of ionic solid. The sodium ion (Na+) has a charge of +1, and the chlorine ion (Cl-) has a charge of -1. The melting point of NaCl is 801 °C, which is very high!
Molecular Solids
Molecular solids have intermolecular forces between the individual molecules keeping everything together. The attractive forces holding the molecules together depend on their polarity.
- Polar molecular solids have dipole-dipole and London dispersion forces present.
- Nonpolar molecular solids only possess London dispersion forces.
Solids in this group have soft textures because the intermolecular forces holding the molecules together are weak compared to the chemical bonds found in other types of solids. Their melting point (MP) varies but generally is low. Nonpolar molecular solids generally have low melting points, while polar molecular solids tend to have slightly higher melting points.
Carbon dioxide (CO2) in its solid form ( dry ice) is a type of molecular solid. In the picture, the black dots are carbon atoms and the red dots are oxygen atoms.
Covalent Network Solids
Covalent network solids have covalent bonds holding atoms together. These solids have hard textures due to their strong covalent bonds. Covalent network solids also have very high melting points. Usually, solids in this category are insoluble in water. They also tend to be poor conductors of heat and electricity, although there are some exceptions.
CGraphite and Cdiamond are both examples of covalent network solids. Graphite is different from other covalent network solids in that it has a soft texture and is a good conductor of electricity this is due to its special electronic structure.
Metallic Solid
Metallic solids have metallic bonds. As the name suggests, the smallest unit in metallic solids are metal atoms. Metallic solids are shiny, possess variable hardness, and have high melting points They are also good at conducting heat and electricity, are malleable, and ductile. Metallic solids are not soluble in water.
Aluminum (Al) is an example metallic solid. All metallic elements can be classified as metallic solids, even mercury if you just cool it down enough.
Amorphous Solids
In amorphous solids, the smallest unit can be an ion, atoms, molecules, or even polymers. The electrostatic forces present in amorphous solids can vary.
Amorphous solids have no distinct melting points, so parts of an amorphous solid melt at different temperatures. Since its particles can randomly arrange, amorphous solids lack an organized structure/pattern such as the ones we see in crystalline solids. Examples of amorphous solids include glass, obsidian (volcanic glass), and even rubber bands.
To learn more about amorphous solids, read "Amorphous Polymers".
Liquids
Liquids assume the shape of the container, but not the volume. Although liquid particles can move around the container, they are still very close to one another, and there is not much space available. So, liquids are slightly compressible. Liquids also have intermolecular forces, but they are usually slightly weaker than solids.
Liquids can have different viscosity, and the greater the viscosity, the slower the liquid will flow. For example, water and honey are liquids, but water flows faster than honey because honey has a greater viscosity.
Viscosity is a liquid's resistance to flow. When temperature increases, viscosity decreases.
Another property that affects liquids is surface tension. In liquids, the intermolecular forces pull the molecules into the liquid,
Surface tension is the amount of energy needed to increase the surface area of a liquid. When the strength of intermolecular forces increases, surface tension also increases.
Gases
Gases assume the shape and volume of the container. Since gas particles are spread out in the container, they are highly compressible when pressure is applied. In this state of matter, gases vibrate and can move freely in random directions.
Some factors affecting gases include temperature, pressure, and volume.
- When the temperature rises, particles move faster.
- When pressure increases, volume decreases since the particles are now closer together.
- When volume increases, pressure decreases.
Kinetic Energy in Solids, Liquids, and Gases
Anything that moves has kinetic energy. For example, water falling down a waterfall contains kinetic energy, and so does a bird flying! And, why does atoms and molecules have kinetic energy? Because they are always moving!
Kinetic energy - the amount of energy an object has due to its motion.
The kinetic energy of gases can be calculated using the following equation:
Let's look at a simple example!
A gas particle with a mass of 16 g can travel at 6.30 m/s. Calculate the particle's kinetic energy.
All we have to do is put the given numbers into the kinetic energy equation:
When a particle has high kinetic energy, it will be able to move around faster.
- Solids have very low kinetic energies because their particles are tightly packed and do not vibrate as much.
- Liquids have intermediate kinetic energies because they can move between each other in the little empty space.
- Gases tend to have high kinetic energies because they can freely move around in the empty space.
If you want to learn more about gases and the ideal gas law, check out "Ideal Gas Laws"!
Activities of Solids, Liquids, and Gases
Another term that you need to be familiar with is the activity (α) of a species, chemists use the term activity to describe the deviation of ideal gases and solutions from ideal behavior.
When dealing with gases, they depend on pressure. So, the activity of gas is considered to be the ratio of the actual partial pressure of the gas to its ideal partial pressure.
This can sound confusing, so let's look at an example. Let's say that you were asked to calculate the pressure for one mole of ethane at 295.15 K behaving as an ideal gas (using the ideal gas law equation) and as a non-ideal gas (using the real gas equation). You found that the pressure for ethane as ideal gas was 24.47 atm, whereas the pressure for ethane as a real gas was 20.67 atm. The activity of ethane would be:
Now, when it comes to solids and liquids, we deal with concentrations. Therefore, activity is used to tell chemists the difference between how many particles appear to be present in the solution, and the number of particles actually present in the solution. The activity of solutions can be estimated using concentration. In general, for relatively dilute solutions, the activity of a substance and its molar concentration is roughly equal.
The activity of pure solids and liquids is always equal to 1 because, per unit volume, the concentration of a pure solid or liquid is always the same. For example, the activity of liquid water is 1, and the activity of 5 grams of aluminum metal is also 1.
Comparing Solids, Liquids, and Gases
To make this simple and easier, let's make a table comparing the three states of matter using the information discussed in this article.
| Solids | Liquids | Gases |
| Shape & Volume | Fixed volume and shape | Fixed volume, but takes the shape of the container | Assumes the volume and the shape of the container |
| Compressibility | Can not be compressed | Slightly compressible | Compressible |
| Motion of molecules | Particles can only vibrate in place (not able to move around), very slow. | Random, faster than solids but slower than gases. Can only move limited distances. | Random and very fast. There is a lot of empty space for particles to move. |
Solids, Liquids, and Gases - Key takeaways
- The three states of matter are solid, liquid, and gasous.
- Solids can be divided into two categories: crystalline and amorphous solids.
- Crystalline solids have a well-arranged shape and pattern, while amorphous solids lack an organized structure.
- Most elements are found as solids at standard room temperature and pressure.
- Gases have the highest amount of kinetic energy.
References
- Moore, J. (2021). 5 Steps to a 5 AP Chemistry. McGraw Hill Professional.
- Zumdahl, S. S., Zumdahl, S. A., & DeCoste, D. J. (2016). Chemistry. Cengage Learning.
- Brown, T. L. (2009). Chemistry: the central science. Pearson Education.
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