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Properties & States of Matter (Physics): An Overview

The physical properties of matter underlie much of physics. In addition to understanding states of matter, phase changes and chemical properties, when discussing matter, it is important to understand physical quantities such as density (mass per unit volume), mass (amount of matter) and pressure (force per unit area).

Atoms and Molecules

The everyday matter than you are familiar with is made of atoms. This is why atoms are commonly called the building blocks of matter. There are more than 109 different types of atoms, and they represent all elements on the periodic table.

The two main parts of the atom are the nucleus and the electron shell. The nucleus is the heaviest part of the atom by far and is where most of the mass is. It is a tightly bound region at the center of the atom, and despite its mass, it takes up relatively little space compared to the rest of the atom. In the nucleus are protons (positively charged particles) and neutrons (negatively charged particles). The number of protons in the nucleus determines which element the atom is, and different numbers of neutrons correspond to different isotopes of that element.

The electrons are negatively charged particles that form a diffuse cloud or shell around the nucleus. In a neutrally charged atom, the number of electrons is the same as the number of protons. If the number is different, the atom is called an ion.

Molecules are atoms that are held together by chemical bonds. There are three major types of chemical bonds: ionic, covalent and metallic. Ionic bonds occur when a negative and positive ion are attracted to each other. A covalent bond is a bond in which two atoms share electrons. Metallic bonds are bonds in which the atoms act like positive ions embedded in a sea of free electrons.

The microscopic properties of atoms and molecules give rise to the macroscopic properties that determine the behavior of matter. The response of the molecules to changes in temperature, the strength of the bonds and so on all lead to properties like specific heat capacity, flexibility, reactivity, conductivity and many others.

States of Matter

A state of matter is one of many possible distinct forms that matter can exist in. There are four states of matter: solid, liquid, gas and plasma. Each state has distinct properties that distinguish it from the other states, and there are phase transition processes by which matter changes from one state to another.

Properties of Solids

When you think of a solid, you probably think of something hard or firm in some way. But solids can be flexible, deformable and malleable as well.

Solids are distinguished by their tightly bound molecules. Matter in its solid state tends to be more dense than when it is in its liquid state (though there are exceptions, most notably water). Solids hold their shape and have a fixed volume.

One type of solid is a ​crystalline​ solid. In a crystalline solid, the molecules are arranged in a repeating pattern throughout the material. Crystals are easily identifiable by their macroscopic geometry and symmetries.

Another type of solid is an ​amorphous​ solid. This is a solid in which the molecules are not arranged in a crystal lattice at all. A ​polycrystalline​ solid is somewhere in between. It is often composed of small, single crystal structures, but without a repeating pattern.

Properties of Liquids

Liquids are made of molecules that can flow easily past each other. The water you drink, the oil you cook with and the gasoline in your car are all liquids. Unlike solids, liquids take the shape of the bottom of their container.

Though liquids can expand and contract at different temperatures and pressures, these changes are often small, and for most practical purposes, liquids can be assumed to have a fixed volume as well. The molecules in a liquid can flow past each other.

A liquid’s propensity to be slightly “sticky” when attached to a surface is called ​adhesion​, and the ability of liquid molecules to want to stick together (such as when a water droplet forms a ball on a leaf) is called ​cohesion​.

In a liquid, pressure depends on depth, and because of this, submerged or partially submerged objects will feel a buoyant force due to the difference in pressure on the top and bottom of the object. Archimedes' principle describes this effect and explains how objects float or sink in liquids. It can be summarized by the statement that “the buoyant force is equal to the weight of displaced liquid.” As such, the buoyant force depends on the density of the liquid and the size of the object. Objects that are more dense than the liquid will sink, and those that are less dense will float.

Properties of Gases

Gases contain molecules that can move easily around each other. They take the full shape and volume of their container and very easily expand and contract. Important properties of a gas include pressure, temperature and volume. In fact, these three quantities are sufficient to completely describe the macroscopic state of an ideal gas.

An ideal gas is a gas in which the molecules can be approximated as point particles and in which it is assumed that they don’t interact with each other. The ideal gas law describes the behavior of many gases and is given by the formula


where ​P​ is pressure, ​V​ is volume, ​n​ is the number of moles of a substance, ​R​ is the ideal gas constant (​R​ = 8.3145 J/molK) and ​T​ is temperature.

An alternative formulation of this law is


where ​N​ is the number of molecules and ​k​ is Boltzmann's constant (​k​ = 1.38065 × 10-23 J/K). (A skeptical reader can verify that ​nR = Nk​.)

Gases also exert buoyant forces on objects immersed in them. While most everyday objects are denser than the air around us, making this buoyant force not very noticeable, a helium balloon is a perfect example of this.

Properties of Plasma

Plasma is a gas that has become so hot that the electrons tend to leave the atoms, leaving positive ions in a sea of electrons. Because there are an equal number of positive and negative charges in the plasma overall, it is considered quasi-neutral, although the separation and local clumping of charges causes the plasma to behave very differently than a regular gas.

Plasma is influenced significantly by electric and magnetic fields. These fields do not need to be external either, as the charges in the plasma itself create electric fields and magnetic fields as they move, which influence each other.

At lower temperatures and energies, the electrons and ions want to recombine into neutral atoms, so for a plasma state to be maintained generally requires high temperatures. However, so called non-thermal plasma can be created where the electrons themselves maintain a high temperature while the ionized nuclei do not. This happens in mercury-vapor gas in a fluorescent lamp, for example.

There isn’t necessarily a distinct cut off between a “normal” gas and plasma. The atoms and molecules in a gas can become ionized by degrees, displaying more plasmalike dynamics the closer the gas gets to being fully ionized. Plasma is distinguished from standard gases by its high electrical conductivity, the fact that it acts like a system with two distinct types of particles (positive ions and negative electrons) as opposed to a system with one type (neutral atoms or molecules), and particle collisions and interactions that are much more complex than the 2-body “pool ball” interactions in a standard gas.

Examples of plasma include lightning, the Earth’s ionosphere, fluorescent lighting and gases in the sun.

Phase Changes

Matter can undergo a physical change from one phase or state to another. The main factors that affect this change are pressure and temperature. As a general rule, a solid must become warmer to turn into a liquid, a liquid must become warmer to turn into a gas, and a gas must become warmer to become ionized and become a plasma. The temperatures at which these transitions occur depend on the material itself as well as the pressure. In fact, it is possible to go straight from a solid to a gas (this is called sublimation) or from a gas to a solid (deposition) under the right conditions.

When a solid is heated to its melting point, it becomes a liquid. Heat energy must be added to heat the solid up to the melting temperature, and then additional heat must be added to complete the phase transition before the temperature can continue to rise. The ​latent heat of fusion​ is a constant associated with each particular material that determines how much energy is required to melt a unit mass of the substance.

This works in the other direction as well. As a liquid cools, it must give off heat energy. Once it reaches the freezing point, it must continue to give off energy in order to undergo the phase transition before the temperature can continue to lower.

Similar behavior occurs when a liquid is heated to its boiling point. Heat energy is added, causing the temperature to rise, until it begins to boil, at which point the added heat energy is used to cause the phase transition, and the temperature of the resulting gas will not rise until all of the liquid has changed phase. A constant called the ​latent heat of vaporization​ determines, for a particular substance, how much energy is required to change the substance’s phase from liquid to gas per unit mass. The latent heat of vaporization for a substance is generally much greater than the latent heat of fusion.

Chemical Properties

Chemical properties of matter determine what types of chemical reactions or chemical changes can occur. Chemical properties are distinct from physical properties in that they require some sort of chemical change in order to measure them.

Examples of chemical properties include flammability (how easy it is for a material to burn), reactivity (how easily it undergoes chemical reactions), stability (how likely it is to resist chemical change) and types of bonds the material can form with other materials.

When a chemical reaction occurs, the bonds between atoms are altered and new substances are formed. Common types of chemical reactions include combination (in which two or more molecules combine to form a new molecule), decomposition (in which a molecule breaks apart into two or more different molecules) and combustion (in which compounds combine with oxygen, releasing significant amounts of heat – more commonly referred to as “burning”) to name a few.

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