Matter exists in distinct forms, known as states or phases, which describe how its particles are arranged and move. The physical state of any substance is determined by the balance between two opposing factors: the kinetic energy of its particles and the intermolecular forces holding them together. Kinetic energy pushes particles apart, causing them to move rapidly. Intermolecular forces are the attractive forces that pull the particles closer together.
When kinetic energy overcomes the attractive forces, a substance transitions to a more energetic state. Conversely, when attractive forces dominate, the substance settles into a more ordered state. Temperature and pressure are the primary external conditions that drive these transitions, allowing matter to switch between its various forms.
The Three Common States of Matter
The states of matter most familiar in daily experience are solid, liquid, and gas, which represent a progression of increasing particle energy. In a solid, the particles possess the lowest thermal energy, meaning their motion is restricted to vibrating in fixed positions. This strong dominance of intermolecular forces gives solids a definite shape and a definite volume, as seen in a block of ice.
A liquid forms when enough energy is added to partially overcome these attractive forces, allowing the particles to move and slide past one another. The particles remain closely packed, maintaining a definite volume, but their fluid motion means a liquid will take the shape of any container it occupies. Water is the classic example of this intermediate state.
When further energy is supplied, the particles gain sufficient kinetic energy to completely break free from the attractive forces, resulting in a gas. Gas particles move rapidly and randomly, separated by vast amounts of empty space, meaning they have neither a definite shape nor a definite volume. Steam, the gaseous form of water, will expand to fill the entire volume of its container.
The Fourth State: Plasma
Adding significantly more energy to a gas can push matter into its fourth state, known as plasma. This is a superheated, highly energetic state where the atoms have been stripped of one or more of their electrons, a process called ionization. The result is an electrically neutral, chaotic soup of positively charged ions and negatively charged free electrons.
Because plasma contains a substantial fraction of these charged particles, it becomes an excellent conductor of electricity and is highly responsive to magnetic fields. The energy required to create plasma is immense, often involving temperatures of 10,000 degrees Celsius or more.
Although rare on Earth, plasma is considered the most common state of matter in the visible universe, making up an estimated 99.9% of all ordinary matter. Stars, including the Sun, are massive spheres of plasma, and natural phenomena like lightning and the auroras are terrestrial examples. Artificially, plasma is used in technology such as neon signs and flat-screen televisions.
The Fifth State: Bose-Einstein Condensate
The fifth state of matter, the Bose-Einstein Condensate (BEC), exists at the opposite extreme of the energy spectrum, requiring temperatures just a fraction above absolute zero. Predicted in the 1920s by physicists Satyendra Nath Bose and Albert Einstein, the BEC was first successfully created in a laboratory in 1995. It is formed by cooling a cloud of certain types of atoms, known as bosons, to temperatures measured in billionths of a degree above zero Kelvin.
At these ultracold temperatures, the atoms virtually stop moving, and their quantum identities begin to overlap. According to Bose-Einstein statistics, a large number of these atoms collapse into the lowest possible quantum state. They effectively lose their individuality and begin to act as a single, collective entity, sometimes referred to as a “super-atom”.
To achieve this state, scientists employ specialized techniques, including laser cooling to slow the atoms and magnetic traps to hold them in place. The BEC allows researchers to observe quantum mechanical phenomena, which are usually confined to the microscopic world, on a much larger, macroscopic scale. The collective behavior of the BEC is linked to exotic properties such as superfluidity, where a substance can flow without any friction.