STATES OF MATTER
STATES OF MATTER
GLOSSARY
amorphous solid A solid whose atoms or molecules are not arranged in an ordered, repeating, three-dimensional array.
covalent bond The joining of atoms by the sharing of one or more electrons.
crystalline solid A solid whose atoms or molecules are arranged in a well-ordered, repeating, three-dimensional array.
dipole force The attractive force that exists between two or more polar molecules due to an asymmetric distribution of charge.
dispersion force The attractive force that exists between atoms and molecules due to temporary dipoles that develop because of charge fluctuations.
homogeneous mixture A mixture containing two or more components that has the same composition throughout.
intermolecular forces Attractive forces that exist between atoms and molecules.
ionic bond The joining of two atoms by the transfer of an electron from one to the other.
‘like dissolves like’ principle The idea that polar molecules mix well with other polar molecules, but don’t mix well with nonpolar molecules. Mostly applicable to aqueous solutions.
macroscopic properties of gases Properties such as temperature, volume, pressure and number of moles of a sample of a gas.
osmotic pressure The pressure required to stop osmotic flow. Osmotic flow is the flow of water through a semipermeable membrane from a more dilute solution to a more concentrated one.
polar bond A chemical bond with asymmetric charge distribution.
polar molecule A molecule containing polar bonds that also has an asymmetric charge distribution over the entire molecule.
polar solute A solute (component of a solution) that has an asymmetric charge distribution. A nonpolar solute has a highly symmetrical charge distribution.
single liquid phase A liquid mixture with uniform composition.
solute The minority component of a solution.
solvent The majority component of a solution.
vapour pressure The pressure of the vapour of a liquid in equilibrium with its liquid.
THE FORCES THAT HOLD MATTER TOGETHER
the 30-second chemistry
Matter exists in three states: solid, liquid and gas. In the gas state, the particles that compose matter are separated by large distances and do not interact with one another very much. In the solid and liquid states, by contrast, the particles interact strongly, held together by attractive forces. Some solids, such as a diamond, are held together by covalent chemical bonds between atoms (which is what makes diamond so strong). Other solids, such as table salt, are held together by ionic bonds between ions. Still other solids, such as ice, and many liquids are held together by attractive forces that exist between molecules. These forces are known as intermolecular forces. Intermolecular forces exist because the electron distribution in a molecule can be either temporarily asymmetrical, resulting in the dispersion force, or permanently asymmetrical, resulting in the dipole force. In either case, the asymmetrical electron distribution causes part of the molecule to be positively charged (either temporarily or permanently) and another part to be negatively charged. The positive and negative ends of neighbouring molecules are then attracted to one another much like opposite poles of a magnet are attracted to one another. These attractions must be overcome for a substance to melt or boil.
3-SECOND NUCLEUS
Solid and liquid matter exist because the particles that compose them have strong attractions to one another.
3-MINUTE VALENCE
Why are some substances solid at room temperature, while others are liquids or gases? Because the particles that compose matter are attracted to one another in varying degrees. Strong attractions between particles are responsible for solids at room temperature, moderately strong attractions result in liquids and weak attractions result in gases. The higher the temperature, the stronger the attractions between particles must be to maintain the liquid and solid state.
RELATED TOPICS
See also
3-SECOND BIOGRAPHY
JOHANNES DIDERIK VAN DER WAALS
1837–1923
Dutch physicist who was among the first to postulate forces between molecules
30-SECOND TEXT
Nivaldo Tro
The strength of the intermolecular forces between water molecules in ice determines its melting point.
THE GASEOUS STATE
the 30-second chemistry
Gases have the unique property – unlike solids and liquids – of always completely filling the volume of their container. By the end of the eighteenth century, the relationships between the volume, pressure and amount of gas had been empirically described. Hot-air balloonists Jacques Charles (of Charles’ law fame) and Joseph Gay-Lussac not only set new altitude records for ballooning, but also used these adventures to collect data on the temperature-volume relationship of gases, which indicated that the volume occupied by a gas increased with temperature. Robert Boyle demonstrated that the volume occupied by a gas is inversely proportional to its pressure, a relationship that became known as Boyle’s law. Amedeo Avogadro hypothesized that equal volumes of gas were occupied by equal numbers of molecules as part of his theory that gases consisted of molecules that in turn were composed of atoms. His theory was largely ignored at the time. Further progress in understanding the origin of these relationships required the acceptance by chemists of the theory of the particle nature of atoms and molecules. The current model to explain these properties is kinetic-molecular theory.
3-SECOND NUCLEUS
Kinetic-molecular theory explains the macroscopic properties of gases based on the behaviour of gas particles.
3-MINUTE VALENCE
Kinetic-molecular theory is based on three postulates. The sizes of the particles that comprise a gas are negligibly small, so that gas particles occupy essentially none of the volume of a gas. Gas particles are in constant motion and have an average kinetic energy proportional to the temperature of the gas. Collisions between gas molecules are perfectly elastic in that energy can be transferred but not lost in the collisions.
RELATED TOPICS
See also
ENTROPY & THE SECOND LAW OF THERMODYNAMICS
3-SECOND BIOGRAPHY
JAMES CLERK MAXWELL
1831–79
Scottish mathematician who, while best known for his work in electromagnetism, also worked on developing a statistical means of explaining the properties of gases
30-SECOND TEXT
John B. Vincent
The properties of gases make both hot-air ballooning and scuba diving possible.
THE LIQUID STATE
the 30-second chemistry
The molecules that compose a liquid are like dancers in a crowded night club. The dancers have so much energy that they move around the floor constantly interacting with different people. They are attracted to everyone else in the club and want to dance with everyone. Similarly, molecules in liquids have attractive forces with all the other molecules around them, but they have so much energy that they don’t stay still and are constantly moving past one another. As a whole, the people in the dance club have the same shape as the club. If they all moved from a square club to a circular one, their overall shape would change. Similarly, as a whole, water molecules flow to assume the shape of their container. Not every molecule in a liquid (or person in a dance club) has the same amount of energy. Some have more, some have less. A few have a lot more energy – so much that they can break free of their attraction to the other molecules in the liquid and fly out on their own as a gas molecule (they essentially dance right out of the club). This is how liquids evaporate.
3-SECOND NUCLEUS
Liquids are made up of molecules with enough energy to flow past one another, but generally not enough to overcome their mutual attraction entirely.
3-MINUTE VALENCE
Pour water into a glass – it fills to a certain level, and the shape of the water is the same as the inside of the glass. If you pour the water into a square-shaped glass, it assumes that shape. If you leave the water to stand for a few days, it slowly evaporates away. How do we explain this behaviour from a particulate viewpoint?
RELATED TOPICS
See also
THE FORCES THAT HOLD MATTER TOGETHER
3-SECOND BIOGRAPHIES
ROGER JOSEPH BOSCOVICH
1711–87
Ragusan (now Croatia) physicist who predicted that the states of matter depended on forces between their particles
FRANÇOIS-MARIE RAOULT
1830–1901
French chemist who explored the properties of solutions
30-SECOND TEXT
Jeff C. Bryan
The molecules that compose the liquid state are in constant motion, not unlike dancers in a crowded night club.
THE SOLID STATE
the 30-second chemistry
The particles that compose a solid are like the dancers in the nightclub from the previous entry (on liquids), except that they have less energy relative to the strength of their attractions. The dancers are still shaking, but they are not moving around each other because they are strongly attracted to those currently surrounding them. Similarly, the attractions between the particles in a solid are so strong compared to the energy they possess that the particles don’t flow past one another as they do in a liquid. The particles that compose a crystalline solid are not only stuck in one place, but also arranged in an orderly fashion like bricks in a wall. In contrast, the particles that compose an amorphous solid are arranged in a more haphazard way, like a pile of macaroni. Crystalline solids (such as salt or ice) tend to be less flexible than amorphous solids (such as plastic or glass). Although the particles that compose solids do not move past or around one another, they do wiggle and shake. When heated, they get more energy. Eventually, when heated enough, they start moving past one another and the solid melts. The amount of energy (temperature) needed for melting depends on how strongly the molecules are attracted to each other.
3-SECOND NUCLEUS
Solids have a definite size and shape because the particles that compose them are stuck in place.
3-MINUTE VALENCE
Solids behave differently from liquids or gases. They have a fixed shape and size and don’t assume the shape of their container like a liquid, nor are they compressible like a gas.
RELATED TOPICS
See also
THE FORCES THAT HOLD MATTER TOGETHER
3-SECOND BIOGRAPHIES
WILLIAM LAWRENCE BRAGG
1890–1971
Australian-born British physicist and winner of the 1915 Nobel Prize in Physics who discovered how to peer in at the structures of solids
LINUS PAULING
1901–94
American winner of the 1954 Nobel Prize in Chemistry, who developed our understanding of how atoms and molecules are attracted to each other
30-SECOND TEXT
Jeff C. Bryan
The molecules in the solid state are like dancers who are stuck in one place on the dance floor.
CERAMICS
the 30-second chemistry
Ceramics are among the most important materials in human civilization. They are solids held together by networks of ionic or covalent bonds extending throughout the material. They differ from metals in that the bonds are to some extent directional and so must be broken in order for planes of atoms to slip past one another. As a result, ceramics cannot be easily deformed, as metals can, but instead tend to be brittle and hard. Ceramics may be made by mixing finely powdered minerals and heating them until their atoms are moving fast enough to move into each other or until the minerals melt into a single liquid phase. When cooled, the atoms in the resulting ceramic are often aligned in the neat rows of a crystal. Glasses may be produced if the cooling is fast enough so that the atoms are frozen in a snapshot of the chaotic liquid from which they were made. Many alumino-silicate mineral ceramics consist of covalently bonded chains or sheets that can absorb water and metal ions in between the layers. These include clays that swell considerably when they take up water, as well as the mineral kaolinite, which is a primary component of fine china or porcelain.
3-SECOND NUCLEUS
The technologically useful properties of ceramic materials depend on the 3D arrangement of their atoms and the nature of the chemical bonds holding them together.
3-MINUTE VALENCE
The first ceramic figurines and pots were made more than 20,000 years ago, long before metal tools. Later artisans used tough ceramics like porcelain, clear ceramics like glass and the cements that dominate cityscapes today. Ceramic scientists continue to produce new technologically useful materials: recent examples include silicon carbide cutting tools, boron-nitride lubricants, silicon computer chips and bioglass-based medical implants made from silica and hydroxyapatite.
RELATED TOPICS
See also
THE LEWIS MODEL FOR CHEMICAL BONDING
3-SECOND BIOGRAPHIES
HERMANN SEGER
1839–93
German chemist who pioneered the scientific study of ceramics using the periodic table
RUSTUM ROY
1924–2010
Indian-born scientist who developed the sol-gel method for preparing ceramics from liquid chemical precursors
W. DAVID KINGERY
1926–2000
American material scientist who first applied solid state chemistry principles to ceramic synthesis and processing
30-SECOND TEXT
Stephen Contakes
The structure and bonding in ceramic materials determine their many useful properties.
SOLUTIONS
the 30-second chemistry
Solutions are homogeneous mixtures formed when one substance (a solute) dissolves in another substance (a solvent). Ocean water, air and sugar water are common examples of solutions. Aqueous solutions are those in which water is the solvent. Solutions show different properties from the components that compose them. For instance, a salt-water solution has a lower freezing temperature than pure water (which is one reason why freshwater lakes freeze more easily than oceans). Similarly, a salt-water solution has a higher boiling temperature, lower vapour pressure and higher osmotic pressure when compared to pure water. Such solution characteristics are known as ‘colligative properties’. They were first studied experimentally by Richard Watson, a Professor of Chemistry at Cambridge University, who observed the freezing time of a series of 18 aqueous solutions of various salts exposed to an unusually cold (-14°C/6.8°F) February atmosphere in Cambridge. He realized that the primary factor determining the degree of lowering in the freezing point of a solution is the number of solute particles (concentration) and not the type of salt. Calcium chloride (CaCl2) is more effective in treating icy roads than sodium chloride (NaCl) because it provides more solute particles (ions) when mixed with the icy surface.
3-SECOND NUCLEUS
When the particles of one substance are dissolved in a second (the solvent), they interfere with the way the solvent molecules interact, changing the properties of the solution.
3-MINUTE VALENCE
The nature of intermolecular forces in all states of matter partly determines whether one substance dissolves in another. The ‘like dissolves like’ principle is helpful in determining solubility in water: polar solutes tend to be most soluble in water (since water is polar). For example, salt is soluble in water but grease (mostly nonpolar) is not.
RELATED TOPICS
See also
3-SECOND BIOGRAPHIES
DAVID BERNOULLI
1700–82
Swiss mathematician whose work Hydrodynamica provided the first qualitative discussion of aqueous salt solutions
RICHARD WATSON
1737–1816
Professor of Chemistry at the University of Cambridge who first carried out experimental measurements studying the properties of aqueous salt solutions
30-SECOND TEXT
Ali O. Sezer
Salt water solutions have a lower freezing point than pure water, which is why salt is used to reduce snow and ice build-up on roads and pathways.
