Chemistry Vocabulary (R-Z)

Because there’s a lot of chemistry vocabulary out there, I’ve decided to break it down into several pages so you don’t have to scroll so much.  This page has vocabulary words that start with the letters R-Z, as I’m sure you probably already figured out.

Incidentally, if you see an asterisk (*) after the definition, there will be more information about the topic at the bottom of the page when you scroll down.  I’ve included what I think is most important in the main definition, but figured that some of you might need a bit of supplemental information.

(Updated 10-23-15)


racemic:  A mixture that contains both enantiomers of a compound.*

radiation:  There are two definitions for this: 1) Electromagnetic radiation refers to radiation such as light, where small bits of energy called photons move from one place to another.  2) Ionizing radiation consists of particles given off by unstable nuclei – the term “ionizing” refers to the fact that when these particles collide with other things, they are capable of knocking off electrons and forming an ion.*

radioactive:  An element that has an unstable nucleus is said to be radioactive. Radioactive nuclei give off several types of ionizing radiation to stabilize the nucleus.

Raoult’s law:  The vapor pressure of a solution is directly proportional to the mole fraction of the solvent:


rate-determining step (rds):  For any multistep process, this is the slowest of the steps that occur. It’s called the rate-determining step because the overall rate of reaction is determined solely by the rate of this step.*

rate law:  A mathematical expression for how the speed of a chemical reaction varies as the concentrations of the reagents change:

rate law

In this statement, r is the rate of reaction, k is the rate constant, [A] and [B] are the molarities of reagents A and B, respectively, and x and y are the “orders” of these reagents. For equilibria involving gases, the molarity terms are replaced with the partial pressures of each gas.  The terms x and y are values that reflect the rate-determining step for the reaction and need to be determined experimentally.*

redox reaction:  This term is short hand for “reduction-oxidation reaction.”  Reduction is the process by which something gains electrons to end up with less positive charge, and oxidation is when something loses electrons to end up with more positive charge.  You cannot have a reduction reaction without an oxidation reaction, because the electrons that are given to one thing have to come from another.  In a redox reaction, the atom that is oxidized is called a “reducing agent” (it causes the other thing to gain electrons and become reduced) while the compound that is reduced is called an “reducing agent” (it causes the other thing to lose electrons and become oxidized).*

reduction:  When something gains electrons to decrease its charge.  For example, if an iron(III) ion gains an electron to become an iron(II) ion, it will have been reduced.

resonance structures:  When more than one valid Lewis structure can be drawn for a molecule, and these structures differ only in where the bonds and lone pairs are (and not the order in which the atoms are arranged), these structures are said to be resonance structures of the molecule.  When looking at resonance structures, remember that none of the resonance structures are completely correct – rather, all of them tell a small part of the story about how the molecule behaves.  For example, the resonance structures of the nitrate ion look like this:


None of these structures is entirely correct.  However, if you take the average of these structures:


You can see that the nitrogen atom in the center has a full positive charge, while the negative charge is spread partially over all three oxygen atoms.  This means that the nitrate ion does not have uneven electron density on the three oxygen atoms, so they should all react identically.*

reversible reaction:  A reaction that can proceed either in the forward or reverse direction. While all reactions are technically reversible, in practice many reactions are not.*

root mean square (RMS) velocity:  This value gives you an idea of how fast the average molecule is a gas will be travelling.*

saline:  Refers to a solution containing sodium chloride and water.  The concentration of such a solution is referred to as its “salinity.”

salt:  An ionic compound formed when an acid and base neutralize one another.  It’s not at all uncommon, however, for people to use the word salt synonymously with ionic compound.  Used alone, the term “salt” is used to identify table salt, or sodium chloride.

saturated: The solution formed when a solvent has dissolved the largest possible quantity of solute. Saturated solutions will have different concentrations at different temperatures, as the solubility of solutes varies as the temperature changes.*

Schrodinger equation:  A mathematical expression that describes the behavior (energy/position/shape) of an electron that is bound to an atom.

second law of thermodynamics:  The randomness/entropy of the universe is always increasing. Though you wouldn’t think it’s possible, there is an energy value (entropy) associated with this randomness, and it plays a significant role in determining whether or not a reaction will take place (link).  Though there are many processes that result in a decrease of randomness for the thing being manipulated (i.e. putting books on a shelf), there must be a corresponding increase in randomness elsewhere to compensate for it (i.e. the heat given off when you do this causes molecules in the room to move more quickly.)*

semiconductor: A substance that conducts electricity poorly at room temperature, but has increasing conductivity as both temperature and applied voltage are increased. Metalloids are typically good semiconductors, and are used as switches in computers for this property.

shielding effect: Electrons in orbitals close to the nucleus repel electrons farther from the nucleus, causing them to be bound less tightly to the atom.  This effect is responsible in explaining why electronegativity decreases as you move down a group, as well as why ionization energies drastically decrease moving down a group.

sigma bond: A single bond formed by the overlap of hybrid orbitals (or in the case of hydrogen, s-orbitals).  You can think of this as the “first bond” that occurs between two atoms (whereas pi-bonds would be responsible for multiple bonding).

significant figure: The number of digits in a measurement that give you useful information. For example, if you weigh yourself as 149 pounds on a bathroom scale, it has three significant figures because the 1, 4, and 9 all give you information about your weight.*

single-displacement reaction:  A reaction in which a pure element switches place with another element in a compound.  These reactions have the general form A + BC → B + AC, indicating that elements A and B switch places.  Reactions such as this are necessarily also redox reactions, as the oxidation state of A and B have to change in a process like this.

sol:  A colloid in which solid particles are found within a liquid or gas.

solubility: A measurement of how much of a solute can be dissolved in a liquid. An important distinction, however, should be made between solubility and concentration: While solubility tells you the maximum theoretical quantity of solute that can be dissolved, concentration tells you the amount that has actually been dissolved in some particular case.  Solubilities are often given in units of “g / 100 g of water”, but are other times just indirectly indicated by solubility product constants.

solubility product constant (Ksp):  The solubility product constant of a compound is a measure of how much stuff can be dissolved in a solvent (usually water).  The larger the solubility product constant, the more soluble the compound.  The Ksp term  is closely related to the K values for other equilibrium constants, as solubility is an equilibrium between the solid and aqueous forms of a compound.

solute:  The thing that gets dissolved in a solution.  In salt water, it’s the salt. In solutions containing two liquids (such as the rubbing alcohol you buy at the store, which contains both isopropanol and water), neither is said to be the solvent or solute – instead, the two liquids are said to be “miscible” with one another, which means they dissolve into each other.

solvent: The thing into which something gets dissolved in a solution.  In salt water, it’s the water.

specific heat capacity (C): The amount of energy needed to raise the temperature of one gram of something by one degree Celsius.  This term is more often just referred to as “specific heat” or “heat capacity.”*

spectator ions:  The ions in a chemical reaction that don’t actually do anything.  For example, when sodium chloride (NaCl) reacts with silver nitrate (AgNO3), silver chloride solid and dissolved sodium nitrate are formed.  Because the sodium and nitrate ions started off dissolved in solution and remained dissolved, they are said to have been the spectator ions.

spontaneous change:  A process in which the free energy of the system decreases will be spontaneous.  How is that measured?  With the Gibbs free energy!

standard temperature and pressure (STP): Standard temperature is zero degrees Celsius, and standard pressure is, officially, 100 kPa.  However, in science classes, standard pressure is almost always taken to be 1 atm, which is only 1% different.*

steric hindrance: The slowing of a reaction that occurs when big bulky functional groups get in the way of the active sites of a reaction.  Think of it like a fat guy standing on your doorstep – it will take longer to enter your house than if a skinny guy was there. This concept is frequently used when describing the kinetics of organic reactions, as differently-sized substituents can lead to drastically-different reaction rates.

stoichiometry:  The use of the relationships between reagents and products to figure out how much stuff you’ll either make in a reaction or need to perform the reaction.  It’s pronounced “stoy-key-ah-meh-tree.”

strong acid:  An acid that fully dissociates in water.*

strong nuclear force: The force that holds the nucleus of an atom together.  It is, as you might guess, strong.

structural formula:  A picture that shows you what a molecule looks like.  The most familiar example of structural formulas are the Lewis structures you’ve doubtlessly seen in chemistry class.

sublimation: When a solid changes into a gas without first becoming a liquid.  You see this happen with dry ice.*

substituent: A substituent is some functional group that’s present in a chemical compound. This term is used almost exclusively in organic chemistry, where these groups are seen as having “substituted” for hydrogen atoms on the main carbon chain.

supercooling: When you cool something below its usual freezing point.  A supercooled liquid can only be created if nucleation centers are absent (i.e. places for the liquid to freeze) and if the solution is kept very still.  These solutions are referred to as “metastable”, because they can be stored, but will freeze if given a chance.

supersaturated: When more of a solute is dissolved in a solvent than theoretically expected. This occurs because of a lack of nucleation centers (i.e. there are no good places for crystals to form) and because a saturated solution has been cooled so that the solubility of the solute decreases.  Such solutions are “metastable” because, if given a chance, the additional solute will crystallize out, forming a mass of crystals and a saturated solution.

surface tension:  A measurement of how much the molecules at the surface of a liquid like to stick to one another.  This is the reason that water molecules tend to make big drops as opposed to spread out:  They prefer to stick to each other than not.

surroundings: Whatever is interacting with the thermodynamic system of interest. Together, the system and surroundings make the “universe.”

suspension: A mixture that appears homogeneous at first, but when allowed to sit will settle out.  An example of this is mud, because a mud puddle will tend to separate into dirt and water over time.

synthesis: When a more complex compound is made from simple ones.  This can be anything from the formation of a compound from its elements to the formation of a very complex organic molecule from smaller ones.

system: The thing you’re interested in at the moment, when doing thermodynamic calculations. This is in contrast to the “surroundings”, which are not the item of interest but interact with it.

temperature: A scale that’s used to measure how hot or cold something is. You can think of it as being a sort of indirect measurement of kinetic energy, as higher temperatures result in the greater movement of particles.  The accepted unit of temperature is the Kelvin.

theoretical yield: When you perform a chemical reaction, the theoretical yield is the amount of product that stoichiometric calculations predict should be made.  Please note that this value is a theoretical construct that assumes you’ve done everything perfectly (which is generally not a good assumption).*

thermodynamics:  The study of energy and where it goes.

third law of thermodynamics: The entropy of a system at 0 K is zero.  This law is true only for perfect crystals.  Unfortunately, since it’s impossible according to the Nernst-Simon statement to reach absolute zero, it’s not going to happen.

titration: Where the concentration of an acid (or base) is determined using a neutralization reaction.  Well, this is true for general chemistry, anyhow.  The basic idea can be extended to other types of reactions as well, such as redox reactions.*

transition state: The more common term for “activated complex.”  The transition state of a chemical reaction is the highest-energy point in the reaction, where the reaction is teetering on the brink of either occurring or not occurring.  One example of the transition state of a reaction is shown below, where a hydroxide ion is displacing a bromide ion on ethane: transitionstateAs you can see, the transition state (in brackets) has reacted about halfway, with the O-C bond not yet formed and the C-Br bond not quite broken.  Transition states cannot be directly observed.

triple bond:  When two atoms are connected to one another via three covalent bonds / three shared pairs of electrons.

triple point:  The conditions of pressure and temperature on a phase diagram at which all three phases of matter are in equilibrium.

Tyndall effect:  When light is shone through a colloid, the light is reflected – this is in contrast to solutions, where there is no such reflection.  This reflection is called the Tyndall effect, and can be seen in many horror movies where you can see the flashlight beams of the heroes before they get their heads cut off by monsters.

unit cell: The smallest part of a crystal structure that can be repeated over and over again to give the overall structure of the crystal.  You can see lots of pictures here.

universe: The system + surroundings in a thermodynamic system. In a literal sense, the universe does, in fact, encompass the entire universe.  However, we generally treat the universe as being the system plus whatever surroundings actually interact with the system in a meaningful way.

unsaturated:  It has two meanings:

  • A solution in which it would be possible to dissolve more of the solute.  Any solution that’s less-than-saturated is, by definition, unsaturated.
  • A hydrocarbon with double or triple bonds.  “Saturation”, in this case, refers to the hydrogenation of these bonds.  “Saturated” and “unsaturated” fats use this meaning of the word saturated.

unshared electron pair: Two electrons that are not involved in bonding, shown as two dots on Lewis structures.  Also referred to as a “lone pair.”

valence electron: The outermost electrons in an atom.  It is the valence electrons that are involved in chemical reactions.

Van der Waals force:  The intermolecular force that connects two nonpolar atoms or molecules.  This occurs when one of the atoms/molecules becomes partially polarized due to a temporary movement of electrons, causing a neighboring atom/molecule to also become partially polarized.  The resulting interaction between these is called the Van der Waals force.

vapor pressure: All liquids evaporate a very small amount, with liquid molecules turning into gas molecules.  Because these gases, like all gases, exert pressure on their surroundings, this pressure is called the “vapor pressure” of the liquid.  Vapor pressure increases with temperature because more energy is available to evaporate the liquid – this is why your bathroom mirror fogs up when you take a hot shower but not when you take a cold one.

vaporization: When a liquid is turned into a gas.  Vaporization occurs via evaporation (in which this transition occurs with just a few molecules below the boiling point) or boiling (in which the molecules all have plenty of energy to vaporize).

viscosity:  The “thickness” of a liquid, in terms of how syrupy it is.  For example, molasses has a high viscosity because it is very thick and sticky, while rubbing alcohol has a very low viscosity.

volatile: Something with a high vapor pressure.

volt:  Equal to 1 J/coulomb, a volt is a measure of electrical potential.

VSEPR (valence shell electron pair repulsion) theory: A theory which states that electrons in a molecule will repel one another whenever possible.  This statement gives rise to our familiar molecular shapes, in which the distance between the bonds is maximized to give you the most “spread-out” molecules possible.

wavefunction:  An equation that describes the location/shape of an electron within an atom.

work:  Work is equal to the amount of force applied to an object over some distance, in units of Joules.

X-ray:  Electromagnetic with very high frequency / short wavelength.  X-ray diffraction is one method for determining the structure of a crystal by bombarding it with X-rays and measuring how the beam has been shifted.

yield:  The amount of product formed during a chemical reaction.  In this sense, the “theoretical yield” is the amount of product that stoichiometry predicts should have been formed, while the “actual yield” is the amount that is actually formed.

zwitterion:  An ion that has an overall charge of zero.  This is due to the fact that it contains both an anionic and cationic portion, the charges of which cancel each other out.

Footnotes and explanations

racemic:  In many cases, only one of the enantiomers of a chiral compound can serve as a medication.  When this happens, it’s frequently the case that drug companies sell the racemic mixture first, and then switch to the enantiopure version to increase the amount of time they can make money from it.

radiation: There are a few points that should be made about radiation:

  • Generally speaking, the ionizing radiation you’re likely to run into (i.e. various medical waste and such) is far more dangerous than the electromagnetic radiation you’re likely to see (i.e. TV/radio signals, sunlight, and cell-phone signals).  This is not to say that there are no sources of dangerous EM radiation (gamma rays will kill you dead in a hurry, while IR light from the sun can cause skin cancer).  However, the concerns about EM radiation from both power lines and cell-phone signals are completely unsupported at this time.
  • Interestingly, there are some forms of electromagnetic radiation that are also considered electromagnetic radiation.  Basically, anything with higher frequency/energy will ionize anything it hits; however, it’s not very common for people to specifically think of EM radiation as ionizing unless a nuclear process produces it (gamma radiation).
  • If somebody says that they want to remove all radiation sources from their lives, point out to them that there is always background radiation present, no matter where in the universe they go.

rate-determining step (rds):  Here’s how this works.  Imagine that you are going to give an eight hour lecture to the United Nations explaining why you think you should be declared emperor of the world.  When doing so, here’s what you do:  1)  Take a sip of water; 2) Speak for eight hours.  So, what determined the total amount of time spent in this activity?Clearly, it’s the step where you talk for eight hours, because taking a sip of water takes basically no time at all.  Similarly, if you have a chemical mechanism where one step occurs nearly instantaneously and another takes a long time, it’s reasonable to ignore the very fast step when talking about overall reaction rate because it doesn’t really play much of a role.

rate law:  It’s possible to write an integrated rate law though the use of integral calculus – this law will describe how the rate of the reaction varies with time.  The order of the reaction usually indicates how many reactive species combine simultaneously in the rate-determining step;  for this reason, there are very few third-order reactions and only a handful of known fourth-order reactions.

redox reaction:  I struggled a bit to try and figure out how to describe oxidizing and reducing agents.  While it’s true that various atoms in each either gain or lose electrons during this process, it’s usually the entire compound in which the atom resides that’s said to be the oxidizing or reducing agent.  Another thing:  You can’t have oxidation without reduction, because the electrons that are taken from one thing have to go somewhere – they are given to another.

resonance structures:  The reason you don’t just see the final drawing with everything spread out is that it’s hard to visualize for small chemical species and darned near impossible to understand if you have much larger chemical species (particularly ions based on aromatic compounds).  The use of resonance structures, while not individually telling us the whole story, allows us to visualize the different possibilities for reactivity in chemical species.

reversible reaction:  The principle of microscopic reversibility states that all processes can be reversed via the same series of steps by which they occurred in the first place.  However, this makes the assumption that all of the chemical species needed to do so are present, and that there’s enough energy to make it happen.  Though the combustion of gasoline is reversible, it’s difficult to imagine everything coming together in just the right way to see this happen.

root mean square (RMS) velocity:  Please don’t send me hate mail because I put that definition up there.  Though not exactly correct, it does do a pretty good job of describing to the layman what RMS velocity is and what it’s used for.  Just between you and me, the complete definition for the RMS velocity is that it’s the square root of the average of the squares of the velocities of each particle in a gas.  The equation for RMS velocity involves terms for the temperature (things move faster at high temperatures) and the mass of the gas molecules (because big things move slower than small ones at a specific temperature). You can find out more here.

saturated:  Gases usually have lower solubilities at high temperatures, while solids have greater solubilities.  Of course, there are exceptions.

second law of thermodynamics: Those who support the concept of intelligent design (i.e. that some intelligent creator made mankind) often point to the second law of thermodynamics, making the point that if it were always obeyed, the evolutionary processes that undeniably result in more-ordered systems would not be possible.  Though this is true, it fails to take into account the idea that the world is not a closed system and has solar energy pumped into it.  Though the order on our planet may be increasing, there is more than enough disorder created by the processes that made this energy to overcome this.  I did, however, read an interesting article in which an example was given to explain why the second law still wouldn’t hold true:  It allows for a pile of metal to spontaneously turn into a computer in this room as long as two computers rust away in the next room to make up for it.  Unfortunately, the author of this article is mistaking direct causality with thermodynamic possibility – though this very event could happen (and theoretically would, given an infinite period of time), you probably won’t see it happen unless somebody makes it happen.

significant figures: Significant figures are only used in conjunction with measurements, because they’re the only time that you’d have uncertainty when figuring out what’s going on.  If you’re in a math class and you do some fancy calculation, you’re living in the magical world of numbers where everything is exact and nothing is screwy.  However, in the real world, our instruments aren’t infinitely good and we screw up a lot, making it necessary for us to tell people how good our data are. How do we do it?  Significant figures.

specific heat capacity (C):  There are a lot of terms related to specific heat, so I thought I’d mention them:

  • heat capacity:  The amount of energy needed to raise the temperature of something by one degree Celsius. The heat capacity of something refers to that specific amount of stuff, and is an extensive property that depends on the amount of stuff present.
  • specific heat capacity:  What I said before.  Also called specific heat.
  • molar heat capacity: The amount of energy needed to raise the temperature of one mole of something by one degree Celsius.

standard temperature and pressure (STP):  There is some disagreement even among science types about what to use for STP.  Though IUPAC says that 100 kPa is the standard pressure, NIST says that it’s 1 atm.  Other folks still use 1 atm as the standard pressure but use other temperature values for standard temperature.  For now, it’s probably best to use zero degrees and 1 atm unless your teacher indicates otherwise.

strong acid: Or, to be more specific, any acid with a pKa value less than -1.74.

sublimation: Other cases where you may have seen this are common in your freezer.  If you’ve ever seen old ice cubes that look all weird and deformed, that’s because some of the water molecules have sublimed and stuck themselves elsewhere.  Additionally, “freezer burn” occurs when water molecules within the food sublime and redeposit somewhere else in the container; this is why old ice cream has ice crystals on the top and feels kind of rubbery.

theoretical yield:  In reality, the actual yield of a reaction will depend not only on how much should have been made, but on other factors such as impurities, spills, and other sources of error.  Ideally, the actual and theoretical yields for a reaction should be identical, but in reality this is impossible.

titration:  The idea behind a titration is this:  If you don’t know how much stuff is in a solution, keep adding something else until it’s all neutralized.  At this point, the amount of stuff that you added will equal the amount of stuff that was in the solution in the first place.  By analogy, consider a box full of hungry alligators.  If you don’t know how many alligators are in there, hang a poisoned chicken from a string into the box – the alligator will take it and die.  Even though you don’t know how many alligators were there in the first place, you can infer from the fact that twelve chickens were eaten that there were 12 alligators initially in the box.

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Several of the images on this page were not produced by me, but are all reproduced as public domain images.  Some info:

  • The transition state photo is in the public domain and can be found here.
  • The nitrate resonance structures are in the public domain, made by Ben Mills.  Link.
  • The averaged nitrate structure is also in the public domain, from Ben Mills.  Link.
  • The rate equation was found here. Copyright information unknown, but presumed public domain according to the principle that works of common property are not copyrightable (link to US Copyright office explanation).

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