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Home :Glossary

Glossary: Chemical change

An analyte is the sample constituent whose concentration is sought in a chemical analysis*.

anode. Compare with cathode*.
The electrode at which oxidation* occurs in a cell. Anions* migrate to the anode.

aqueous. (aq) aqueous solution.
A substance dissolved in water.

balanced equation. balanced.
A description of a chemical reaction that gives the chemical formulas of the reactants* and the products* of the reaction, with coefficients introduced so that the number of each type of atom and the total charge is unchanged by the reaction. For example, a balanced equation for the reaction of sodium metal (Na(s)) with chlorine gas (Cl2(g)) to form table salt (NaCl(s)) would be 2 Na(s) + Cl2(g) = 2 NaCl(s), NOT Na(s) + Cl2(g) = NaCl(s).

Brösted acid. Compare with acid*.
A material that gives up hydrogen ions in a chemical reaction.

Brösted base. Compare with base*.
A material that accepts hydrogen ions in a chemical reaction.

catalyst. catalyze; catalysis.
A substance that increases the rate of a chemical reaction, without being consumed or produced by the reaction. Catalysts speed both the forward and reverse reactions, without changing the position of equilibrium*. Enzymes* are catalysts for many biochemical reactions.

cation. Compare with anion*.
A cation is a positively charged ion. Metals* typically form cations.

chemical change. reaction; chemical reaction. Compare with physical change*.
A chemical change is a dissociation, recombination, or rearrangement of atoms.

chemical equation.
A compact notation for describing a chemical change. The formulas of the reactants* are added together on the left hand side of the equation; the formulas of the products* are added together on the right side. Coefficients are inserted before the formulas to ensure that the equation is balanced*. The phase* in which each substance is found is usually indicated in parentheses after each formula. For example, 2 H2(g) + O2(g) = 2 H2O(g) indicates that 2 moles of hydrogen gas combine with one mole of oxygen gas to produce two moles of steam.

combination reaction.
A reaction in which two or more substances are chemically bonded together to produce a product. For example, 2 Na(s) + Cl2(g) rightarrow 2 NaCl(s) is a combination reaction.

combustion. combustion reaction.
A chemical reaction between a fuel and an oxidizing agent* that produces heat (and usually, light). For example, the combustion of methane is represented as CH4(g) + 2 O2(g) = CO2(g) + 2 H2O(ell).

complete ionic equation. total ionic equation. Compare with net ionic equation*.
A balanced equation* that describes a reaction occurring in solution, in which all strong electrolytes* are written as dissociated ions*.

compound Compare with element* and mixture*.
A compound is a material formed from elements chemically combined in definite proportions by mass. For example, water is formed from chemically bound hydrogen and oxygen. Any pure water sample contains 2 g of hydrogen for every 16 g of oxygen.

decomposition. decompose; decomposable; decomposition reaction. Compare with synthesis*.
A reaction in which a compound is broken down into simpler compounds or elements. Compounds sometimes decompose if heated strongly or if subjected to a strong electric current* (electrolysis*).

displacement. displacement reaction; replacement reaction; replacement.
A reaction in which a fragment of one reactant is replaced by another reactant (or by a fragent of another reactant). Displacement reactions have the same number of products as reactants, and are described by equations of the form A + BC rightarrow AB + C (single displacement) or AB + CD rightarrow AC + BD (double displacement).

double displacement. double displacement reaction; double replacement; double replacement reaction; double exchange; exchange; metathesis.
A double displacement or metathesis is a reaction in which two reactants trade fragments:
AB + CD = AC + BD
Most commonly, the fragments are ions, e. g.
AgNO3(aq) + NaCl(aq) = AgCl(s) + NaNO3(aq)

A substance that dissociates fully or partially into ions* when dissolved in a solvent, producing a solution that conducts electricity. See strong electrolyte*, weak electrolyte*.

endpoint. end point. Compare with equivalence point*.
The experimental estimate of the equivalence point* in a titration*.

equivalence point. Compare with end point*.
The equivalence point is the point in a titration* when enough titrant* has been added to react completely with the analyte*.

formation. formation reaction.
A reaction that forms one mole of a compound from its elements in their most stable forms. For example, the formation reaction for water is H2(g) + ½O2 rightarrow H2O(ell).

A substance that undergoes an sharp, easily observable change when conditions in its solutions change. See, for example, acid-base indicator* and redox indicator*.

insoluble. insolubility. Compare with soluble*.
Refers to a substance that does not dissolve in a solvent* to any significant degree. Compounds with solubilities of less than 1 g per liter of water are often referred to as 'insoluble', even though they do dissolve to a small extent.

ionic dissociation. ionize; ionization.
When ionic substances dissolve, their ions are surrounded by solvent molecules and separated from each other. This phenomena is also called ionization.

ionic equation. complete ionic equation . Compare with net ionic equation* and molecular equation*.
An ionic equation is a balanced chemical equation in which strong electrolytes are written as dissociated ions. For example, Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) = AgCl(s) + Na+(aq) + NO3-(aq) is an ionic equation; AgNO3(aq) + NaCl(aq) = AgCl(s) + NaNO3(aq) is not.

limiting reactant. limiting reagent.
The reactant that limits the amount of product produced in a chemical reaction. For example, mixing one mole of H2(g) with one mole of O2 produces one mole of steam (H2O(g)), with half a mole of O2(g) remaining. The hydrogen gas limits the amount of steam produced in this case.

molecular equation. Compare with ionic equation*.
A molecular equation is a balanced chemical equation in which ionic compounds are written as neutral formulas rather than as ions. For example, AgNO3(aq) + NaCl(aq) = AgCl(s) + NaNO3(aq) is a molecular equation; Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) = AgCl(s) + Na+(aq) + NO3-(aq) is not.

net ionic equation. Compare with net ionic equation* and molecular equation*.
A net ionic equation is an ionic equation* with all DEFINE[spectator ions">spectator ions eliminated. For example, Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) = AgCl(s) + Na+(aq) + NO3-(aq) is an ionic equation; the net ionic equation would be Ag+(aq) + Cl-(aq) = AgCl(s) because the sodium and nitrate ions are spectators (they appear on both sides of the ionic equation.

1. having no net electrical charge. Atoms are electrically neutral; ions are not. 2. A solution containing equal concentrations of H+ and OH-.

neutralization reaction. neutralization; acid-base reaction.
A chemical change in which one compound aquires H+ from another. The compound that receives the hydrogen ion is the base; the compound that surrenders it is an acid.

A nonelectrolyte is a substance which does not ionize* in solution.

percent yield. percentage yield. Compare with theoretical yield* and actual yield*.
Percent yield equals experimental yield divided by theoretical yield times 100%.

polar molecule. polar. Compare with covalent compound*, ionic compound* and polar bond*.
An asymmetric molecule containing polar bonds*. H2O, NH3, and HCl are examples of polar molecules. Non-examples are CO2, CCl4, and BCl3 which contain polar bonds but are nonpolar because they have symmetric shapes. Alkanes* are usually asymmetric but are nonpolar because they contain no polar bonds. Polar molecules are electric dipoles* and they attract each other via dipole-dipole forces*.

precipitate. (downarrow) ppt.
An insoluble* substance that has been formed from substances dissolved in a solution. For example, mixing silver nitrate and sodium chloride solutions produces a precipitate, insoluble silver chloride (along with soluble sodium nitrate.

Precipitation is the conversion of a dissolved substance into insoluble form by chemical or physical means.

product. Compare with reactant*.
A substance that is produced during a chemical change*.

reactant. Compare with product*.
A substance that is consumed during a chemical change*.

rearrangement reaction. isomerization; isomerize.
A reaction in which a reactant and product are isomers* of each other. Chemical bonds within the reactant are broken and reformed to produce the product.

redox reaction. electrochemical reaction; oxidation-reduction reaction; redox.
A reaction that involves transfer of electrons from one substance to another. Redox reactions always involve a change in oxidation number for at least two elements in the reactants.

saturated solution. Compare with supersaturated solution*.
A solution which does not dissolve any more solute. When a saturated solution is placed in contact with additional solute, solute neither dissolves nor is deposited from a saturated solution.

solubility. solubilities; equilibrium solubility; solubleness.
The solubility of a substance is its concentration in a saturated solution*. Substances with solubilities much less than 1 g/100 mL of solvent are usually considered insoluble. The solubility is sometimes called "equilibrium solubility" because the rates at which solute dissolves and is deposited out of solution are equal at this concentration.

spectator ion.
A spectator ion is an ion that appears as both a reactant and a product in an ionic equation*. For example, in the ionic equation
Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) = AgCl(s) + Na+(aq) + NO3-(aq)
the sodium and nitrate ions are spectator ions.

standard solution.
A solution of precisely known concentration.

1. Ratios of atoms in a compound. 2. Ratios of moles of compounds in a reaction. 3. A branch of chemistry that quantitatively relates amounts of elements and compounds involved in chemical reactions, based on the law of conservation of mass* and the law of definite proportions*.

strong acid. Compare with weak acid*.
A strong acid is an acid* that completely dissociates into hydrogen ions and anions in solution. Strong acids are strong electrolytes*. There are only six common strong acids: HCl (hydrochloric acid), HBr (hydrobromic acid), HI (hydroiodic acid), H2SO4 (sulfuric acid), HClO4 (perchloric acid), and HNO3 (nitric acid).

strong base.
A strong base is an base* that completely dissociates into ions in solution. Strong bases are strong electrolytes*. The most common strong bases are alkali metal and alkaline earth metal hydroxides.

strong electrolyte. Compare with weak electrolyte*.
A strong electrolyte is a solute that completely dissociates into ions in solution. Solutions of strong electrolytes conduct electricity. Most soluble ionic compounds are strong electrolytes.

supersaturated solution. supersaturated.
A supersaturated solution has concentration of solute that is higher than its solubility*. A crystal of solute dropped into a supersaturated solution grows; excess solute is deposited out of the solution until the concentration falls to the equilibrium solubility.

synthesis. synthesize; synthetic reaction. Compare with decomposition*.
Formation of a complex product from simpler reactants. For example, water can be synthesized from oxygen and hydrogen gas: H2(g) + ½O2(g) rightarrow H2O(ell).

theoretical yield. maximum yield; stoichiometric yield. Compare with actual yield* and percent yield*.
The amount of product obtained when all of the limiting reagent* reacts.

The substance that quantitatively reacts with the analyte* in a titration*. The titrant is usually a standard solution* added carefully to the analyte until the reaction is complete. The amount of analyte is calculated from the volume of titrant required for complete reaction.

titration curve.
A plot that summarizes data collected in a titration*. A linear titration curve plots moles of analyte (or, some quantity proportional to moles of analyte) on the Y axis, and the volume of titrant* added on the X axis. Nonlinear plots use the log of the concentration of the analyte instead. Nonlinear titration curves are often used for neutralization titrations (pH vs. mL NaOH solution). Logs are used to exaggerate the rate of change of concentration on the plot, so that the endpoint* can be determined from the point of maximal slope.

A procedure for determining the amount of some unknown substance (the analyte*) by quantitative reaction with a measured volume of a solution of precisely known concentration (the titrant*).

weak acid. Compare with strong acid*.
An acid* that only partially dissociates into hydrogen ions and anions in solution. Weak acids are weak electrolytes*. Recognize weak acids by learning the six common strong acids*; any acid that doesn't appear on the list of strong acids is usually a weak acid.

weak base. Compare with strong base*.
A base* that only partially dissociates into ions in solution. Weak bases are weak electrolytes*. Ammonia is an example of a weak base; the reaction NH3(aq) + H2O(l) = NH4_+(aq) + OH-(aq) is reversible.

weak electrolyte. Compare with strong electrolyte*.
A weak electrolyte is a solute that incompletely dissociates into ions in solution. For example, acetic acid* partially dissociates into acetate* ions and hydrogen ions, so that an acetic acid solution contains both molecules and ions. A solution of a weak electrolyte can conduct electricity, but usually not as well as a strong electrolyte because there are fewer ions to carry the charge from one electrode to the other.

yield. experimental yield; actual yield. Compare with theoretical yield* and percent yield*.
The amount of product actually obtained in a chemical reaction.

General Chemistry Online! Chemical change

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/reactions/glossary.shtml