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People Behind the Science

Johannes Nicolaus Brönsted (1879–1947)

ohannes Brönsted was a Danish physical the behavior of indicators. He discovered a hydroxyl ions. A strong acid is completely
Jchemist whose work in solution chem- method of eliminating potentials in the mea- ionized (dissociated) and produces many
istry, particularly electrolytes, resulted in a surement of hydrogen ion concentrations hydrogen ions, whereas a weak acid is only
new theory of acids and bases. and devised a simple equation that connects partly dissociated and produces few hydro-
Brönsted was born on February 22, the activity and osmotic coefficients of an gen ions. Conductivity measurements con-
1879, in Varde, Jutland, the son of a civil electrolyte, as well as another that relates firm the theory, as long as the solutions are
engineer. He was educated at local schools activity coefficients to reaction velocities. not too concentrated.
before going to study chemical engineering From the absorption spectra of chromic— In 1923, Brönsted published (simultane-
at the Technical Institute of the University chromium(III)—salts, he concluded that ously with Thomas Lowry in Britain) a new
of Copenhagen in 1897. He graduated two strong electrolytes are completely dissoci- theory of acidity, which has certain important
years later and then turned to chemistry, ated and that the changes of molecular con- advantages over that of Arrhenius. Brönsted
in which he qualified in 1902. After a short ductivity and freezing point that accompany defined an acid as a proton donor and a base
time in industry, he was appointed an assis- changes in concentration are caused by the as a proton acceptor. The definition applies
tant in the university’s chemical laboratory electrical forces between ions in solution. to all solvents, not just water. It also explains
in 1905, becoming professor of physical and In 1887, Svante Arrhenius had pro- the different behavior of pure acids in solu-
inorganic chemistry in 1908. posed a theory of acidity that explained its tion. Pure dry liquid sulfuric acid or acetic
Brönsted’s early work was wide ranging, nature on an atomic level. He defined an (ethanoic) acid does not change the color of
particularly in the fields of electrochemis- acid as a compound that could generate indicators, nor does it react with carbonates
try, the measurement of hydrogen ion con- hydrogen ions in aqueous solution and an or metals. But as soon as water is added, all
centrations, amphoteric electrolytes, and alkali as a compound that could generate these reactions occur.

Source: From the Hutchinson Dictionary of Scientific Biography. © Research Machines plc 2003. All Rights Reserved. Helicon Publishing is a division of Research Machines.

SUMMARY
A water molecule consists of two hydrogen atoms and an oxygen atom Water solutions that carry an electric current are called electrolytes,
with covalent bonding. Oxygen has more positive protons than either of and nonconductors are called nonelectrolytes. In general, ionic substances
the hydrogens, so electrons spend more time around the oxygen, pro- make electrolyte solutions, and molecular substances make nonelectro-
ducing a polar molecule, with centers of negative and positive charge. lyte solutions. Polar molecular substances may be ionized by polar water
Polar water molecules interact with an attractive force between the nega- molecules, however, making an electrolyte from a molecular solution.
tive center of one molecule and the positive center of another. This force The boiling point of a solution is greater than the boiling point of
is called a hydrogen bond. The hydrogen bond accounts for the decreased the pure solvent, and the increase depends only on the concentration
density of ice, the high heat of fusion, and the high heat of vaporization of the solute (at a constant pressure). For water, the boiling point is in-
of water. The hydrogen bond is also involved in the dissolving process. creased 0.521°C for each mole of solute in each kg of water. The freezing
A solution is a homogeneous mixture of ions or molecules of two point of a solution is lower than the freezing point of the pure solvent,
or more substances. The substance present in the large amount is the and the depression also depends on the concentration of the solute.
solvent, and the solute is dissolved in the solvent. If one of the compo- Acids, bases, and salts are chemicals that form ionic solutions in
nents is a liquid, however, it is called the solvent. Fluids that mix in any water, and each can be identified by simple properties. These proper-
proportion are called miscible fluids, and immiscible fluids do not mix. ties are accounted for by the modern concepts of each. Acids are pro-
+
Polar substances dissolve in polar solvents but not nonpolar solvents, ton donors that form hydronium ions (H 3 O ) in water solutions. Bases
–
and the general rule is like dissolves like. Thus oil, a nonpolar substance, are proton acceptors that form hydroxide ions (OH ) in water solutions.
is immiscible in water, a polar substance. Strong acids and strong bases ionize completely in water, and weak acids
The relative amount of solute in a solvent is called the concentra- and weak bases are only partially ionized. The strength of an acid or
tion of a solution. Concentrations are measured in (1) parts per million base is measured on the pH scale, a power of ten notation of the hy-
(ppm) or parts per billion (ppb); (2) percent by volume, the volume of dronium ion concentration. On the scale, numbers from 0 up to 7 are
a solute per 100 volumes of solution; (3) percent by weight, the weight acids, 7 is neutral, and numbers above 7 and up to 14 are bases. Each
of solute per 100 weight units of solution; and (4) salinity, the mass of unit represents a tenfold increase or decrease in acid or base properties.
salts in 1 kg of solution. A salt is any ionic compound except those with hydroxide or
A limit to dissolving solids in a liquid occurs when the solution oxide ions. Salts provide plants and animals with essential elements.
is saturated. A saturated solution is one with equilibrium between The solubility of salts varies with the ions that make up the compound.
solute dissolving and solute coming out of solution. The solubility Solutions of magnesium or calcium produce hard water, water in which
of a solid is the concentration of a saturated solution at a particular it is hard to make soap lather. Hard water is softened by removing the
temperature. magnesium and calcium ions.

11-19 CHAPTER 11 Water and Solutions 293