V.C.E Chemistry
Year 11 Revision Notes
Behaviour of Solutions
Explain the process of solution of Solids in liquids.
Factors Affecting Solubility of a Solid in a Liquid
The Nature of the Solid ( Solute)
The Nature of the Liquid (Solvent)
Define saturated unsaturated and super saturated
solutions.
Solutions
can be formed in many different ways, e.g. when a solid crystalline dissolves in
water, when a gas dissolves in a liquid etc. At this stage we are mainly
concerned with the dissolution of a solid cystalline substance into a liquid to
form a homogenous mixture.
Terms
Solute
– the solid which dissolves in the liquid
Solvent
– the liquid in which the solid dissolves
Dissolution
– the process of dissolving of the crystalline solid in the liquid
Crystallization
– the reforming of the crystalline solid from the mixture
The
particles of the solid are torn from the crystal lattice by the molecules of the
solvent. These solvated particles are then dispersed randomly through out the
solvent. Increasing the temperature weakens the bonds with in the solid
(increased vibrations) and increases the Kinetic Energy of the solvent particles
so that more energy is available for removing the solute particles.
Ionic
substances are, in general soluble in ionic or polar substances
So
ionic NaCl is soluble in H2O which is polar but is insoluble in CCl4
(carbon tetra chloride which is a non polar solvent
Covalent
substances are in general soluble in covalent or non polar liquids
So
covalent I2 is insoluble in polar H2O and soluble in non
polar CCl4
Ionic or polar liquids will dissolve ionic or polar
solids
Covalent liquids will dissolve covalent solids
Energy is required to “tear” the solid particles from
their crystal lattice. Ay higher temperatures, more heat energy is available for
this purpose and as a result more solid will dissolve more easily, there are
however exceptions. The process of dissolving a solid in a liquid involves two
factors
a)
the
tearing of the solute particle from the solid lattice, i.e. overcoming solute /
solvent attractions involving the use of energy
b)
the
solvation (surrounding) of the solute particle by solvent molecules, i.e. solute
/ solvent attraction, resulting in the release of energy to the system
If these two processes are in balance, no heat will be
evolved or absorbed by the system. If they are not, heat my be either absorbed
or evolved depending on the solute or solvent
In general, agitation of the solvent will increase the
rate at which a solute will dissolve in a solvent.
A
solution’s concentration is determined by the relative amounts of solute and
solvent in that solution. There are several ways of expressing concentration,
some of them being
i)
gram
of solute per 100 gm of solvent
ii)
mass
of solute present in 100 gm of solution (written as a % as in 10% w/w)
iii)
mass
of solute present in 100 cm3 of solution (written as %
as in 10% w/v)
iv)
very
low concentration are expressed as parts per million (p.p.m.)
v)
Amount
of substance of solute per cubic decimetre (litre) of solution mol-1
dm3 (the moist common. This is called molar concentration
There
is a limit to the amount of a solute which can be dissolved in a given amount of
solvent at a given temperature. Additional solute will not dissolve and the
solid and the liquid solution are said to be in equilibrium. That is, the number
of particles leaving lattice = the number of particles re-entering the lattice
Is
a solution which is in equilibrium with undissolved solute at a given
temperature
Is
a solution containing less solute than the amount required to saturate the
solution at a given temperature
It
is possible to form solutions in which the amount of solute exceeds that
required for saturation. Such a solution is said to be supersaturated. These
solutions are very unstable and normally recrystallize very easily and rapidly.
The
solubility of a solute in a particular solvent is defined a
The
number of grams of solute which will dissolve in 100 gm of solvent at a
particular temperature.
The
solubility of a solute in a solvent varies with temperature. A graph drawn to
show this relationship for a particular compound is called a solubility curve.
Using these curves we can determine the solubility at a particular temperature
as well as the variation of solubility with temperature. We can also use
solubility curves aand tables to indicate whether a given solution is saturated,
unsaturated or supersaturated
i.e.
above the curve = supersaturated
on the curve = saturated
below the curve = unsaturated
The
term solution is not confined to solids in liquids but includes systems of
solids, liquids or gases in liquids. A solution is a homogenous mixture obtained
by the dissolving of a solute in a solvent. The particles in a solution can not
be filtered from the liquid and do not settle out when allowed to stand. These
particles are extremely small, being less than a millionth of a millimetre in
diameter. The dispersed particles of a solution are indivual small particles or
ions, the smallest possible units of the dissolved substance. It is difficult to
make any generalization about the solubility of substances in liquids, but one
useful working rule is as follows.
Covalent
substances are usually soluble in covalent liquids
Ionic
substances are not usually soluble in covalent liquids but usually soluble in
polar liquids such as water, which allows the ions to dissociate.
In
a suspension, the dispersed particles are large enough to be filtered out from
the liquid and will eventually settle out as a sediment, e.g. mud in water. The
particles of a suspension are at least one thousandth of a millimetre in
diameter.
Like
solutions, colloids cannot be filtered to remove particle, nor will the
dispersed particles settle out. On the other hand, colloids have one important
property in common with suspensions, they scatter light of an incident light
beam, so that the path of the beam through the colloid is marked by a bright
cloudiness – known as the Tindall Effect. This cloudiness is not observed with
solutions. Colloidal sized particles, (between one millionith and one thousandth
of a millimetre in diameter) are usually large molecules or clusters of ions.
Some colloids are very stable indeed.
Alloys
are mixtures of two or more metals, or of a metal and some other material. Most
alloys contain a large amount of one metal, called the base metal, and smaller
amounts of one or more other metals or non-metals. Many pure metals are too
soft, rust too easily or other mechanical or chemical disadvantages to be useful
themselves But usually these disadvantages can be overcome if the metal is
combined with other metals into alloys. Two metals can be mixed in various
proportions to produce alloys with quite different characteristics. Three, four
or more different metals may be mixed together to form s single alloy.
Most
alloys are made by heating (and melting) two or more metals, mixing them
together and allowing them to harden. Some alloys can be formed without melting,
e.g. powdered mixtures of metal are heated, and the fine particles may join
together to form a compact mass.
Is
a preparation of one liquid evenly dispersed in another. The two liquids do not
dissolve in each other. Rather, tiny drops of the dispersed liquid remain
suspended in the other liquid. These drops range in size form 0.1 to 20
thousandths of a millimetre. Some common substances such as cosmetic lotions,
foods, lubricants, medicines and paints are emulsions. Emulsions are not
permanently stable. The liquids separate from each other after a period of time.
To help keep them mixed and emulsifying agent is needed, e.g. milk is an
emulsion of butter fat in water, the protein casein is the emulsifier that keeps
the butter fat suspended in the milk.