Solutions
A solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.
Properties of Solutions
· Reqular distribution of atoms (or molecules or ions) solutes in solvent.
· Ease of recovery of solute or solvent in solution
Classification of Solutions:
Solutions can be classified into particles size and solute concentration.
· Solute particles size
true solution
A true solution is a homogeneous solution in which the solute particles have diameters less than 10-7cm. i.e., the solute particles are of molecular dimensions. The particles are invisible even under powerful microscopes.
For example, sodium chloride in water is a true solution. Most ionic compounds form true solutions in water. Organic compounds like sugar and urea also form true solutions in water.
Suspensions
A suspension is a heterogeneous mixture in which one or more components have a particle size greater than 10-5cm. Very often these particles are visible to the naked eye.
For example, sand in water, sodium chloride in benzene, turmeric in water etc.
Colloids
A colloid is a solution in which the particle size ranges between 10-7 and 10-5 cm. For example, milk, blood, honey, smoke, ink, gum, starch solution etc.
Difference between true solutions, suspensions and colloids
Property / True solutions / Colloidal solutions / Suspensions /Particle size / Less than 10 - 7 cm / Between 10 - 5 and 10 - 7 cm / Greater than 10 - 5 cm
Visibility of particles / Invisible to naked eye not visible under powerful microscope / Invisible to naked eye. Visible under powerful microscope / Easily visible
Sedimentation of particles / Do not settle down / Settle down under high centrifugation / Settle down due to gravity
Filtration through filter paper / No residue is formed / No residue is formed / Residue is formed
· Solute Concentration
1. Saturated solutions have the maximum amount of solute dissolved in them. No more solute can be made to dissolve.
2. Unsaturated solutions have less than the maximum amount of solute dissolved in them. This means that more solute could be added to the solution and the additional solute would still dissolve.
3. Supersaturated solutions have more than the maximum amount of solute dissolved in them. This is not normally possible. Not all substances will form supersaturated solutions. If a crystal of the solute is added to a supersaturated solution, precipitation of all of the extra solute will occur.
Definitions of Acids and Bases
· Arrhenius Acids and Bases
The Arrhenius definition of acids and bases is one of the oldest. An Arrhenius acid is a substance that when added to water increases the concentration of H1+ ions present. The chemical formulas of Arrhenius acids are written with the acidic hydrogens first. An Arrhenius base is a substance that when added to water increases the concentration of OH1- ions present. HCl is an example of an Arrhenius acid and NaOH is an example of an Arrhenius base.
· Bronsted-Lowry Definition of Acids and Bases
We will use the Bronsted-Lowry definitions for acids and bases:
Acids are species that donate a proton (H+).
and
bases are species that accept a proton.
Acid example:
HNO3 (aq) + H2O NO3-(aq) + H3O+(aq)
Keq = a very large number
In this example, HNO3 is an acid and H2O is acting as a base.
NO3- is called the conjugate base of the acid HNO3, and H3O+ is the conjugate acid of the base H2O.
Base example:
NH3 (aq) + H2O NH4+(aq) + OH-(aq)
K = 1.8x10-5
In this example, NH3 is a base and H2O is acting as an acid. NH4+ is the conjugate acid of the base NH3, and OH- is the conjugate base of the acid H2O.
A compound that can act as either an acid or a base, such as the H2O in the above examples, is called amphiprotic.