Acid
An acid: a compound that produces hydrogen ions,
H+(aq), when dissolved in water.
For example, the gas hydrogen chloride can
dissolve in water to form hydrochloric acid. This is a
dissociation equation, not decomposition.
HCl(g) H+(aq) + Cl-(aq)
Properties of Acids
form electrolytic solutions when dissolved and
release hydrogen ions
taste sour
reactive with metals
reactive with carbonates
Formation of Acids
Acids can dissociate into ions in water. One of the
ions will be the hydrogen ion and the other will be
the anion, which can be a non-metal (in a binary
acid) or a polyatomic ion (in an oxyacid).
Name Formula Example of Use
hydrochloric acid HCl Cleaning agent, flux
sulphuric acid H2SO4 Chemical reagent, battery acid
nitric acid HNO3 Salt preparation, explosives
acetic acid HC2H3O2 Vinegar
phosphoric acid H3PO4 Fertilizers, additive
carbonic acid H2CO3 Produces the “bubbles” in pop
Formation of acids
To form an oxyacid, non-metals can react with
oxygen to form a non-metal oxide.
E.g. Reaction between nitrogen gas and oxygen
gas
N2(g) + 2 O2(g) 2 NO2(g)
These non-metal oxides are often gases or liquids. The
non-metal oxide can then react with water to form
an oxyacid.
E.g. Reaction between nitrogen dioxide gas and
water
3 NO2(g) + H2O(l) 2 HNO3(aq)+ NO(g)
Reactivity of Acids
Acids can react with metals to produce hydrogen
gas.
E.g. Reaction between magnesium and
hydrochloric acid
Mg(s) + 2 HCl(aq) H2(g) + MgCl2(aq)
They can also typically react with carbonate
compounds to produce carbon dioxide gas.
E.g. Reaction between vinegar (acetic acid) and
baking soda (sodium bicarbonate) to produce
carbon dioxide, water, and sodium acetate
HC2H3O2(aq) + NaHCO3(aq) CO2(g) + H2O(l) + NaC2H3O2(aq)
BasesA base: A compound that forms hydroxide ions, OH-
(aq),
when dissolved in water.
E.g. Sodium hydroxide dissolved in water.
NaOH(s) Na+(aq) + OH-
(aq)
Property of Bases
form electrolytic solutions when dissolved and
release hydroxide ions
taste bitter
feels slippery
Formation of Bases
Bases can also dissociate into ions in water. One of
the ions will be the hydroxide ion. The other will be
the cation, which is usually a metal.
Name Formula Use
sodium hydroxide NaOH Drano
potassium hydroxide KOH soap manufacturing
ammonium hydroxide NH4OH cleaning solutions
calcium hydroxide Ca(OH)2 mortar, cement
magnesium hydroxide Mg(OH)2
milk of magnesia (antacid)
Formation of Bases
There are two types of common reactions to form a base.
If the metal is reactive enough, it can be reacted with
water to form a base but hydrogen is also formed.
e.g. An alkali metal such as potassium is dropped in
water.
K(s) + H2O(l) KOH(aq) + H2(g)
If the metal is not as reactive in water, a metal oxide can
be formed first. This metal oxide can then be reacted with
water to produce a base.
e.g. calcium oxide is reacted in water to form a dilute
solution of calcium hydroxide (limewater)
CaO(s) + H2O(l) Ca(OH)2(aq)
The pH Scale The term pH stands for ‘power of hydrogen’, in
terms of the hydrogen ions in solution.
The pH scale goes from 0 to 14, with 7 being neutral.
Acids have a pH less than 7 and bases have a pH
greater than 7.
A pH less than 7 is acidic.
A pH greater than 7 is basic.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
increasing acidity increasing basicity
increasing H+,
decreasing OH-
increasing OH-,
decreasing H+
The pH Scale
The pH scale is a logarithmic scale, meaning that
each step on the pH scale represents a 10 fold
change in concentration of hydrogen ions.
For example, pH 3 is 10x more acidic than pH 4
and pH 3 is 100 x more acidic than pH 5
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
10 x 10 x
100 x
Indicators A pH meter is commonly used to determine the pH
of a solution. The probe of the pH meter is placed in
the solution and is connected to a computer which
will display the pH.
A pH indicator is a
substance that changes
colour to show the
concentration of hydrogen
ions (H+) and hydroxide
ions (OH-) in a solution.
pH Indicator Colour in acid Colour in base
Litmus paper red blue
Bromothymol blue yellow blue
Phenolphtahlein colourless pink
Universal Indicator
The pH indicators above change colour around a
neutral pH and can be used to determine if a
solution is acidic or basic.
To more accurately determine the pH of a solution,
several indicators must be used. Universal indicator
is a mixture of several indicators that can be used to
determine the approximate pH of a solution.
The universal indicator can also be coated on
paper to generate pH indicator paper which can
also be used.