Acids, Bases,

and pH

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.

Universal Indicator

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Red orange yellow green blue purple