Acids, Bases and Indicators

Acids, Bases and Indicators

An acid was defined n the early times as a substance that tastes sour. Vinegar, which contains acetic acid, is sour, and so is lemon juice, which contains citric acid. Acids were also known to change the colors of organic dyes. The dye present in red cabbage, for example, turns from reddish purple to brighter red color when the cabbage is marinated with vinegar. A common organic acid dye used to identify acids is litmus, which is derived from various lichens. Litmus turns from blue to red when mixed with an acidic solution.

Bases, or alkalis, also known from ancient times, have a bitter taste and slippery feel. Bases turn red litmus back to blue, and turn the otherwise colorless organic dye phenolphthalein a bright pink.

The Arrhenius theory of acids and bases defines an acid as a substance which, when placed in water, generates protons (H+ ions), and base as a substance which, when placed in water, generates hydroxide ions (OH-).

Many compounds may be reorganized from their formulas. In the case of acids, the available hydrogen atoms which are responsible for the acidity are written first, followed by the symbols of their other elements in the formula. Similarly, some bases (but not all) may be recognized by their formulas as an hydroxide ion combined with a positive metal ion.

Examples:                                                          ACIDS

Hydrochloric acid              HCL

Nitric acid                            HNO3

Sulphuric acid                    H2SO4

Acetic acid                           HC2H3O2

Oxalic acid                           H2C2O4

Carbonic acid                     H2CO3


Sodium hydroxide           NaOH

Potassium hydroxide     KOH

Calcium hydroxide           Ca(OH)2

Ammonia                            NH3


According to Arrhenius theory, the solutions of hydrochloric acid and sodium hydroxide may be represented by their respective equations:


HCl(aq)     =         H+(aq) + Cl-(aq)

NaOH(aq)         =           Na+(aq) + OH-(aq)


The hydrogen ion, present in all aqueous acid solutions is species responsible for all the acidic properties from the sour taste to the color of litmus. Likewise, the hydroxide ion is responsible for all the basic properties of a solution.

Water behaves as both acid and as a base, as illustrated by the equation:

H2O +H2O                =           H3O+     +    OH-

 One water molecule has donated a proton (acted as an acid) and another water molecule has accepted a proton (acted as a base). Since hydronium ions and hydroxide ions are produced in equal concentrations, water is neutral. Aqueous solutions contain both hydronium and hydroxide ions. In acidic solutions, the concentration of hydronium ions is greater than that of hydroxide ions. In basic solutions, the concentration of hydroxide ions is greater than that of hydronium ions.


Substances used to determine whether a solution is acidic or basic are known as indicators. Indicators are usually organic compounds whose color depends on the concentration of H+ ions. The table below lists several common indicators, their color in acid and in base solutions, and the approximate pH (given as a range) at which the color changes.

Since:   pH = -log (H+ concentration

An indicator can also be used to determine the approximate pH of a given solution.

Methyl orangeMethyl redBromothymol bluePhenolphthalein

Litmus paper











There are several characteristic reactions of acids and bases:

Neutralization is a reaction of an acid and a base to form a salt and water:


e.g:                                                                        HCl(aq) + NaOH(aq)     =          H2(g) + NaCl(aq)

In neutralization reaction, the hydrogen ion of the acid solution unites with the hydroxide from the basic solution to form water molecule. The net ionic equation is:

H+(aq) + OH(aq)      =         H2O(l)

For the above reaction, when NaOH solution is added to HCL solution containing a few drops of phenolphthalein, the solution turned from colorless to pink when the acid is neutralized and one more drop of the base is abed.

The reaction of an active metal with acids forms a salt and hydrogen gas (a SINGLE REPLACEMENT REACTION):

Mg(s) + 2HCl(aq)        =            MgCl2(aq) + H2(g)

The reaction of a carbonate with an acid forms a salt, carbon dioxide and water:

CaCO3(s) + 2HCL(aq)         =          CO2(g) + H2O(l) + CaCl2(aq)

The reaction of a strong base with ammonium salts forms ammonia gas, water and salt:

2KOH(aq) + (NH4)2 SO4 (aq)       =         2NH3(g)  + 2H2(l) + K2SO4(aq)

The reaction of a transition metal salt with a base results in precipitation of the transition metal hydroxide and another salt:

3 Sr(OH)2(aq_ + 2FeCl3(aq)        =        2Fe(OH)3(s) +3SrCl2(aq)

Be sure to see our lab on indicators in the Labs and Experiments page.


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