Every day, we encounter liquids that have acidic and basic properties. The only exception, in this case, is water. The particular acid-base characteristics set them apart from each other. Carl-Axel Arrhenius defines an acid as a compound that yields hydrogen cations when dissolved in water. The strength of an acid or a base is determined by its pH level.
The abbreviation pH is used for the phrase ‘power of hydrogen’. On a pH scale, the mid-point is referred to as neutral. If the pH of a certain compound comes below the midpoint in the scale, that compound is considered acidic.
On the contrary, if the pH of a compound on the scale is above the midpoint, it is considered basic or alkaline.
Characteristics of Acids and Bases
Acid-Base characteristics can be broadly categorised under two headings – physical and chemical.
Physical properties of acids and base
Physical properties of acids – The following physical characteristics can identify acids.
They have a pH value of below 7.
They are sour. The word acid has been derived from a Latin word for ‘sour’. This is so because it is one of the physical properties of acids that makes it very distinguishable.
Strong acids are good conductors of electricity, while weak acids are weak conductors of electricity.
Mineral acids are colourless, while organic acids are white.
Sometimes, when strong acids and bases react, the reaction results in the formation of water and salts. So it can be said that when acids react with water, they produce neutral substances.
Physical properties of bases – Bases are identified by the following physical characteristics –
Bases are bitter to taste.
An aqueous solution of bases acts as electrolytes.
If touched, bases feel very slippery in texture.
Bases have a pH that is higher than 7.
Except for hydroxides of copper and iron, all bases are colourless.
Some bases are soluble in water.
Let us now compare the physical properties of acids and bases.
Solubility in water
Soluble in water
Few bases are soluble in water
Sometimes, due to impurities, mineral acids tend to turn yellow. But originally, they were colourless. At the same time, the organic compounds are white.
Bases are colourless. The exception, in this case, is hydroxides of copper and iron.
Acids tend to turn blue litmus into red.
Bases tend to turn red litmus to blue.
Chemical properties of acids and bases
Chemical properties of acids
Reaction with metals – Hydrogen is displaced from the acid, and salts are produced along with the release of hydrogen gas when an acid reacts with metal.
Example: M + H2SO4 → MSO4 + H2
Reaction with metal oxides – Salt and water are produced when acids react with metal oxides.
Example: CuO + 2HCl → CuCl2 + H2
Reaction with metal carbonates or bicarbonates – Salt, carbon dioxide, and water are produced in this type of reaction.
Example: Na2CO3 + HCl (aq) → 2NaCl (aq) + H2O + CO2
Chemical properties of bases
Reaction with metal – Bases dissolved in water, which forms alkali’s, when reacted with metals, forms hydrogen gas and salts.
Example: 2NaOH + Zn → Na2ZnO2 + H2
Reaction with non-metal oxides – Bases react with non-metal oxides to form salt and water.
Example: CO2 + Ca(OH)2 → CaCO3 + H2O
A solution containing equal concentrations of hydronium and hydroxide ions results in the formation of a neutral solution. An acid-base neutralisation reaction occurs when we mix solutions of an acid and a base. It either contains an excess of hydronium ions, or it can happen that it contains an excess of hydroxide. Whether the solution is acidic, neutral, or basic depends upon the nature of the salt formed.
An example of a neutralisation reaction is –
HCl (aq) + NaOH (aq) ⇌ NaCl (aq) + H2O (l)
This concept of neutralisation has many applications. For instance, we suffer from acidity sometimes. Our stomach contains a solution of about 0.03 M HCl. Now, it irritates when this acid escapes from the valves and comes up to the oesophagus. In such cases, we are given antacids, which are bases. It neutralises the stomach acid and forms CO2 gas. The most common form of antacid is calcium carbonate.
The reaction of this is –
CaCO3(s) + 2HCl (aq) ⇌ CaCl2(aq) + H2O(l) + CO2(g)
Comparison between Acid and Bases based on their characteristics
Let us draw a general comparison between acids and bases that can summarise the entire concept.
Soapy and slippery
Less than 7
Greater than 7
– Produces H2 gas when reacted with metals
– Produce CO2 when reacted with carbonated compounds
– Does not typically react with metals or with carbonated compounds
– Does react with oils and fats
Conducts electricity in water
vinegar (ethanoic acid), lemon juice (citric acid)
Baking soda (sodium bicarbonate), ammonia water (ammonia hydroxide)
The many characteristics of both acids and bases are important and should be learned. This way, different solutions can be identified and used properly in the laboratory. Knowing about different acid-base characteristics also helps us prepare for mishaps in a chemistry laboratory. Learning about the concepts of acids and bases has also helped us find relief from acidity. It has been made possible only because of the knowledge of both physical and chemical characteristics.