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Many acids and bases occur naturally in nature. We use many of these in our day to day life. The concept of acid, bases and salts are very old ones that have undergone several major refinements as chemical science has ivolved.

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Acids are sour in taste and it turns blue litmus to red. Acids are substances which can increase the concentration of hydrogen (H⁺) ions in an aqueous solution. Acid solutions conduct electricity.
Examples:- Sulphuric acid (H₂SO₄) , Nitric acid (HNO₃) , Acetic acid (CH₃COOH) etc.
On the bases of occurance acids are divided into two types.
(i) Natural Acids :- Acids which are obtained from natural sources are called natural acids or organic acids.
Example:- Acetic acid (CH₃COOH) , Methanoic acid (HCOOH) etc.
(ii)Mineral Acids :- Acids that are prepared from minerals are known as mineral acids.
Examples:- Sulphuric acid (H₂SO₄) , Nitric acid (HNO₃) etc.

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Acids react with reactive metals to form hydrogen gas.

Example:-     Zn + 2HCl ----> ZnCl₂ + H2
The gas evolved after the reaction of metals with acids can be tested by bringing a burning match stick at the mouth of the test tube. If the gas burns with a pop sound, then it confirms the evolution of hydrogen gas.

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(ii) Reaction of acids with metal carbonates.

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When acids react with carbonates, carbondioxide gas is liberated.
Example:- CaCO₃ + 2HCl ---> CaCl₂ + H₂O + CO2
The evolved gas after the reaction of acids with carbonates can be determined by passing it through a lime water. If the lime water turns milky then it shows the liberation of carbon dioxide.

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The number of hydrogen ions that can be donated by one molecule of an acid is its basicity.
1)If the basicity is 1, it is called monobasic acid.
2)If the basicity is 2, it is said to be dibasic acid.
3)If the basicity is 3, the acid is called a tribasic acid.

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In industrial areas and townships the chances of air pollution are very high. In such regions, gases like SO2 and NO₂ reach the atmosphere in large amounts. These gases dissolve in rain water and reach the soil as acids. This is known as ' acid rain '.
Acid rain lose the ability of plants to produce carbohydrates through photosynthesis and it destroys the greenery of a region.

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Bases are bitter in taste and have soapy touch. It turns red litmus to blue and these are substances which can increase the concentration of hydroxide ions (OH⁻) in an aqueous solution.
Example:- Sodium hydroxide (NaOH), Calcium hydroxide (Ca(OH)₂), Potassium hydroxide (KOH) etc.
All bases are not alkalies. The bases that dissolve in water are called alkalies.

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1) ARRHENIUS' THEORY
Acids are substances which liberate H⁺ ions in aqueous solution and bases are substances which liberate OH⁻ ions in aqueous solutions.
2)LOWRY BRONSTED THEORY
Bronsted acid is an H+ ion donor and Bronsted base is an H⁺ ion acceptor.

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Acid and alkali react with each other to nullify their individual properties and form salt and water. Such chemical reactions are called neutralisation reactions.

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1) pH Scale is the method used to express the acidic or basic nature of a substances based on the amount of H⁺ ions present in their aqueous solutions.
2) pH value of a neutral solution is 7.
3) pH vaiue of acids is less than 7 and that of bases is greater than 7.

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A salt is a combination of an anion of an acid and a cation of a base. They are electrically neutral.
Example:- KCL, NaNO₃, CaSO4
It is usually prepared by the neutralisation reaction of an acid and a base.