Acid

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Introduction
    We use colored solution to detect acidity or basicity of substance in the lab. These substances can be prepared by mixing colored parts of plant body in ethanol (C₂H₅OH). The color in plant leave is due to the presence of pigments such as chlorophyll and chromoplast. Colored solution is used widely in chemistry to detect acidity and basicity.

Indicator             An indicator is a chemical substance which has capacity to show the termination of a chemical reaction by changing its color. The indicator does not take part in chemical reaction and does not effects the reaction showing change in color. The indicators are widely used to identify whether a substance is acid or base. Colored solution prepared from red-rose and beet-root is examples of indicator. Litmus paper is the most common in lab ot identify whether a substance is base or acid. Litmus is prepared from lichen plant.
            Blue litmus changes into red in acidic medium and red litmus changes into blue in basic medium. Phenolphthalein, methyl orange, turmeric, etc are examples of indicator.

Litmus can detect whether a solution is acid or base but it cannot show the strength of acidity or basicity. Universal indicators show the strength of an acid or base. Universal indicator is a mixture of ordinary indicators. Universal indicator is a mixture of ordinary indicators. Universal indicator changes the color itself according to the strength of an acid or base.

pH value of some substances:

Substance	pH
Water Salt solution Sugar Ethanol Lemon juice Toothpaste Ammonia solution Apple Butter Soda Milk of magnesium Washing soda Kerosene Vinegar	7 7 7 7 2.5 9 10 3 6 8.5 9 11.5 7 3

From above table we come to know that in food materials also acidic and basic property exists. Basicity is present in sodium hydrogen carbonate (baking soda) and lemon juice is acidic. Universal indicator is used to measure the pH scale of solution. This scale is called pH meter.

Acid
            An acid is a substance which gives hydrogen ion when dissolved in water. An acid neutralizes the base.
            HCl = H⁺ + Cl^-
            H₂SO₄ = 2H⁺ + SO^{- -}₄
This hydrogen of an acid can be replaced partially or completely b y metals like zinc and magnesium.
Example: Zinc + Sulphuric acid = Zinc sulphate + Hydrogen
                Zn + H₂SO₄ = ZnSO₄ + H₂

Some common acid:

Name of the acid	Molecular Formula
Hydrogenchloric acid Sulphiric acid Nitric acid Phosphoric acid Carbon acid	HCl H2SO4 HNO3 H3PO4 H2CO3

Characteristic of Acid:

1. Acids posses sour taste. 
2. Acids turn blue litmus into red.
3. Acids neutralize base.                                                                    HCl + NaOH = NaCl + H₂O  
4. Hydrogen of an acid is displaced by metals.                                    Zn + H₂SO₄ = ZnSO₄ + H₂
5.  Acid produces salt and water when treated with basic oxide        CaO + 2HCl = CaCl₂ + H₂O  
6. Many carbonates evolve carbon dioxide when treated with acids.                                                                                               E.g. CaO + 2HCl = CaCl₂ + H₂O

  Preparation of Acids

         Acids can be prepared by following methods.

1. By direct combination of elements.

H₂ + Br₂ = 2HBR

2. By action of water on acid anhydride. When acid anhydride is treated with water, corresponding acids are produced.

CO₂ + H₂O = H₂CO₃

3. By oxidation of non-metals. When nitric acid acts on phosphorous, and sulphur, phosphoric and sulphuric acids are formed.

P + 5HNO₃ = H₃PO₄ + 5NO₂ + H₂O