ACIDS, BASES AND SALTS

Acid: - Word acid has been derived from a Latin word ‘acidus’ which means sour. The substances that taste sour are called acidic substances. e.g. vinegar, lemon, curd etc.

Base: - Bases are those substances which are bitter in taste. The substances that that taste bitter are called basic substances. e.g. soap, washing soda, baking soda, etc.

Alkali: - All bases cannot dissolve in water. Those bases that dissolve in water are known as alkali.

Neutral substance: - Materials that are neither acidic nor basic.

ACIDS

The word acid is derived from the Latin word “acere” meaning sour. Substances that taste sour contain acids. The chemical nature of such substances is “Acidic”.
Example: Curd, lemon juice, tamarind, spinach and vinegar.

Properties of Acids

·       Acids have corrosive action on skin.

·       Acids neutralize bases to form salt and water.

·       Acids are good conductors of electricity.

Classification of Acids

Acids can be classified as natural and mineral acids.

Natural acids are present in food. Example: Lactic acid, citric acid, acetic acid and tartaric acid.

Common laboratory acids like hydrochloric acid, sulphuric acid and nitric acids are called mineral acids or inorganic acids.

Naturally occurring acids with their source

Acid name	Naturally found in …
Acetic acid	vinegar
Citric acid	Citrus fruits (Orange, lemon etc.)
Ascorbic acid	Amla, Citrus fruits (Orange, lemon etc.)
Lactic acid	Curd
Oxalic acid	Spinach,tomato
Tartaric acid	Grapes, tamarind, unripe mango
Malic acid	Apple

BASES

Basic substances taste bitter. The chemical nature of such substances is “basic”. Bases are slippery to touch. Example: Milk of magnesia, Soaps, Lime water, Household cleaners.

Properties of Bases

·       Bases are bitter to taste, soapy to touch.

·       Bases are good conductors of electricity in aqueous solution where they release ions, which conduct electricity.

·       Bases liberate heat on dilution.

NEUTRAL substance:  Substance that does not show acid or base properties are called neutral substance. Example: Water.

Differences between Acid and bases

Properties	Acids	Bases
Taste	Sour	Bitter
Dissolved in water	Give H+ ion	Give OH‑ ion
Litmus test	Turn blue litmus into red	Turn red litmus into blue
Action with Phenolphthalein	Don’t give any colour	Give pink colour
China rose petal solution	Shows dark pink	Shows green colour
Naturally found	Vinegar, Curd, Citrus fruits etc.	Soap, Milk of magnesia, Baking soda

INDICATOR

It is not always possible to know the nature of substance by testing them because it may be harmful or sometimes lethal. Therefore a special kind of substance is used to know the acidic or basic nature of substance in laboratory, which is known as indicator. Thus Indicator is a substance which changes its color on coming in contact with other substances. Indicators show different colour in acidic, basic or neutral solutions.

Types of Indicator: It can be divided into two groups.

Natural Indicators: These are naturally occurring substances e.g. Litmus, turmeric, China rose etc.

Synthetic Indicators: These are made in the laboratory. It is also called man-made indicators e.g. phenolphthalein, methyl orange, methyl red etc.

Litmus:

It is extracted from lichens. Lichen is not a single organism. It is a symbiosis between algae and fungi.  Litmus has purple colour in distilled water. Litmus is available as red and blue litmus paper.

Blue litmus paper turns into red when dipped in acidic solution while red litmus paper turns into blue when dipped in basic solution

Turmeric:

Turmeric has yellow colour. Turmeric paper does not change its colour with acid while it turns into red when it is dipped into basic solution.

Phenolphthalein:

It becomes colourless in acidic medium. But it gives pink colour in basic medium.

Classification of simple salts
 

Salts can be classified into acidic, basic and neutral salts.

When a strong acid reacts with a weak base, an acidic salts is formed. Example: Ammonium chloride.

When a strong base reacts with a weak acid, a basic salts is formed.
Example: Sodium acetate.

When a strong acid reacts with a strong base, a neutral salt is formed.
Example: NaCl, KNO3, CaBr2.

Properties of salts

Melting and boiling points

Most of the salts are solids. And they have high melting and boiling points.

Solubility in water

Generally salts are soluble in water.

Examples:  Sodium chloride, Potassium sulphate, Ammonium carbonate and aluminium nitrate are soluble in water.

Silver chloride, lead chloride, copper carbonate, etc., are the examples for water insoluble salts.

Water of crystallization

Some salts have a fixed number of water molecules as an essential part of their crystal. These water molecules which form the part of the crystal are called water of crystallization and such salts are called hydrated salts. These water molecules are responsible for the crystal its shape and color of some crystals. In general they exist as dry in pure form. These salts on heating lose water molecules in them and forms anhydrous salts.

For example, in laboratory, the blue coloured copper sulphate is copper sulphate penta hydrate. When heated, this salt loses its water molecules, as a result the copper sulphate crystal lose its shape and color and changes to a powdery substance.
CuSO4.5H2O (On heating) → CuSO4 + 5H2O

NEUTRALIZATION

The reaction between an acid and a base is known as neutralization. When solution of acid is mixed with the solution of base; both of them neutralize each other’s effect. In neutralization reaction, salt along with water are produced. Heat is also evolved. Reactions in which heat is evolved are known as exothermic reactions. Thus, neutralization reaction is an exothermic reaction.

Acid + Base → Salt + Water

 

e.g.                HCl   +   NaOH → Nacl + H2O

 

Ø  HCl (Hydrocloric Acid): A strong acid

Ø  NaOH (Sodium Hydroxide): A strong base

Ø  NaCl (Sodium Chloride): Common salt (Neutral in nature)

Salt formed because of neutralization reaction may be acidic or basic or neutral in nature. Nature of salt depends upon the strength of acid and base.

Neutralization Reaction in Everyday Life

Many neutralization reactions are used in everyday life.

Indigestion: Optimum quantity of Hydrochloric acid maintains the ideal environment for enzymes to digest proteins into smaller units that the body can use and thus helps in the digestion of food. Sometime our stomach produces more hydrochloric acid due to excess food intake or any other reasons.  Production of more hydrochloric acid in the stomach is a symptom of indigestion. This causes pain in the stomach.

Antacid is used to neutralize excessive acid release in our stomach. A medicine made of milk of magnesia (a base) is used to get rid of excess acid present in stomach. Milk of magnesia, being a base neutralizes the hydrochloric acid and gives relief from pain because of indigestion.

Ant sting: Ant sting contains formic acid (Methanoic acid).  Injection of acid by ant causes pain and irritation. Rubbing baking soda over the skin gives relief from pain. Baking soda, which is a base, neutralizes the effect of acid injected by ant. Calamine solution is another base used to neutralize an ant sting.

Soil Treatment: Excess use of fertilizers makes soil acidic or basic. Plant grows well in neutral soil but not in acidic or basic soil. Acidic soil is treated with slaked lime (Calcium hydroxide) or quick lime (Calcium oxide). Use of slaked lime or quick lime neutralizes the acidic nature of soil. Organic matter releases acid and hence are used to neutralize the basic nature of soil.

Factory Waste: The nature of factory wastes is acidic. Release of such kind of acidic wastes in the environment is detrimental for the aquatic organisms. Thus, factory wastes must be treated with basic substances to neutralize the acid present in them before being flushed in the environment.

Acid Rain: Oxides like carbon dioxide, sulphur dioxide and nitrogen dioxide which are released from vehicles, chimneys and refineries, when mixed with droplets of rain, turn the rain water acidic. Acid rain may cause damage to historical monuments, animals and plants.

Industries located in and around Agra like rubber processing, automobile, chemicals and especially the Mathura oil refinery, have been responsible for producing oxides that causes acid rain. This acid makes the marble of Taj Mahal to corrode

INTERESTING FACTS

·       Tooth decay is due to release of acids by bacteria present in mouth.

·       Sodium hydroxide is found in soaps.

·       Ammonium Hydroxide is found in window cleaners.

·       Calcium hydroxide is found in lime water.

·       Sodium bicarbonate is found in baking powder.

·       Rain is slightly acidic in nature.

·       Distilled water is neutral in nature.

·       Chemical name of Vitamin C is ascorbic acid.

·       Calamine solution contains Zinc oxide.