# Acid Base and Salt Notes

The taste of the food varies because each food item has a unique chemical composition. Based on their chemical nature, substances can be categorized into acids, bases and salts.

## Acids:

Acids are compound in which one of the elements that makes up the acid molecule is always the hydrogen element. For example, hydrochloric acid (HCl).
Substances that contain acids are known as acidic substances. The word acid comes from the Latin word 'acere' meaning sour.

Types of acids:
Based on the sources they are obtained from, acids are of two types:
Organic  acids: Acids that are naturally obtained from plants and animal sources are called organic acids.

Mineral acids: Acid that are derived from an inorganic material or source are called mineral acids. For example, nitric acid, hydrochloric acid and sulphuric acid.

Depending upon the amount of water present in acids, they are categorized into concentrated and dilute acids.
Concentrated acids: Acids that contain only a small quantity of water are called concentrated acids.

Dilute acids: Acids that contain more water than the concentrated acid are called dilute acids. They can be obtained by adding water to a concentrated acid.

Properties of acids:
• Acids are sour to taste.
• Acids turns blue Litmus to red.
• Acids can corrode metals like aluminium and iron due to their corrosive nature.  That is why acids are stored in glass containers and not in metal containers.
• Acids are soluble in water.
• Acids react with metal to form a salt and hydrogen gas.
$Mg + 2HCl \rightarrow MgCl_2 + H_2$
$\text {Magnesium} + \text{ Hydrochloric acid} \rightarrow \text{Magnesium chloride} + \text{hydrogen Gas}$
• Acids react with bases to form a salt and water.
$NaOH + HCl \rightarrow NaCl + H_20$
$\text {Sodium hydroxide} + \text{Hydrochloric acid} \rightarrow \text{Sodium chloride} + \text{water}$
• Acids react with carbonates to form a salt and carbon dioxide gas.
$CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2$
$\text {Calcium Carbonate} + \text{Hydrochloric acid} \rightarrow \text{Calcium chloride} + \text{water} + \text {Carbon dioxide}$
 Strong acids Weak acids Hydrochloric acid  sulphuric acid, nitric acid, phosphoric acid citric acid, lactic acid, Acetic Acid, carbonic acid

Uses of acids:
Acids are widely used in industries and present in everyday products. Hydrochloric acid  which is present in our  stomach, help to digest our food. Uses of some acids are given in the following table;

## Bases:

Bases are compounds which contain oxygen or  oxygen along with hydrogen. A base that contain oxygen is called an oxide while a base that contains oxygen along with hydrogen is called the Hydroxide.
Substances that contain bases are called basic substances.

Types of Bases:
Based on how they take part in a reaction, bases are of two types: strong bases and weak bases.
Strong bases: Some of the bases are corrosive in nature and may result in skin burns. Such bases are called strong bases. For example, Sodium hydroxide, calcium hydroxide and potassium hydroxide.
Weak bases: Some of the bases are not corrosive in nature. Such bases are called weak bases. For example, Magnesium hydroxide, Ammonium hydroxide, copper hydroxide.

Properties of bases:
• Bases are bitter to taste.
• Bases turn red litmus to blue.
• Some of the bases are soluble in water they are called alkalis (potassium hydroxide and sodium hydroxide).
• Bases react with acids to form a salt and water.
$NH_4OH + HCL \rightarrow NH_4Cl + H_20$
$\text {Ammonium Hydroxide} + \text{Hydrochloric acid} \rightarrow \text{Ammonium chloride} + \text{water}$
• Bases react with metals to form salt and hydrogen gas
$2NaOH + Zn \rightarrow Na_2ZnO_2 +H_2$
$\text{Sodium hydroxide} + \text{Zinc} \rightarrow \text{Sodium Zincate} + \text {H_2}$
Uses of bases:
Sodium Hydroxide is a strong base and it is used to drain cleaners. Strong bases dissolve grease and help in removing dirt.
Uses of some bases are given in the following table:

## Salts

A salt is formed when an acid and a base react.
The reaction in which acids react with bases resulting in the formation of salt and water are called neutralization reactions.

Types of salts:

A salt can be acidic basic or neutral.
Acidic salts: Acidic salt are formed when strong acids react with weak bases. These salts have a pH value of less than 7. For example, ammonium chloride ($NH_4Cl$), aluminium chloride ($AlCl_3$).

Basic salts: Basics salts are formed when strong bases react with weak acids. These salts have a pH value of more than 7. For example, sodium carbonate ($Na_2CO_3$), Sodium acetate ($CH_3COONa$).

Neutral salts: Neutral salts are formed when strong acids react with strong bases. These salts have a pH value of 7. For example, sodium chloride (NaCl), Potassium Nitrate (KNO3).

Properties of Salts:
• Most of the salts are soluble in water.
• Solution of salts in water act as good conductor of electricity.
• Some salts are white crystal whereas some are colored. For example, copper sulphate is blue in color and ferrous sulphate is green.
• Some salts contain water molecules trapped inside them, this is referred to as water of crystallization. The salts that contain water of crystallization are referred to as hydrated salts. For example, Blue vitriol (CuSO4.5H2O).
• Two or more salts that crystallize to form a single substance are referred to as mixed salts. For example, Alum K2SO4.Al2(SO4)3.24H2O.

Uses of salts

## Neutralization reaction:

When the bases react with acid to produce salt and water. This is a neutralization reaction. In a neutralization reaction, the acid loses its acidity  and the base its alkalinity. As a result, a neutral solution is obtained.

Uses of neutralization reactions:
Following are some neutralization reaction that we observe in our everyday life.
In the treatment of ant sting: Some people are highly allergic to ant string as it releases formic acid. It is neutralized by rubbing moist baking soda (sodium bicarbonate) or Calamine solution, which contains zinc carbonate on the affected area.
In the treatment of indigestion: Hydrochloric acid secreted in the stomach helps in the digestion of food. However, its excess secretion could lead to acidity and indigestion. Milk of magnesia is used to neutralize the acid.
In the treatment of soil: When the soil is too acidic, it is treated with slaked lime or quicklime, which is a base. If the soil is too basic, it is treated with organic matter such as fertilizers that are obtained from vegetables or fruits as they are acidic in nature.
In the treatment of sewage waste:  several Industries produce acids as waste. If they are allowed to flow into water bodies, these wastages would affect aquatic organisms. Slaked lime (Calcium Hydroxide) is often used to neutralize this acidic waste let out from the factories.

Protecting teeth: One of the best common applications of neutralization of acids is the toothpaste. Toothpastes contain an alkali that neutralizes the weak acid produced by bacteria and prevent tooth damage.

## Indicators:

Substance that assist in determining whether a given substance is acidic or basic, with the help of a colour change are called indicators.

Natural indicators:
Litmus: Litmus is the most commonly used natural indicator extracted from lichen. Two types of Litmus are available: in the form of a solution, referred to as Litmus solution and a strip of paper called Litmus Paper. Acid turns blue Litmus red. Bases turn red litmus blue.
Turmeric: In acidic and neutral substances  the turmeric solution remains yellow in colour, while in basic substance it turns brick red in colour.
China rose: China rose (Hibiscus) juice obtained from the petals is used as a natural indicator. if acidic substance are added, it turns dark pink (Magenta) in colour, while adding basic substances make the juice green.
other commonly used acid base indicators

 Indicators Colour in acid Colour in base Methyl orange Red orange Orange Phenolphthalein Colourless Pink

Universal indicator:
Universal indicators are mixture of dyes or compounds that show a gradual change in colour to indicate the acidity or basicity for pH values from 1 to 14.

pH:
pH scale is a numerical representation of the acidity or basicity of substances. It varies  from 1 to 14. Solution with a pH value of less than 7 are referred to as 'acids; the ones with the pH value of more than 7 are referred to as 'bases' and those with  a pH value of 7 are referred to as 'neutral'.