Acid

 

The word ‘Acid’ came from Latin word ‘Acidus or Acere’ which means sour.

Sour taste is the most common characteristic of acid. Acid turns blue litmus paper red. There are many substances which contain acid and hence taste sour, such as curd, tamarind, lemon, etc.

Types of Acids:

Acids are divided into two types on the basis of their occurrence – Natural acids and Mineral acids.

Natural Acid: Acids which are obtained from natural sources are called natural acid or organic acid.

Mineral Acids: Acids that prepare from mineral are known as mineral acids, inorganic acids, man-made acids or synthetic acid, such as hydrochloric acid, sulphuric acid, nitric acid, etc.

Organic Acids & Source

Acid

Source

Acetic acid

Vinegar

Ascorbic acid

Guava, amla

Citric acid

Lemon, orange and other citrus fruits

Lactic acid

Sour milk, curd

Methanoic acid

Ant sting, nettle sting

Oxalic acid

Tomato

Tartaric acid

Tamarind

Chemical Properties of Acid

Reaction of acids with metal:

Acids give hydrogen gas along with respective salt when they react with a metal.

Metal + Acid ⇨ Salt + Hydrogen

Example: Hydrogen gas and zinc chloride are formed when hydrochloric acid reacts with zinc metal.

Zn + 2HCl ⇨ ZnCl2 + H2

Base

Base is bitter in taste and feels soapy on touch. A base turns red litmus paper blue.

Sodium hydroxide (caustic soda), calcium hydroxide, sodium carbonate (washing soda), lime (Calcium oxide), potassium hydroxide (caustic potash), etc. are examples of base.

 Base can be divided into two types – Water soluble and water insoluble.

The ionic salt of alkali and alkaline earth metals are soluble in water. These are also known as alkali. For example – sodium hydroxide, magnesium hydroxide, calcium hydroxide, etc. Alkali is considered as strong base.

Salts

Salts are the ionic compounds which are produced after the neutralization reaction between acid and base. Salts are electrically neutral. There are number of salts but sodium chloride is the most common among them. Sodium chloride is also known as table salt or common salt. Sodium chloride is used to enhance the taste of food. Characteristics of salt are as below:

  1. Most of the salts are crystalline solid
  2. Salts may be transparent or opaque
  3. Most of the salts are soluble in water
  4. Solution of salts conducts electricity. Salts conduct electricity in their molten state also
  5. The salt may be salty, sour, sweet, bitter and umami (savoury)
  6. Neutral salts are odourless
  7. Salts can be colorless or coloured

Salts having common acidic or basic radicals are said to belong to same family.

Example: 

Acid 

Base

Salt 

Example 

Strong

Strong

Neutral

NaOH + HCl → NaCl + H2O

Strong

Weak

Acidic

HCl + NH4OH → NH4Cl + H2O

Weak

Strong

Basic

CH3COOH + NaOH → CH3COONa + H2O

Weak

Weak 

Neutral

CH3COOH + NH4OH → CH3COONH4 + H2O

The most common salt is sodium chloride or table salt which forms by the combination of sodium hydroxide (base) and hydrochloric acid. Other examples include Epsom salts (MgSO4) used in bath salts, ammonium nitrate (NH4NO3) used as fertilizer, and baking soda (NaHCO3) used in cooking.

pH – Measurement of strength of Acid and Base

The strength of acid or base depends upon the hydrogen ion concentration. If the concentration of hydrogen ion is greater than hydroxide ion, the solution is called acidic. If the concentration of hydrogen ion is smaller than the hydroxide ion, the solution is called basic. If the concentration of hydrogen ion is equal to the concentration of hydroxide ion, the solution is called neutral solution.

pH is a scale which quantifies the concentration of hydrogen ion in a solution. The range of pH scale is between 0 to 14.

The pH value decreases with increase in hydrogen ion concentration. If the value of pH is 0, this shows maximum hydrogen ion concentration. pH value equal to 14 shows lowest hydrogen ion concentration. pH value equal to 7 shows the hydrogen ion concentration is equal to hydroxide ion concentration.

A neutral solution, such as distilled water has value of hydrogen ion concentration equal to 7 on pH scale. The acidic solution has value of hydrogen ion concentration less than 7 on pH scale. The basic solution has value of hydrogen ion concentration greater than 7 on pH scale.

In pH scale, ‘p’ stands for ‘potenz’. Potenz is a German word which means ‘power’ or ‘potential’. Here; ‘H’ stands for hydrogen ion. Thus, pH means the potential of hydrogen or power of hydrogen.

pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity (a(H+ )), in a solution.

Indicator:

Substances which show the acidic or basic behavior of other substance by change in colour is known as indicator.

Type of Indicator: There are many types of indicators. Some common types of indicators are

  1. Natural
  2. Olfactory Indicator
  3. Synthetic Indicator
  4. Universal Indicator

Natural Indicator: Indicators obtained from natural sources are called natural indicators. Litmus, turmeric, red cabbage, China rose, etc. are some common natural indicators used widely to show the acidic or basic character of substances.

Litmus: Litmus is obtained from lichens. The solution of litmus is purple in colour. Litmus paper comes in two colour – blue and red.

  • An acid turns blue litmus paper red.
  • A base turns red litmus paper blue.
  • Turmeric: Turmeric is another natural indicator. Turmeric is yellow in colour. Turmeric solution or paper turns reddish brown with base. Turmeric does not change colour with acid.
  • Red cabbage: The juice of red cabbage is originally purple in colour. Juice of red cabbage turns reddish with acid and turns greenish with base.
  • Olfactory Indicators: Substances which change their smell when mixed with acid or base are known as olfactory indicators. For example onion, vanilla, clove, etc.
  • Onion: Paste or juice of onion loses its smell when added with base. It does not change its smell with acid.
  • Vanilla: The smell of vanilla vanishes with base, but it’s smell does not vanishes with an acid.
  • Olfactory indicators are used to ensure the participation of visually impaired students in laboratory.
  • Synthetic Indicator: Indicators that are synthesized in laboratory are known as synthetic indicators. For example, phenolphthalein, methyl orange, etc.
  • Phenolphthalein is a colourless liquid. It remains colourless with acid but turns into pink with a base.
  • Methyl orange is originally orange in colour. It turns into red with acid and turns into yellow with base.

Indicator

Original colour

Acid

Base

Red litmus

Red

No change

Blue

Blue litmus

Blue

Red

No change

Turmeric

Yellow

No change

Reddish brown

Red cabbage juice

Purple

Reddish

Greenish yellow

Phenolphthalein

Colourless

Colourless

Pink

Methyl orange

Orange

Red

Yellow

Onion

n/a

No change

Smell vanishes

Vanilla

n/a

No change

Smell vanishes

Universal Indicator:

  • Using a litmus paper, phenolphthalein, methyl orange, etc. only the acidic or basic character of a solution can be determined, but use of these indicators does not give the idea about the strength of acid or base. So, to get the strength as well as acidic and basic nature of a given solution universal indicator is used.
  • Universal indicator shows different colour over the range of pH value from 1 to 14 for a given solution. Universal indicator is available both in the form of strips and solution. Universal indicator is the combination of many indicators, such as water, propanol, phenolphthalein, sodium salt, sodium hydroxide, methyl red, bromothymol blue monosodium salt, and thymol blue monosodium salt. The colour matching chart is supplied with universal indicator which shows the different colours for different values of pH.

Bleaching Powder (CaOCl2 )

Bleaching powder is also known as chloride of lime. It is a solid and yellowish white in colour. Bleaching powder can be easily identified by the strong smell of chlorine.

When calcium hydroxide (slaked lime) reacts with chlorine, it gives calcium oxychloride (bleaching powder) and water is formed.

Ca(OH)2 + Cl2 ⇨ CaOCl2 + H2O

Aqueous solution of bleaching powder is basic in nature. The term bleach means removal of colour. Bleaching powder is often used as bleaching agent. It works because of oxidation. Chlorine in the bleaching powder is responsible for bleaching effect.

Use of bleaching powder:

  • Bleaching powder is used as disinfectant to clean water, moss remover, weed killers, etc.
  • Bleaching powder is used for bleaching of cotton in textile industry, bleaching of wood pulp in paper industry.
  • Bleaching powder is used as oxidizing agent in many industries, such as textiles industry, paper industry, etc.

Baking Soda (NaHCO3)

Baking soda is another important product which can be obtained using byproducts of chlor-alkali process. The chemical name of baking soda is sodium hydrogen carbonate (NaHCO3 ) or sodium bicarbonate. Bread soda, cooking soda, bicarbonate of soda, sodium bicarb, bicarb of soda or simply bicarb, etc. are some other names of baking soda.

Baking soda is obtained by the reaction of brine with carbon dioxide and ammonia. This is known as Solvay process.

NaCl + CO2 + NH3 + H2O ⇨ NH4Cl + NaHCO3

In this process, calcium carbonate is used as the source of CO2and the resultant calcium oxide is used to recover ammonia from ammonium chloride.

Properties of sodium bicarbonate:
  • Sodium bicarbonate is white crystalline solid, but it appears as fine powder.
  • Sodium hydrogen carbonate is amphoteric in nature.
  • Sodium hydrogen carbonate is sparingly soluble in water.
  • Thermal decomposition of sodium hydrogen carbonate (baking soda).
  • When baking soda is heated, it decomposes into sodium carbonate, carbon dioxide and water.

2NaHCO3 + heat ⇨ Na2CO3 + CO2 + H2O

Sodium carbonate formed after thermal decomposition of sodium hydrogen carbonate; decomposes into sodium oxide and carbon dioxide on further heating.

Na2CO3 ⇨ Na2O + CO2

This reaction is known as dehydration reaction.

Use of Baking Soda:

  • Baking soda is used in making of baking powder, which is used in cooking as it produces carbon dioxide which makes the batter soft and spongy.
  • Baking soda is used as antacid.
  • Baking soda is used in toothpaste which makes the teeth white and plaque free.
  • Baking soda is used in cleansing of ornaments made of sliver.
  • Since sodium hydrogen carbonate gives carbon dioxide and sodium oxide on strong heating, thus it is used as fire extinguisher.

Baking powder

Baking powder produces carbon dioxide on heating, so it is used in cooking to make the batter spongy. Although baking soda also produces carbon dioxide on heating, but it is not used in cooking because on heating; baking soda produces sodium carbonate along with carbon dioxide. The sodium carbonate; thus produced; makes the taste bitter.

2NaHCO3 + heat ⇨ Na2CO3 + CO2 + H2O

Baking powder is the mixture of baking soda and a mild edible acid. Generally, tartaric acid is mixed with baking soda to make baking powder.

NaHCO3 + C4H6O6 ⇨ CO2 + H2O + Na2C4H4O6

When baking powder (mixture of baking soda and an edible acid) is heated, the sodium carbonate formed because of heating of baking soda neutralizes after reacting with tartaric acid and sodium tartarate salt is formed. The smell of sodium tartarate is pleasant and taste is good. This makes the cake or any other food tasty.

Washing Soda (Sodium carbonate)

Sodium carbonate is manufactured by the thermal decomposition of sodium hydrogen carbonate obtained by Solvay process.

NaCl + CO2 + NH3 + H2O ⇨ NH4Cl + NaHCO3

NaHCO3 + C4H6O6 ⇨ CO2 + H2O + Na2C4H4O6

The sodium carbonate obtained in this process is dry. It is called soda ash or anhydrous sodium carbonate. Washing soda is obtained by rehydration of anhydrous sodium carbonate.

Na2CO3 + 10H2O ⇨ Na2CO3.10H2O

Since there are 10 water molecules in washing soda, hence it is known as Sodium bicarbonate decahydrate.

Sodium carbonate is a crystalline solid and it is soluble in water when most of the carbonates are insoluble in water.

Use of sodium carbonate:

  • It is used in cleaning of cloths; especially in rural areas.
  • In making of detergent cake and powder.
  • In removing permanent hardness of water.
  • It is used in glass and paper industries.

Plaster of Paris

Plaster of Paris is obtained by heating of gypsum, a hydrated salt of calcium.

CaSO4.2H2O + Heat ⇨ CaSO4.(0.5)H2O + (1.5)H2O

After addition of water Plaster of Paris is again converted into gypsum.

CaSO4.(0.5)H2O + (1.5)H2O ⇨ CaSO4.2H2O

Plaster of Paris is used in making of toys, designer false ceiling, etc. Doctors use Plaster of Paris to set the fractured bone.