Introduction
‘Sour’ and ‘Bitter’ taste of food is due to acids and bases. If someone has acidity you give them basic solution such as a baking soda, ENO etc. which are basic in nature and neutralize the acid. The acids and bases nullify each others effect.
Acids are sour in taste - They turn Blue Litmus Red
Bases are bitter in taste - They turn Red Litmus Blue.
Natural Indicators - Litmus, Turmeric, Cabbage Leaves, Petals of flowers such as Petunia , Hydrangea and Geranium
Litmus is a purple dye - It is extracted from lichens (Fungi+Bacteria) . It is purple when the solution is neither basic nor acidic.
Turmeric Behavior - Yellow Turns Reddish Brown In Base
Synthetic Indicators - Methyl Orange, Phenolphthalein
Methyl Orange - Red for low pH acid (strong acids) ——> Moved to Orange and Yellow as pH increases. (Less Acidic)
Phenolphthalein - Turns Colorless in Acidic Medium, Magenta/Pink in Basic Medium
Olfactory(Nose) Indicators: Change in smell occurs when in Basic or Acidic
medium.
Vanilla, Onion and Clove (laung) are olfactory indicators
Acids
Acids contain H+ ion as cation and anion such as Cl– in HCl, NO3– in HNO , SO2- in H2SO4 ,CH3COO– in CH3COOH. Since the cation present in acids is H+, this suggests that acids produce hydrogen ions, H+(aq), in solution, which are responsible for their acidic properties.
Bases
Bases contain OH- ion as anion and cation such as Na+ in NaOH, Ca2+ in Ca(OH)2 etc. Since the anion present in bases is OH-, this suggests that bases produce hydrogen ions, OH- (aq), in solution, which are responsible for their basic properties.
Reaction With Metals and Metal Salts
Acid/Base + Metal → Salt + Hydrogen gas
Metal displaces hydrogen atoms from the acids and bases as hydrogen gas and forms a compound called a salt.
2NaOH(aq) + Zn(s) → Na2ZnO2(s) + H2(g)
(Base) Sodium Zincate (Burns with Pop Sound)
H2SO4(aq) + Zn(s) → ZnSO4(s) + H2(g)
(Acid) Zinc Sulphate (Burns with Pop Sound)
Such reactions are not possible with all metals.
Metal Carbonate and Metal Hydrogencarbonate + Acids
Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water
Na2CO3 (s) + 2 HCl(aq) → 2NaCl(aq) + H2O(l) + CO2 (g)
(Sodium Carbonate) (Turns Lime Water Milky)
NaHCO3 (s) + HCl(aq) → NaCl(aq) + H2O(l) + CO2 (g)
(Sodium Hyderogencarbonate) (Turns Lime Water Milky)
The carbon dioxide produced turns lime water milky due to formation of a white precipitate of Calcium Carbonate.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
(Lime Water) (White Precipitate)
The white precipitate disappears on passing excess Carbon Dioxide due to formation of water soluble calcium hydrogencarbonate.
CaCO3(s)+ H2O(l) + CO2(g) → Ca(HCO3 )2(aq)
(Soluble in Water)
Reactions Between Acids and Bases
Base + Acid → Salt + Water
H+(aq) + OH– (aq) → H2O(l)
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
It is also known as neutralization reaction as bases and acids neutralize each other. The process of dissolving an acid or a base in water is a highly exothermic one.
Metal Oxides + Acids
Metal oxide + Acid → Salt + Water
CuO + 2HCl ————>. CuCl2 + H2O
(Blue Green)
The blue-green colour of the solution is due to the formation of copper(II) chloride in the reaction.
Since metallic oxides react with acids like bases they are also called as Basic Oxides
Non-Metallic Oxides + Base
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
Since non-metallic oxides react with bases like acids they are also called as Acidic Oxides
ACIDS AND BASES IN WATER SOLUTION
An acid or base releases their H+ and OH- ions in presence of water.
Acid + Water ———-> Hydronium Ion (H3O+) or H+(aq)
HCl+H2O→H3O+ +Cl–
Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H+(aq) or hydronium ion (H3O+).
H+ + H2O → H3O+
Base + Water ——- Hydroxide Ion
NaOH(s) + H2O ————> Na+ (aq) + OH– (aq)
KOH(s) + H2O ————> K+ (aq) + OH– (aq)
MgOH2(s) + H2O ————> Mg2+ (aq) + OH– (aq)
The bases which are soluble in water are called Alkalis.
Alkalis are soapy to touch, bitter and corrosive.
Power of Acids and Bases
Mixing an acid or base with water results in decrease in the concentration of ions (H3O+/OH–) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.
The strength of an acid or base can be measured on a universal indicator that shows different colors at different concentrations of hydrogen ions. A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for ‘potenz’ in German, meaning power.
The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH– ion concentration in the solution, that is, increase in the strength of alkali
Acids that give rise to more H+ ions are said to be strong acids, and acids that give less H+ ions are said to be weak acids.
pH in Everyday Life
Acid Rain: pH of rain water less than 5.6 . When it flows into rivers, it makes it acidic and difficult for aquatic life to survive.
pH range for Humans: Human Bodies function well within a pH of 7 to 7.8. Human stomach has Hydrochloric Acid (HCl) that helps in digestion. When stomach produces too much acid, it causes pain and irritation.
To neutralize this acidity, we used bases such as ‘Milk of Magnesia’ - Mg(OH)2 .These bases are called antacids.
Tooth Decay: The tooth enamel is made up of calcium hydroxyapatite which is the hardest substance in our body. It does not dissolve in water but gets corroded when the pH of mouth is less than 5.5. The low pH is due to degradation of sugar and food particles by bacteria.
This is why toothpastes are used which are basic in nature help neutralize the excess acid that can cause tooth cavity.
Self Defense by Animals and Plants: Insect bites such as a bee sting or stinging hair of nettle leaves inject acid into our body which causes pain and irritation. Use of milk baking soda gives relief on stung area.
Nettle Leaves inject Methanoic Acid
Some Naturally occurring Acids
Natural Source | Acid Name | Natural Source | Acid Name |
Vinegar | Acetic acid | Sour milk (curd) | Lactic acid |
Orange | Citric acid | Lemon | Citric acid |
Tamarind | Tartaric acid | Ant sting | Methanoic acid |
Tomato | Oxalic acid | Nettle sting | Methanoic acid |
Family of Salts
- NaCl and Na2SO4 are salts of Sodium (Na) family.
- Similarly, NaCl and KCl are salts of Chloride salts and belong to the chloride family.
Salts with pH > 7 are basic and those with pH < 7 are acidic.
Some Important Salts
NaCl
Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt. Beds of rock salt were formed when seas of bygone ages dried up. Rock salt is mined like coal.
It is an important raw material for various materials of daily use, such as sodium hydroxide, baking soda, washing soda, bleaching powder and many more.
Sodium hydroxide - NaOH
2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide.
The process is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide.
Bleaching powder - CaOCl2
Chlorine gas is used for the manufacture of bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)2].
Uses of Bleaching Powder
1. For bleaching cotton and linen in textile industry. For bleaching washed clothes. For bleaching wood pulp in paper industry.
2. As an oxidizing agent
3. As a disinfectant to make drinking water germ free.
Baking Soda - NaHCO3
NaCl+H2O+CO2 +NH3 →NH4Cl + NaHCO3
Added to food for faster cooking and making crispy edible food products. It is a noncorrosive basic salt and is thus used to control acidity by neutralizing it. When it is heated it becomes Sodium Carbonate.
2NaHCO3 ⎯⎯HEAT⎯⎯→Na2CO3 +H2O+CO2
Used in making baking powder when mixed with mild edible acid such as tartaric acid.
NaHCO3 + H+ → CO2 + H2O + Sodium salt of acid
- The carbon dioxide released helps in making the cake or bread spongy and soft.
- Works as an antacid - Reduces acidity
- Used in fire-extinguishers - provides soda for production of CO2
Washing Soda - Na2CO3.10H2O
- Used in glass, soap and paper industry.
- Used in manufacture of Borax - Borosil glass
- Used as a cleaning agent
- Used for removing permanent hardness of water.
Plaster of Paris - CaSO4.1/2H2O
Produced by heating Gypsum (CaSO4.2H2O) at 373K
CaSO4.2H2O → CaSO4.1/2H2O + 11/2H2O
(Gypsum)
Used as plaster for supporting fractured bones in the right position. making toys, materials for decoration and for making surfaces smooth.
Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.
CaSO4.1/2H2O + 1 1/2H2O → C a S O 4 . 2 H 2 O