Acids - Notes, Chemistry | General Test Preparation for CUET

Acids and Bases

Acid 

• a substance which when added to water produces hydrogen ions [H⁺].

Base 

• a substance which when added to water produces hydroxide ions [OH^-].

Properties

Acids 

• react with zinc, magnesium, or aluminium and form hydrogen (H_2(g))
• react with compounds containing CO₃^2- and form carbon dioxide and water
• turn litmus red
• Taste sour (lemons contain citric acid) 

Bases 

• feel soapy or slippery
• turn litmus blue
• they react with most cations to precipitate hydroxides
• Taste bitter (ever get soap in your mouth) 

Strength of Acids and Bases

• Acids 

1. Strong Acids: 

• completely dissociate in water, forming H⁺ and an anion.

2. Weak acids: 

• a weak acid only partially dissociates in water to give H⁺ and the anion 

• Bases 

1. Strong Bases: 

• They dissociate 100% into the cation and OH^- (hydroxide ion).

2. Weak Bases: 

• Most weak bases are anions of weak acids.
• Weak bases do not furnish OH^- ions by dissociation.
• When a weak base reacts with water the OH^- comes from the water and the remaining H⁺ attaches itself to the weak base, giving a weak acid as one of the products. 

How do Acids and Bases React with Metals

Acid + Metal  → Salt + Hydrogen gas 

How do Acids and Bases React with each other

Base + Acid → Salt + Water

NaoH+ HCl → NaCl + H₂o

HOW STRONG ARE ACID OR BASE SOLUTIONS

• 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. 
• On the pH scale we can measure pH from 0 (very acidic) to 14 (very alkaline). 
• pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. 
• Higher the hydronium ion concentration, lower is the pH value.
• 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.

pH in our digestive system

• stomach produces hydrochloric acid. 
• It helps in the digestion of food without harming the stomach.
• During indigestion the stomach produces too much acid and this causes pain and irritation. 
• To get rid of this pain, people use bases called antacids. Magnesium hydroxide (Milk of magnesia), a mild base, is often used for this purpose. 

pH change as the cause of tooth decay

• Tooth decay starts when the pH of the mouth is lower than 5.5. 
• Tooth enamel, made up of calcium phosphate is the hardest substance in the body. 
• It does not dissolve in water, but is corroded when the pH in the mouth is below 5.5. 
• Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating. 
• The best way to prevent this is to clean the mouth after eating food. 
• Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent tooth decay. 

Self defence by animals and plants through chemical warfare

• Bee-sting leaves an acid which causes pain and irritation. 
• Use of a mild base like baking soda on the stung area gives relief. 
• Stinging hair of nettle leaves inject methanoic acid causing burning pain. 

MORE ABOUT SALTS

Common salt — A raw material for chemicals

• The common salt thus obtained 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

• 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. 

• Chlorine gas is given off at the anode, and hydrogen gas at the cathode.
• Sodium hydroxide solution is formed near the cathode. The three products produced in this process are all useful. 

Bleaching powder

• chlorine is produced during the electrolysis of aqueous sodium chloride (brine). 
• This 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]. 
• Bleaching powder is used –
• (i) for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry;
• (ii) as an oxidising agent in many chemical industries; and
• (iii) for disinfecting drinking water to make it free of germs 

Baking soda

• The soda commonly used in the kitchen for making tasty crispy pakoras is baking soda.
• The chemical name of the compound is sodium hydrogencarbonate (NaHCO3). 
• It is produced using sodium chloride as one of the raw materials. 

Uses of Baking Soda (NaHCO3)

(i) For making baking powder, which is a mixture of baking soda (sodium hydrogencarbonate) and a mild edible acid such as tartaric acid. Carbon dioxide produced during the reaction causes bread or cake to rise making them soft and spongy.

(ii) Sodium hydrogencarbonate is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.

(iii) It is also used in soda-acid fire extinguishers. 

Washing soda

• Another chemical that can be obtained from sodium chloride is washing soda. 
• Sodium carbonate can be obtained by heating baking soda; recrystallisation of sodium carbonate gives washing soda. 
• It is also a basic salt. 

Uses of washing soda

(i) Sodium carbonate (washing soda) is used in glass, soap and paper industries.

(ii) It is used in the manufacture of sodium compounds such as borax.

(iii) Sodium carbonate can be used as a cleaning agent for domestic purposes.

(iv) It is used for removing permanent hardness of water 

Plaster of Paris                   

• On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate .

• This is called Plaster of Paris, the substance which doctors use as plaster for supporting fractured bones in the right position. 
• Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.
• Plaster of Paris is used for making toys, materials for decoration and for making surfaces smooth.