Competitive Notes on Acids and Bases – In this post, we will learn all the important points of Acids, Bases, Alkalis, Salts and Indicators for different competitive exams.
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Notes on Acids and Bases
Acid
Acids are defined as compounds which contain one or more hydrogen atoms and when dissolved in water, produce hydronium ions (H3O+) the only positively charged ions.
At first, the H+ ion produces in an aqueous solution but the H+ ion cannot exist independently. Therefore, it combines with a water molecule to form a hydronium ion (H3O+).
- Strongest Acid – Sulphuric acid
- Weakest Acid – Peroxyacetic acid
Types of Acid
- Organic Acid – Acids which are obtained usually from plants are called organic acids. They are weak acids and they do not ionize completely in solution. Examples – Oxalic Acid, Acetic Acid.
- Inorganic Acid – Acids which are obtained usually from minerals are known as inorganic acids. They are strong acids. Examples are hydrochloric acid, Sulphuric acid, Nitric acid etc.
Acids present in different sources
| Natural Source | Acid present |
|---|---|
| Vinegar | Acetic acid |
| Orange | Citric acid |
| Tamarind | Tartaric acid |
| Tomato/ spinach | Oxalic acid |
| Sour milk (Curd) | Lactic acid |
| Lemon | Citric acid |
| Ant sting/Nettle sting | Methanoic acid (Formic acid) |
| Amla, citrus fruits | Ascorbic acid |
| Venus atmosphere | Sulphuric acid |
| Rancid butter | Butyric acid |
| Gastric juice | Hydrochloric Acid |
| Apple | Maleic Acid, Tartaric acid |
| Olive oil | Oleic acid |
| Fats | Stearic acid |
| Grapes, Tamarind | Tartaric acid |
| Urine | Uric acid |
| Soft drinks | Carbonic acid |
Carbonic acid is a weak mineral acid, used in soft drinks because of its non-corrosive character.
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What is the meaning of Concentrated and Dilute acid?
- An acid that contains a very small amount of water or no water, is called a concentrated acid.
- An acid that contains far more amount of water than its own mass is known as dilute acid.
- In order to dilute acid, we should pour acid into the water slowly and continuously stir.
Properties of Acid
- Sour taste
- Some acids are liquid and some are solid at room temperature.
- They change the colour of the Indicator.
- They contain one or more non-metals in their chemical composition.
- They corrode most metals.
- They liberate carbon dioxide from carbonates and hydrogen carbonates (i.e., bicarbonates)
- They lose their acidic property when they react with what are known as bases (i.e., metal oxides and hydroxides and also ammonium hydroxides).
- Can donate a proton (as per Bronsted and lowry’s concept)
- Can accept electron (Lewis theory)
Solid Acids – Boric acid, Oxalic acid, Tartaric acid, Citric acid, Phosphoric acid
Indicators
Indicators are complex substances that acquire separate colours in acidic and basic mediums.
Those substances whose smell (or odour) changes in acidic or basic solutions are called OLFACTORY INDICATORS. For example – onion, vanilla and clove oil.
The Most commonly used natural indicator is litmus. It is extracted from lichens. A lichen is a composite organism that emerges from algae or cyanobacteria (or both) living among filaments of a fungus in a mutually beneficial (symbiotic) relationship.
| Indicator | Colour change in acidic medium | Colour change in Basic medium |
|---|---|---|
| Litmus | Blue to Red | Red to Blue |
| Methyl Orange | Orange to pink | |
| Phenolphthalein | Remains colourless | Pink colour |
| Turmeric juice | Remains yellow or neutral | Reddish to deep brown |
| Red cabbage juice | Purple to Red | Purple to green |
Basicity of an Acid
The number of removable hydrogen ions from an acid is called the basicity of that acid.
- Monobasic acid – HCl, HNO3
- Dibasic acid – H2SO4, H2CO3, H3PO3
- Tribasic acid – H3PO4
General Uses of some Acids
- Boric acid – eye wash/ antiseptic
- Citric acid – food preservation/ vitamin C preparation
- Oxalic acid – Ink stain remover
- Carbonic acid – Flavoured drinks
- Tartaric acid – Baking Powder
- Acetic acid – Table vinegar and cooking
- Hydrochloric acid – cleaning of metal items
- Benzoic Acid – Preservation of food and making of perfumes and medicines
- Nitric acid – Use in Explosives
- Phosphoric acid – used in fertilizers
Use of HCL
- Present in gastric Juice and responsible for digestion.
- Used as a bathroom cleaner.
- As a pickling (process of cleaning the metal surface) agent before galvanization.
- In the tanning of leather.
- In the dying and textile industry
- In the manufacture of gelatin from bones
Use of HNO3
- In the manufacture of fertilizers like ammonium nitrate
- In the manufacture of explosives like TNT (Trinitrotoluene), TNB (Trinitro Benzene), Picric acid etc
- It forms nitrates in the soil.
- Manufacture of rayon (type of textile fibre)
- Manufacture of dyes and drugs
Use of H2SO4
- In a lead storage battery
- In the manufacture of HCL
- In the manufacture of Alum
- In the manufacture of fertilizers, drugs, detergents and explosives.
- In car battery
Acid Rain
Acid rain is a by-product of a variety of human activities that releases oxides of sulphur and nitrogen into the atmosphere. Burning of fossil fuels, coal, oil, petrol and diesel produces sulphur dioxide and nitrogen oxide. They all pollute the air. Polluted air also contains many oxidising agents which produce oxygen due to excessive heat. This oxygen further combines with the oxides of sulphur and nitrogen. When it rains, oxides of sulphur and nitrogen react with rainwater and form acids.
Sulphuric acid and nitric acid formed are washed down to the earth with water. This is Acid Rain. The highly acidic rainwater has harmful effects.
- Acid rain damages the leaves of plants and trees.
- It removes basic nutrients such as calcium from the soil.
- It causes respiratory ailments in human beings and animals.
- It affects plant and animal life in aquatic ecosystems.
- It corrodes water pipes resulting in the leaching of heavy metals, such as iron, lead and copper, into the drinking water.
- Acid Rain damages buildings as it reacts with stone or metals.
- Hydrochloric acid is used in digestion
- Nitric acid is used in the purification of gold and silver.
- Conc. H2SO4 and HNO3 are used to wash iron for its galvanization.
- Oxalic acid is used to remove rust spots.
- Boric acid is a constituent of eyewash.
- Formic acid is present in red ants.
- Uric acid is present in the urine of mammals.
Base
A base is either a metallic oxide or a metallic hydroxide or an ammonium hydroxide which reacts with hydronium ions of an acid to form salt and water only. Examples – Metallic oxide, metallic hydroxide
An alkali is a base soluble in water.
Base Properties
- They are a sharp and bitter taste.
- They are soapy substances and slippery to the touch.
- They are strong electrolytes.
- It gives hydroxyl ions (OH-) in an aqueous solution.
- Can accept a proton (as per Bronsted and lowry’s concept)
- Can donate electrons (Lewis theory)
The acidity of a base
The number of hydroxyl ions [OH]- which can be produced per molecule of the base in an aqueous solution is called the acidity of the base.
- Monoacidic base – NaOH, KOH and NH4OH
- Diacidic base – Ca(OH)2 and Cu(OH)2
- Triacidic base – Al(OH)3 and Fe(OH)3
Use of Bases
- Sodium hydroxide uses for the manufacture of soap.
- Potassium hydroxide uses for the manufacture of salts and soaps and uses in batteries.
- Calcium hydroxide (slaked lime) uses for the manufacture of bleaching powder and softening of hard water.
- Magnesium hydroxide uses as an antacid.
- Aluminium hydroxide uses as a foaming agent in fire extinguishers.
- Ammonium hydroxide uses to remove grease stains from clothes.
- Calcium oxide uses as a drying agent.
pH Scale
Important pH values
- Milk – 6.5
- Blood – 7.4
- Pure water – 7
- Toothpaste – 9.0
- Saliva – 6.5
- Urine – 4.8
- Our body works within the pH range of 7.0 to 7.8
- Acid rain water pH value is always less than 5.6
- The tooth starts to decay when the pH of the mouth is lower than 5.5. Tooth enamel is made up of calcium phosphate which is the hardest substance in the body. It does not dissolve in water but starts to decay when the pH goes down to 5.5.
Neutralisation Reaction
When an acid reacts with a base/alkali it forms salt and water and after the reaction, the solution loses the both property of acid and base.
Common salt
- Chemically known as sodium chloride.
- Produced by the reaction of hydrochloric acid and sodium hydroxide base.
- Also known as Rock salt.
Bleaching Powder
- Produced by the action of chlorine gas on dry slaked lime.
- Chemically known as Calcium hypochlorite.
- Used for – 1. bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in the laundry. 2. as an oxidising agent in many chemical industries 3. for disinfecting drinking water to make it free of germs.
Baking Soda
- Chemically known as sodium hydrogen carbonate.
- Use for faster cooking.
Baking powder
- It is a mixture of baking soda and a mild edible acid tartaric acid.
- It is used in soda-acid fire extinguishers.
Washing Soda
- The chemical name is sodium carbonate.
- Use in glass, soap and paper industries
- Use in the manufacture of sodium compounds such as borax.
- Use as a cleaning agent for domestic purposes.
- It is used for removing the permanent hardness of the water.
Important Questions on Acids and Bases
Which acid is considered a basic chemical in Industry?
(A) H2CO3
(B) HNO3
(C) H2SO4
(D) HCl
Which of the following is present in the maximum amount of acid rain?
(A) HCl
(B) HNO3
(C) H2SO4
(D) H2CO3