Acids are substances with a sour taste. Historically, taste was used to distinguish acids from bases.
The word "acid" comes from the Latin word "acidus," meaning "sour."
Acids were produced by early chemists through the distillation of substances like alum, vitriol, sal ammoniac, and common salt.
Classification of acids based on origin
Organic Acids: are present in animal and plant material (e.g., vinegar, lemon juice).
Mineral Acids: are obtained from minerals in rocks. They are generally corrosive and dangerous.
Properties of Acids
Sour taste.
Corrosive nature.
Acidic solutions conduct electricity.
Indicators
Indicators are substances that change color in the presence of acids or alkalis and are often plant extracts.
Litmus: A common indicator that turns red in acidic solutions and blue in basic solutions. The color change is a chemical reaction where the molecules of the indicator are altered by the acid or base.
Universal Indicator: A mixture of dyes that produces a range of colors to indicate the strength of an acid or alkali.
Turmeric: A natural indicator that remains yellow in acidic solutions and turns pink in basic solutions.
Phenolphthalein: Is colorless in acid and neutral solutions, and turns pink in alkali solutions.
Methyl orange: Is red in acid, orange in neutral solutions and yellow in alkali solutions.
The pH Scale
The pH scale, ranging from 1 to 14, measures the strength of an acid or base.
A pH meter is used to accurately measure pH using an electrode.
Acids in Daily Life
Acids are used in many products, such as dyes, construction materials, and cleaning agents.
Sulphuric acid is called the 'king of chemicals' due to its widespread use. Over 200 billion kg are produced annually.
Common Acids and Their Uses
Ethanoic Acid (CH_3COOH): Found in vinegar (also known as acetic acid).
Methanoic Acid (HCOOH): Found in ant and nettle stings; used in kettle descaler.
Lactic Acid (CH_3CCH(OH)COOH): Found in sour milk and yoghurt.
Citric Acid (C6H8O_7): Found in lemons, oranges, and other citrus fruits; a source of vitamin C.
Carbonic Acid (H2CO3): Found in fizzy soft drinks.
Hydrochloric Acid (HCl): Used in cleaning metal surfaces and found as dilute acid in the stomach.
Nitric Acid (HNO_3): Used in making fertilizers and explosives.
Sulphuric Acid (H2SO4): Used in car batteries and in making fertilizers, paints, and detergents.
Phosphoric Acid (H3PO4): Used in anti-rust paint and in making fertilizers.
Acids and Digestion
Acids like vitamin C (ascorbic acid) aid in the absorption of iron from food.
Lemon juice is often added to spinach salads to enhance iron absorption.
Vitamin C is essential for healthy blood vessels, gums, bones, and muscles, and it helps in wound healing.
Stomach Acid
The stomach lining is protected from stomach acid by a layer of mucus.
Ulcers
Ulcers are primarily caused by Helicobacter pylori (H. pylori) bacteria, which damages the mucus layer, exposing the stomach lining to acid.
H. pylori creates an alkaline environment to protect itself from gastric acid and is treated with antibiotics.
Approximately 50% of world’s population is estimated to be infected with H.pylori.
Lactic Acid
Lactic acid bacteria convert sugar into acid through fermentation.
Lactobacillus is a genus of lactic acid-producing bacteria that converts lactose in milk into lactic acid, causing milk proteins (casein) to solidify into curds.
Fermentation converts sugar to acids or alcohol.
Probiotics
Probiotics are live bacteria and yeasts beneficial for health, especially the digestive system.
More than 260 species of microbial food culture are identified and described for their beneficial use in fermented food products globally.
Probiotics help in digestion, nutrient absorption, and maintaining normal bowel functions.
Sporolac
Lactic Acid Bacillus in SPORLAC tablets are in the form of spores and survive gastric acidity.
Sporolac tablets germinate in the small intestine and make the environment acidic, which inhibits the growth of other harmful bacteria.
These bacteria survive in acidic environments only; elsewhere, they remain in spore form.
Curd Formation
Curd can be made by adding acidic fruits, like tamarind and ginger, to milk.
Ginger contains the protease zingipain, which catalyses the denaturation of protein in milk, changing milk to curd.
Lactic Acid and Exercise
Lactic acid forms in muscles during vigorous exercise, causing muscle pain. It serves as a source of energy for the muscles.
Otto Meyerhof's experiments showed that anaerobic conditions lead to lactic acid formation during work and its removal during rest.
pH and Food Spoilage
A high pH favors the growth of yeasts and fungus. In neutral or alkaline pH foods, such as meats, bacteria are more dominant and food spoils faster.
Curdled Milk
Milk curdles due to a chemical reaction where lactic acid bacteria digest lactose, producing lactic acid.
As lactic acid increases, the pH drops, causing protein (casein) molecules to clump.
Spoiled vs. Fermented Milk
Spoiled milk has a yellowish color and foul smell due to cold-loving bacteria, while fermented milk (curd) has a different flavor and smell, as well as health benefits.
Spoiled milk can harm health due to the presence of harmful microbes.
Sour Food
Sour food can be fermented food like curd, pickles, buttermilk, and yoghurt, which are good for the body and have specific health benefits.
Spoiled food is rotten food and is full of dangerous bacteria and pathogens.
Vinegar
Vinegar consists mainly of acetic acid and has been used since 3rd century BC.
It is commonly used as a flavoring agent and has several medicinal benefits.
Benefits of Vinegar
Vinegar lowers blood sugar level.
Vinegar has antibacterial properties.
Vinegar has antioxidant properties.
Vinegar and Blood Sugar
Vinegar prolongs satiety by maintaining a constant blood sugar level in the body.
It helps in absorption of other minerals in the diet. The acetic acid in vinegar increases the body’s absorption of important minerals from the foods we eat.
Vinegar increases calcium absorption from green vegetables.
Glycaemic Index (GI)
The glycaemic index, or GI, measures how a carbohydrate-containing food raises blood glucose.
Glycaemic index uses a scale of 0 to 100, with higher values given to foods that cause the most rapid rise in blood sugar.
Carbohydrates like table sugar and bread are easier for the body to change into glucose.
Low GI foods take longer to digest and help you feel satisfied for longer.
Medicinal uses for vinegar mentioned in ʽThe Canon of Medicineʼ
It is a powerful clotting agent.
Heals burns and skin inflammations
Relieves headaches caused by heat.
Considered vinegar a good digestive supplement.
Vinegar as Antibacterial
Vinegar strengthens the mouth and kills germs in the stomach and Acetic acid is a potent antimicrobial agent and can kill many types of bacteria.
Hippocrates used vinegar for cleaning wounds over two thousand years ago.
Vinegar is used in the food industry to lower the pH of products, preventing the growth of harmful bacteria.
Vinegar and Tuberculosis
Acetic acid, the active ingredient in vinegar, can effectively kill mycobacteria.
Exposure to a 6\% solution of acetic acid for 30 minutes effectively kills tuberculosis, even strains resistant to almost all antibiotics.
Free Radicals and Antioxidants
Free radicals are atoms with unpaired electrons that are highly reactive and can cause chain reactions.
Antioxidants neutralize free radicals by donating electrons, preventing cell and tissue damage.
Vitamin C and vitamin E are examples of antioxidants found naturally in many fruits and vegetables.
Vinegar and Gallbladder
Vinegar contains chlorogenic acid, a natural compound that functions as an antioxidant. It helps the gallbladder by facilitating the flow of bile and dissolving gallstones.
Vinegar and Heart
Vinegar refreshes the heart. Cardiovascular diseases (heart disease, high blood pressure and stroke) are currently the world’s biggest cause of death.
Vinegar is a good dietary source that contains antioxidants. It prevents the oxidation of low-density lipoprotein (LDL), thus preventing the buildup of plaque and heart attacks.
Moreover, the acetic acid in vinegar also reduces blood pressure.
Equations
acid + base \rightarrow salt + water
Summary Points
An acid is a compound that tastes sour and causes litmus paper to turn red.
Natural indicators are substances that change color in acidic and alkaline solution.
Lactic acid is the reason for the curdling of milk
Strains of lactic acid bacteria are known as friendly bacteria and are used widely in probiotics
Vinegar is an excellent idaam because of the various health benefits it possesses:
Glucose lowering property - a remedy for diabetic and insulin resistant patients
Anti-microbial property - because of acetic acid
Antioxidant property - because of chlorogenic acid.