pH Scale and importance of pH in daily life - West Bengal PCS Exam Notes

Acid

An acid is a substance which forms H+ ions as the only positive ion in aqueous solution.

Examples:-

Hydrochloric acid dissolved in water forms H⁺ and Cl^– ions
	HCl —> H⁺ + Cl^–

Sulphuric acid dissolved in water forms H⁺ and SO₄^2- ions
	H₂SO₄ —>2H⁺ + SO₄^2-

Nitric acid forms H⁺ and NO₃^– ions when dissolved in water
	HNO₃ —> H⁺ + NO₃^–

Ethanoic acid,also know as acetic acid, forms H⁺ and CH₃COO^– ions in water
	CH₃COOH —> H⁺ + CH₃COO^–

Acetone, also known as propanone, doesn’t form any ions in water, so it isn’t an acid.
CH₃COCH₃ just dissolves in water.
Methane, CH₄, doesn’t form any ions in water, so this isn’t an acid either.
So just containing hydrogen doesn’t make something an Arrhenius acid.

Properties of Acids

tastes sour
acids change blue litmus to red
their aqueous (water) solutions conduct electricity (i.e. they are electrolytes)
react with bases to form salts and water as the only products
evolve hydrogen gas (H₂) upon reaction with an active metal, such as alkali metals, alkaline earth metals, zinc, iron, aluminum, forming a salt as the only other product
Evolve carbon dioxide on reacting with metal carbonates.

Base

An alkali is a substance which forms OH- ions as the only negative ion in aqueous solution. A base is an insoluble hydroxide.

Examples:-

Sodium hydroxide, when dissolved in water, forms Na⁺ and OH^– ions
	NaOH —> Na⁺ + OH^–

Calcium hydroxide dissolves in water to give Ca⁺ and OH^– ions
	Ca(OH)₂ —> Ca²⁺ + 2 OH^–

Ethanol CH₃CH₂OH does not form OH^– ions when dissolved in water, so it isn’t a base.
Ethanol just dissolves. Strictly speaking, ethanol actually forms a tiny amount of H⁺ ions in water and is a very weak acid.

Properties of Bases

taste bitter
feel slippery or soapy (But don’t touch them! They react with your skin to form soap.)
bases turn red (acidified) litmus back to blue
their aqueous (water) solutions conduct electricity (i.e. they are electrolytes)
react with acids to form salts and water as the only products

pH Scale

A measure of the degree of the acidity or the alkalinity of a solution as measured on a scale (pH scale) of 0 to 14. The midpoint of 7.0 on the pH scale represents neutrality, i.e., a “neutral” solution is neither acid nor alkaline. Numbers below 7.0 indicate acidity; numbers greater than 7.0 indicate alkalinity. It is important to understand that pH is a measure of intensity, and not capacity; i.e., pH indicates the intensity of alkalinity in the same way temperature tells how hot something is, but not how much heat the substance carries.

The pH scale is logarithmic which means that moving on (unit either way on the pH scale results in a 10 fold increase in the degree of alkalinity or acidity.

Classification, pH, Product

Acidic

0-1 Hydrochloric, Sulfuric, Nitric Acids
1-2 Phosphoric, Sulfamic Acids
2.0 Citrus Fruit
6.0 Milk

Neutral

7.0-7.5 Water, Sugar, Table Salt

Alkaline

8.0 Eggs
11.0 Ammonia
13-14 Caustic Soda, Degreasers

Products at the two extremes (less than pH 1 or greater than pH 13) are extremely oppressive and corrosive. Examples include sulfuric and hydrochloric acid on the acid end, and caustic soda on the alkaline end. Use solutions of phosphoric or sulfamic acid cleaners, typically in the pH range of slightly less than 2, may be described as “safe” acids comparison to the stronger acids. Of course, necessary safety precautions (eye and hand protection) as noted on the Material Safety Data Sheet should always be followed.

As an illustration of their non-aggressive behavior in comparison to stronger acids, products containing sulfamic or phosphoric acid were found to result in no chemical attacks on nylon carpets. On the other hand, a hydrochloric or powder acid-based product would basically dissolve the fibers.

Other than the two pH extremes, the pH scale becomes secondary to the inherent properties of the specific chemical in terms of corrosiveness. As an example, the pH of carbonated cola soda (which contains phosphoric acid) is in the 2.5 range. A concentrated (35%) hydrogen peroxide solution has a pH of approximately 3.5. Carbonated cola soda may be slightly irritating to the eyes, causing no permanent damage. However, a 35% hydrogen peroxide solution will cause chemical burns to the skin or mucous membranes. Thus, it is not the pH factor alone that causes corrosion of products to surfaces.

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