PROPERTIES OF ACIDS AND BASES
In this article we look at the properties of acids and of bases. The Latin word acidus derives from the verb acere, which means "to be sour, to be sharp, to be burning"; acidus means "sour, acidic, sharp in taste", and it passed into chemistry in this form.
The best definition of acids and bases was given by the Swedish scientist Svante August Arrhenius. He defined acids and bases in the Theory of Electrolytic Dissociation that he developed in 1887 and that carries his name. Because that theory fell short of explaining the acidity and basicity of substances whose structure holds neither (H+) nor (OH-), the Bronsted-Lowry and the Lewis acid-base definitions were formed in later years. According to Arrhenius, substances that give water the (H3O+) ion, that is hydronium, when they dissolve in it are acids, and those that give the hydroxyl ion (OH-) are bases. According to Arrhenius the H+ ion has no electron. Since it consists entirely of positive charge, in aqueous solutions it joins with the H2O molecule to form the (H3O+) ion, hydronium.
For a substance to be called an acid or a base, the first condition is that it dissolves ionically in water. Substances that dissolve molecularly in water cannot be acids or bases.
Properties of acids
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When dissolved in water they give the (H+) ion to the medium.
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Their pH is below 7.
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Their pOH is above 7.
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In their aqueous solutions the (H+) concentration is higher than the (OH-) concentration.
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Acids turn blue litmus red.
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They taste sour.
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They are caustic; they give a burning sensation on open wounds and on the hand.
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They are corrosive and irritating.
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Acids and bases give a neutralization reaction with each other, forming a salt and H2O.
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When they dissolve in water, the percentage of ionization sets their strength. An acid that ionizes 100% is strong.
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Their aqueous solutions are electrolytes (substances that conduct electricity) and carry electric current.
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How much electricity they conduct depends on their concentration and their strength. The strong ones conduct better.
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Acids can react with salts that contain carbonate. The result is a salt and water together with (CO₂) gas.
PROPERTIES OF BASES
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When dissolved in water they give the (OH-) ion to the medium.
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NaOH (aq) → Na⁺ (aq) + OH⁻ (aq)
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Their pH is above 7.
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Their pOH is below 7.
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In their aqueous solutions the (OH⁻) ion concentration is higher than the (H+) concentration.
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Bases turn red litmus paper blue.
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They taste bitter.
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They are slippery; they give a slippery feel on the hand.
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They are corrosive and irritating.
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Acids and bases give a neutralization reaction with each other, forming a salt and H₂O.
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When they dissolve in water, the percentage of ionization sets their strength. A base that ionizes 100% is strong.
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Their aqueous solutions are electrolytes (substances that conduct electricity) and carry electric current.
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How much electricity they conduct depends on their concentration and their strength. The strong ones conduct better.
Not every compound that contains H is an acid
C₆H₁₂O₆ (s) → C₆H₁₂O₆ (aq)
C₂H₅OH (l) → C₂H₅OH (aq)
Not every compound that contains OH is a base
Alcohols → R–OH … (example: C₂H₅OH, CH₃OH and the like)
Their structure holds OH, but they are not bases.
Even though the structure holds OH⁻, they are not bases because when they dissolve in water they cannot form the OH⁻ ion.
Indicators
Substances that let us tell whether a material is an acid or a base, and how strong or how weak that acid or base is, are called indicators.
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Indicator - Acid Color - Base Color |
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Litmus - Red - Blue |
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Thymol Blue - Red - Yellow |
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Bromophenol Blue - Yellow - Blue |
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Congo Red - Blue - Red |
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Methyl Orange - Red - Yellow |
Natural indicators
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Natural Indicators - Acid Color - Base Color |
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Kale - Pink - Yellow |
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Lavender - Colorless - Brown |
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Rosehip - Red - Dark Green |
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Cherry - Light Pink - Light Yellow |
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Red Onion - Light Red - Light Brown |
Acids and bases we use often in daily life
Acetic acid: The main component of vinegar (vinegar acid). Used in foods as a flavoring and a preservative.
Citric acid: Found naturally in citrus fruits such as lemon and orange. Used in foods to give sourness and as a preservative.
Sodium bicarbonate: Known as "carbonate" or "baking soda". Used as a raising agent, against heartburn, and in cleaning.
Sodium hydroxide: Known as "caustic soda". Used in making soap, detergent, and strong cleaning products such as drain openers.
Ammonia: A cleaning agent with a sharp smell. Used especially in glass cleaners and grease removers.
Hydrochloric acid: Known as "spirits of salt". It is a very strong acid and is used to clean demanding surfaces such as toilets and bathrooms. It is also what enables digestion in our stomach.
Author:
Industrial Design Engineer