Category: 1. Acids Properties and Examples

An acid is a chemical species that donates protons or hydrogen ions and/or accepts electrons. Most acids contain a hydrogen atom bonded that can release (dissociate) to yield a cation and an anion in water. The higher the concentration of hydrogen ions produced by an acid, the higher its acidity and the lower the pH of the solution.

The word acid comes from the Latin words acidus or acere, which mean “sour,” since one of the characteristics of acids in water is a sour taste (e.g., vinegar or lemon juice).

This table offers an overview of the key properties of acids compared with bases.

Summary of Acid and Base Properties
Property	Acid	Base
pH	less than 7	greater than 7
litmus paper	blue to red	doesn’t change litmus, but can return acid (red) paper back to blue
taste	sour (e.g., vinegar)	bitter or soapy (e.g., baking soda)
odor	burning sensation	often no odor (exception is ammonia)
texture	sticky	slippery
reactivity	reacts with metals to produce hydrogen gas	reacts with several fats and oils

Arrhenius, Brønsted-Lowry, and Lewis Acids

There are different ways of defining acids. A person referring to “an acid” is usually referring to an Arrhenius or Brønsted-Lowry acid. A Lewis acid is typically called a “Lewis acid.” The reason for the differing definitions is that these different acids don’t include the same set of molecules:

• Arrhenius Acid: By this definition, an acid is a substance that increases the concentration of hydronium ions (H₃O⁺) when added to water. You might also consider increasing the concentration of hydrogen ion (H⁺) as an alternative.
• Brønsted-Lowry Acid: By this definition, an acid is a material capable of acting as a proton donor. This is a less restrictive definition because solvents besides water are not excluded. Essentially, any compound that can be deprotonated is a Brønsted-Lowry acid, including typical acids, plus amines, and alcohol. This is the most widely used definition of an acid.
• Lewis Acid: A Lewis acid is a compound that can accept an electron pair to form a covalent bond. By this definition, some compounds that don’t contain hydrogen qualify as acids, including aluminum trichloride and boron trifluoride.

Acid Examples

These are examples of types of acids and specific acids:

• Arrhenius acid
• Monoprotic acid
• Lewis acid
• Hydrochloric acid
• Sulfuric acid
• Hydrofluoric acid
• Acetic acid
• Stomach acid (which contains hydrochloric acid)
• Vinegar (which contains acetic acid)
• Citric acid (found in citrus fruits)

Strong and Weak Acids

Acids may be identified as either strong or weak based on how completely they dissociate into their ions in water. A strong acid, such as hydrochloric acid, completely dissociates into its ions in water. A weak acid only partly dissociates into its ions, so the solution contains water, ions, and the acid (e.g., acetic acid).

Acetylsalicylic acid (also known as aspirin) is a medication used to treat pain, fever, and inflammation. Aspirin, in the form of leaves from the willow tree, has been used for its health effects for at least 2,400 years.

Aspirin is a white, crystalline, weakly acidic substance.

Citric acid (C₆H₈O7) is a weak organic tricarboxylic acid that occurs naturally in citrus fruits. The citrate ion is an intermediate in the TCA cycle (Krebs cycle), a central metabolic pathway for animals, plants and bacteria. Because it is one of the stronger edible acids, the dominant use of citric acid is used as a flavoring and preservative in food and beverages, especially soft drinks.

Formic acid (HCO2H) is the simplest carboxylic acid and is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. The word “formic” comes from the Latin word for ant, formica, referring to its early isolation by the distillation of ant bodies. Formic acid occurs widely in nature as its conjugate base formate.

Carbonic acid is a chemical compound with the chemical formula H2CO3 and is also a name sometimes given to solutions of carbon dioxide in water (carbonated water), because such solutions contain small amounts of H2CO3(aq). Carbonic acid, which is a weak acid, forms two kinds of salts: the carbonates and the bicarbonates.

In geology, carbonic acid causes limestone to dissolve, producing calcium bicarbonate—which leads to many limestone features such as stalactites and stalagmites. Carbonic acid is a polyprotic acid, specifically it is diprotic, meaning that it has two protons which may dissociate from the parent molecule.

When carbon dioxide dissolves in water, it exists in chemical equilibrium, producing carbonic acid:

CO2+H2O↽−−⇀H2CO3

The reaction can be pushed to favor the reactants to generate CO2(g)CO2(g) from solution, which is key to the bubbles observed in carbonated beverages

Nitric acid (HNO3) is a highly corrosive mineral acid and is also commonly used as a strong oxidizing agent. Nitric acid is normally considered to be a strong acid at ambient temperatures. Nitric acid can be made by reacting nitrogen dioxide (NO2(g) with water.

3NO2(g)+H2O(l)→2HNO3(ag)+NO(g)

Nitric acid reacts with most metals, but the details depend on the concentration of the acid and the nature of the metal. Dilute nitric acid behaves as a typical acid in its reaction with most metals (e.g., nitric acid with magnesium, manganese or zinc will liberate H2H2 gas):

Mg+2HNO3→Mg(NO3)2+H2

Mn+2HNO3→Mn(NO3)2+H2

Zn+2HNO3→Zn(NO3)2+H2

Nitric acid is a corrosive acid and a powerful oxidizing agent. The major hazard it poses is chemical burn, as it carries out acid hydrolysis with proteins (amide) and fats (ester) which consequently decomposes living tissue (Figure 14.2.214.2.2). Concentrated nitric acid stains human skin yellow due to its reaction with the keratin

Sulfuric acid is a highly corrosive strong mineral acid with the molecular formula H2SO4. Sulfuric acid is a diprotic acid and has a wide range of applications including use in domestic acidic drain cleaners,^[as an electrolyte in lead-acid batteries, and in various cleaning agents. It is also a central substance in the chemical industry.

Because the hydration of sulfuric acid is thermodynamically favorable (and is highly exothermic) and the affinity of it for water is sufficiently strong, sulfuric acid is an excellent dehydrating agent.

Concentrated sulfuric acid has a very powerful dehydrating property, removing water (H2O) from other compounds including sugar and other carbohydrates and producing carbon, heat, steam. Sulfuric acid behaves as a typical acid in its reaction with most metals by generating hydrogen gas.

M+H2SO4→M(SO4)+H2

Hydrochloric acid is a corrosive, strong mineral acid with many industrial uses. A colorless, highly pungent solution of hydrogen chloride (HCl) in water. Hydrochloric acid is usually prepared by treating HCl with water.

Hydrochloric acid can therefore be used to prepare chloride salts. Hydrochloric acid is a strong acid, since it is completely dissociated in water. Hydrochloric acid is the preferred acid in titration for determining the amount of bases.

Acids are very common in some of the foods that we eat. Citrus fruits such as oranges and lemons contain citric acid and ascorbic acid, which is better known as vitamin C. Carbonated sodas contain phosphoric acid. Vinegar contains acetic acid. Your own stomach utilizes hydrochloric acid to digest food. Acids are a distinct class of compounds because of the properties of their aqueous solutions as outlined below:

1. Aqueous solutions of acids are electrolytes, meaning that they conduct electrical current. Some acids are strong electrolytes because they ionize completely in water, yielding a great many ions. Other acids are weak electrolytes that exist primarily in a non-ionized form when dissolved in water.
2. Acids have a sour taste. Lemons, vinegar, and sour candies all contain acids.
3. Acids change the color of certain acid-base indicates. Two common indicators are litmus and phenolphthalein. Blue litmus turns red in the presence of an acid, while phenolphthalein turns colorless.
4. Acids react with active metals to yield hydrogen gas. Recall that an activity series is a list of metals in descending order of reactivity. Metals that are above hydrogen in the activity series will replace the hydrogen from an acid in a single-replacement reaction, as shown below:
   

Zn(s)+H2SO4(aq)→ZnSO4(aq)+H2(g)

1. Acids react with bases to produce a salt compound and water. When equal moles of an acid and a base are combined, the acid is neutralized by the base. The products of this reaction are an ionic compound, which is labeled as a salt, and water.

It should not be hard for you to name several common acids (but you might find that listing bases is a little more difficult). Below is a partial list of some common acids, along with some chemical formulas:

Chemist Name	Common Name	Uses
hydrochloric acid, HCl	muriatic acid (used in pools) and stomach acid is HCl	Used in cleaning (refining) metals, in maintenance of swimming pools, and for household cleaning.
sulfuric acid, H2SO4		Used in car batteries, and in the manufacture of fertilizers.
nitric acid, HNO3		Used in the manufacture of fertilizers, explosives and in extraction of gold.
acetic acid, HC2H3O2	vinegar	Main ingredient in vinegar.
carbonic acid, H2CO3	responsible for the “fizz” in carbonated drinks	As an ingredient in carbonated drinks.
citric acid, C6H8O7		Used in food and dietary supplements. Also added as an acidulant in creams, gels, liquids, and lotions.
acetylsalicylic acid, C6H4(OCOCH3)CO2H	aspirin	The active ingredient in aspirin.

What exactly makes an acid an acid, and what makes a base act as a base? Take a look at the formulas given in the above table and take a guess.

Many people enjoy drinking coffee. A cup first thing in the morning helps start the day. But keeping the coffee maker clean can be a problem. Lime deposits build up after a while and slow down the brewing process. The best cure for this is to put vinegar (dilute acetic acid) in the pot and run it through the brewing cycle. The vinegar dissolves the deposits and cleans the maker, which will speed up the brewing process back to its original rate. Just be sure to run water through the brewing process after the vinegar, or you will get some really horrible coffee.