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Bases

Any substance, which can accept protons or give a pair of electrons to form a bond, is called a base. Besides this, bases produce hydroxyl ion (OH^–) whey they dissociate in their aqueous solution. See the below chemical reactions, in which a base is dissociating into its constituting ions one of which is hydroxyl ion;

NaOH (aq) → Na⁺ + OH^–

Ca (OH)₂ → Ca²⁺ + 2OH^–

So, we can say that bases tend to donate electrons and accept protons or produce hydroxide ions in aqueous solutions.

Common definitions of bases:

Three theories have been introduced by three different scientists to define acids and bases. Let us see how they defined bases.

• Arrhenius theory: According to this, a substance that dissociates in water and forms hydroxyl ions (OH^–) is a base.
• Bronsted-Lowry theory: This theory says any substance that accepts protons is a base.
• Lewis theory: This theory says that it is a substance that can donate a pair of electrons.

Properties of Bases

Bases show various characteristics properties, some of which are listed below;

• The pH of bases in solutions is greater than 7.
• They taste bitter and less often used in foods.
• They are slippery to touch like soaps.
• In an aqueous solution or a molten state, bases dissociate into ions and thus can conduct electricity. So, their aqueous solutions are also electrolytes.
• The reactions of strong bases with acids and organic substances are very vigorous.
• Bases turn litmus paper blue, turns methyl orange to yellow. However, it does not changes the blue colour of bromothymol.
• They react with metals differently than acids. They produce salt and release hydrogen gas by reacting with metals. For example,

Zn + 2 NaOH → Na₂ZnO₂ + H₂

• They produce a salt and water on reacting with acids. For example,

KOH + HCl → KCl + H₂O

Uses of Bases

• They are present in most cleaners such as ammonia is an important component of many cleaners and the drain cleaners contain sodium hydroxide.
• Sodium hydroxide is also used in the manufacturing of soaps.
• Calcium hydroxide is used in the whitewashing of walls.
• The antacids that are used to treat stomach acidity are made of bases such as sodium hydrogen carbonate or magnesium hydroxide.
• The base ammonium hydroxide is widely used as a reagent in laboratories.
• The soils which are highly acidic are neutralized by using slaked lime.

Classification of bases I Types of bases

Bases are classified based on different factors as described below;

1) Based on their degree of dissociation in solutions and reactivity:

i) Strong base:

It is a base that dissociates completely into ions in water. Besides this, it can remove a hydrogen ion from a weak acid. For example, sodium hydroxide (NaOH).

ii) Weak base:

It does not dissociate completely into ions when dissolved in water. The aqueous solution of a weak base contains both the weak base and its conjugate acid. For example, ammonia (NH3), water (H2O) and pyridine (C₅H₅N).

iii) Superbase:

It is more capable of removing hydrogen ions from weak acids than strong bases. It has weak conjugate acid and can be prepared by mixing an alkali metal with its conjugate acid. Besides this, it cannot stay in an aqueous solution as it is stronger as a base than the hydroxide ion. For example, sodium hydride (NaH).

iv) Neutral Base:

It is a type of base that forms a bond with a neutral acid in such a way that the acid and base share the same pair of electrons from the base.

v) Solid base:

As the name suggests, it is active in solid form. For example, silicon dioxide (SiO₂) and NaOH attached to the alumina.

2) Based on acidity:

i) Monoacidic base:

These types of bases when ionize or dissociate in their aqueous solution produce one hydroxide ion per molecule. For example, NaOH, KOH, etc.

ii) Diacidic base:

These bases give two hydroxide ions when they ionize or dissociate in their aqueous solutions. For example, Ca (OH) ₂, and Mg (OH)₂.

iii) Triacidic base:

It is a base, which produces three hydroxide ions per molecule when it ionizes in water.

3) Based on concentration:

i) Concentrated alkali:

It refers to an alkali that is present in high concentration in its aqueous solution.

ii) Dilute alkali:

It is an alkali or base that is present in a relatively low percentage in its aqueous solution.

How to identify a base in a chemical reaction?

You can easily find out whether a molecule or a substance is base or not in a chemical reaction. You have to count the number of hydrogen atoms in that substance before and after the reaction. If the substance has more hydrogen atoms after the reaction than it has before the reaction, it is a base as a base tends to accept hydrogen ions.

Some commons bases are described below;

1. Rubidium hydroxide

Rubidium hydroxide is a strong base, which is also called rubidium hydrate. It is made of one rubidium ion and one hydroxide ion. Its chemical formula is RbOH. It is highly corrosive and looks like a greyish white solid.

Rubidium hydroxide is formed when rubidium dissolves in water. It is not found naturally, so, it is synthesized in a laboratory from rubidium ion and is available in liquid form. It is used in scientific researches and it may burn the skin or cause irritation in the eyes so protective measures should be taken while handling this chemical.

It is rarely used in industrial methods. However, it is mainly used as a catalyst and as an analytical reagent. Besides this, it is also used as a raw material for various rubidium crystals and so mostly used in glass and ceramic industries.

2. Zinc hydroxide

It is a weak base and looks like a white powder. It is an inorganic compound with the chemical formula is Zn (OH) ₂. The melting point of Zn hydroxide is 125 degrees centigrade.

It is found naturally as well as can be synthesized in a lab by adding sodium hydroxide to a solution of zinc salt. In nature, it is found in three minerals; wulfingite, ashoverite and sweetite.

It is an amphoteric hydroxide, which can react with both acids and bases. Although it is an insoluble hydroxide, it gets dissolved when it reacts with strong acids.

It is used in medical bandages as an absorbent. The dressings are covered with zinc compound to absorb blood from the site of injury. It is also used in the production of pesticides and pigments.

Physical properties of Zinc Hydroxide

• It is odourless.
• It is dull white in appearance.
• Its pH is 8.88 and its oxidation number or state is +2.
• It is insoluble in alcohol and slightly soluble in water.

How is Zinc Hydroxide prepared?

It can be prepared by adding sodium hydroxide to a solution of a zinc salt, as shown in the below chemical reaction;

Zn + 2 OH^– → Zn (OH)₂

3. Potassium hydroxide

Potassium hydroxide is an inorganic compound, which is a strong base with the chemical formula KOH. It has multiple names such as caustic potash, sodium hydrate and potash lye. It is highly corrosive in nature and reacts quickly with acids. Its reaction with water, when it is dissolved in water, is highly exothermic as shown below;

KOH → K⁺ (aq) + OH^– (aq)

It is a white or colourless solid, which is widely used in laboratories and everyday life.

4. Calcium hydroxide

It is also an inorganic compound which is a base. It can be a strong or medium strength base depending on its concentration. It is also known as calcium dihydroxide, hydralime, caustic lime, lime hydrate, milk of lime.

The chemical formula of calcium hydroxide is Ca (OH) ₂. It looks like a white power and may exist as a colourless crystal. The saturated solution of calcium hydroxide is commonly known as lime water.

It is relatively insoluble in water. However, its solubility in water is large enough to make its solutions a base as shown in the below reaction;

Ca (OH) ₂ (s) + Ca2⁺ (aq) + 2 OH^– (aq)

It has many uses such as used in sewage treatment, paper production, food preparation and construction work. Besides this, it is also used in the medical field, e.g., root canal fillings generally contain calcium hydroxide.

Furthermore, calcium hydroxide used in food items is generally safe. However, industrial-grade one is not safe, which may result in calcium hydroxide poisoning and can lead to severe complications or even death.

5. Lithium hydroxide

Lithium hydroxide is an inorganic compound, which is a strong base. It is known by different names such as lithium hydrate and lithium hydroxide. Its chemical formula is LiOH and it is a white crystalline solid. It easily reacts with water, however, less soluble in ethanol.

It is widely used in the making of lubrication grease. The other uses are as an electrolyte in nickel-hydrogen and nickel-cadmium batteries, as a dye, as a carbon dioxide scrubber that protects people from being suffocated in submarines, spaceships, etc.

In the ceramic industry, it is used as a coolant in water reactors and also to prevent corrosion. Besides this, it is very lightweight so can be used in combination with other metals like aluminium, copper, etc., to make lightweight alloys.

6. Ammonia

Ammonia is a weak base with the chemical formula NH₃. It is an inorganic compound that contains one nitrogen atom chemically bonded to three hydrogen atoms. It can exist as a colourless liquid or gas and has a distinctive odour.

It is naturally found in air, soil and water and even in plants, animals and humans. It is formed in the human body when the proteins are broken into amino acids and ammonia.

Its major use is as a fertilizer. It is the main component of ammonium nitrate fertilizer. Besides this, it also has lots of uses such as in household cleaning products that are used to clean surfaces, sinks, toilets, tiles, etc.,; as a refrigerant gas in air-conditioning equipment, in the production of plastics, fabrics, dyes, and also used in the treatment of wastewater.

7. Sodium Hydroxide

It is a strong base with the chemical formula NaOH. It is known by different names such as caustic soda, white caustic, and sodium hydrate. It is an odourless inorganic compound that exists as white crystals at room temperature.

Sodium hydroxide can absorb water and carbon dioxide from the air. It is toxic in nature, so, if someone comes in contact with it for a longer duration, it may burn skin, eyes, and cause temporary hair loss.

It has many uses such as it is used in soap-making, in the cleaning of drains, in the preparation of other chemicals, in increasing the alkalinity of solutions, in the manufacturing of various medicines like aspirin, the medication used to reduce cholesterol, and in anticoagulants that prevent blood clotting.

It is also used in fuel cells that act as batteries to produce electricity and in the treatment of water as it reduces water acidity and reduces heavy metals from water.

8. Magnesium Hydroxide

Magnesium hydroxide is a base with the chemical formula Mg (OH)₂. It is also known as milk of magnesia. It is an inorganic compound and is the main component of antacids. It appears as a white solid and is slightly soluble in water. Its melting point is 350 degrees centigrade.

It has many uses such as;

• It is a good thermal conductor.
• It is used as a food additive and to prevent fire.
• It is used in gold mining and warehouses.
• It is also used as a photographic fixer.

How is Magnesium hydroxide prepared?

It can be formed by reacting a solution of magnesium salts with alkaline water. This reaction initiates the formation of solid magnesium hydroxide, which is the product of this reaction as shown below in this reaction;

Mg2+ 2 OH^– → Mg (OH)₂

On a large scale, it is produced by using lime and seawater. Furthermore, excessive use of Magnesium hydroxide may have an adverse on health. It can cause diarrhoea, nausea, irritation in the stomach, etc.

9. Methylamine

Methylamine is a weak base. It is an organic compound with the chemical formula CH₅N. It is also called methanamine, methyl ammonia, aminomethane, etc. In its purest form, it generally exists as a colourless gas, however, it is also found in liquid form in solution with water, ethanol, etc.

10. Pyridine

Pyridine is a weak base with the chemical formula C₅H₅N and is also known as azabenzene. It is a colourless liquid with a fishy smell. It is soluble in water and is highly flammable. It is commonly added to ethanol to make it inappropriate for drinking. Besides this, it is used in the dyes and pharmaceutical industries.

It is poisonous in nature, so, when inhaled or consumed, it may cause health issues like migraine, laryngitis, cough, etc.

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