Alkalis are substances with unique properties that play a vital role in chemical processes, both in everyday life and in industrial settings alike. With identifiable features like its soluble structure, where it sits on the pH scale, and how they interact on the ionic level with other atoms and molecules set them apart from their counterparts, acids, as well as from neutral chemicals.
In this post:
What is An Alkali?
An alkali in chemistry is a soluble base substance. They neutralise any acids they come into contact with, and have a pH above 7. This makes them indispensable in chemical testing processes.
Alkalis are highly corrosive to organic compounds because they can easily break the bonds between carbon chains and their functional groups. They can denature and dissolve proteins and start the saponification process, or breakdown, of fats.
Alkalis can also refer to the hydroxides of alkaline earth metals, such as calcium hydroxide or Ca(OH)2. Alkaline compounds were identified hundreds of years ago and originally referred to the ashes of plants that contained high amounts of sodium or potassium.
Key Takeaways
Alkalis are bases that dissolve in water
On the pH scale, they sit above 7
They are derived from alkali metals, such as sodium and potassium, or alkaline earth metals, like calcium
Alkalis are highly reactive and corrosive, capable of breaking down organic materials easily
Common alkalis include sodium hydroxide, potassium hydroxide, calcium hydroxide, and ammonium hydroxide
Alkalis have diverse applications, ranging from soap making to industrial cleaning, food processing, and water treatment.
What Is a Base?
While alkalis are a subcategory of bases, not all bases are considered alkalis. While they are generally classified as Brønsted-Lowry bases, not all of them exhibit the characteristics of Brønsted-Lowry bases.
A base is similar to an alkali, in that it has a pH level higher than 7 and reacts with acids to neutralise them and form salts. The difference is that alkalis are soluble in water, whereas bases are not. This means that a solid material, such as zinc oxide, is just as reactive to acids as a liquid base, such as potassium hydroxide.
One other difference is that, when they react, an alkali will release OH– ions and accept protons, whereas a base will simply neutralise acids.
Examples of an Alkali
Alkalis are associated with a group of alkali metals and their compounds. These metals include hydrogen (H), lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr).
Despite its classification as an alkali metal, hydrogen behaves differently, with pure water being its neutral hydroxide. However, the unbalanced ionisation of water can result in basic or acidic conditions, as can be seen in this equation:
2H2O ⇌ H3O+ + OH–
As many are often used as diagnostic reagents, it’s good to know what common alkali examples exist:
Potassium Carbonate
Potassium carbonate (K₂CO₃) is a water-soluble salt that is often referred to as potash. White in colour, it is primarily used in the production of glass where it reduces the melting point of silica and enhances the clarity of the finished product. This makes it crucial in the manufacture of high quality glass products, such as optical lenses.
It is also used in the production of soaps and detergents as its ability to break down grease and oils makes it an effective cleaning agent, and it has applications in the culinary industry where it is used to regulate acidity and improve texture in baked goods and other foods.
Sodium Hydroxide
With the chemical formula NaOH, and more commonly known as caustic soda, sodium hydroxide is important in manufacturing various products, including paper and soap. In paper manufacturing, sodium hydroxide is used to dissolve cellulose, turning it into pulp as this is easier to spread thinly for paper production.
The sodium hydroxide is thoroughly washed away and the pulp is then bleached and washed again. When combined with oil or fat, caustic soda can also be used to make soap and detergent.
Potassium Hydroxide
Potassium hydroxide has the chemical formula KOH and is commonly known as potash lye. Potassium hydroxide has several uses, including agricultural, industrial, and household applications.
For example, it’s commonly used by farmers to balance the pH of acidic soil. In food processing, potassium hydroxide acts as a stabiliser and thickening agent. It also serves as an electrolyte in alkaline batteries and is often used in electroplating and lithography.
Calcium Hydroxide
This alkali has the formula Ca(OH)2 and is known by many other names including hydrated lime, builders’ lime, and slaked lime. Calcium hydroxide is commonly used in food preparation during the pickling process, which is why it’s often called pickling lime.
Arguably the most important, and certainly the oldest, use of calcium hydroxide is in the preparation of lime mortar for plastering walls. It’s also very useful as a flocculant when treating sewage water, as it causes the colloidal particles in water to precipitate.
Ammonium Hydroxide
Represented by the chemical formula NH4OH, ammonium hydroxide is an aqueous solution of ammonia, which is dissolved gas in water. It’s mainly used as a cleaning agent in industries, although it’s also used in manufacturing plastic, rayon, fertilisers, and rubber.
What is the Difference Between an Acid and an Alkali?
Acids and alkalis have many key differences between them, but the most well-known is the pH level. Acids fall below 7, while alkalis sit above this. Other differences that acids show include:
- Releasing hydrogen ions (H⁺) in solution
- A sour taste
- Reacts with metals, producing hydrogen gas
- Turns blue litmus paper red
- Reacts with alkalis to neutralise and form salt and water
Conclusion
Alkalis play a pivotal role in chemistry, acting as soluble bases that produce hydroxide ions (OH⁻) in solution and neutralise acids. Derived from alkali and alkaline earth metals, they exhibit properties such as high reactivity and corrosiveness, enabling them to break down organic materials like fats and proteins.
From potassium carbonate’s role in glassmaking and food processing to sodium hydroxide’s applications in soap and paper production, alkalis prove their versatility across numerous industries.
