Strong Alkali: Fully Ionises to Release OH⁻
Acids, Bases, and the pH Scale
To understand strong alkalis, we first need to know about acids and bases. Substances are often classified by their effect on water. An acid is a substance that donates a proton (H⁺ ion) when dissolved in water. A base is a substance that accepts a proton or, more commonly for our topic, donates a hydroxide ion (OH⁻) in water. The strength of an acid or base is measured by the pH scale, which runs from 0 to 14.
- A pH of 7 is neutral (pure water).
- pH values < 7 indicate acidity.
- pH values > 7 indicate alkalinity (or basicity).
A strong alkali sits at the high end of this scale, typically with a pH of 12 to 14. This high pH is a direct result of the high concentration of OH⁻ ions it releases.
What Does "Fully Ionises" Mean?
This is the core scientific idea. Ionisation (or dissociation) is the process where a compound splits into its constituent ions when dissolved in water. For a base, this means releasing hydroxide ions (OH⁻).
A strong alkali undergoes complete or 100% ionisation. Imagine dropping a teaspoon of a strong alkali like sodium hydroxide pellets into water. Every single molecule of the alkali will break apart (ionise) into positive metal ions and negative hydroxide ions. None of the original molecular compound remains in the solution.
$ MOH_{(s)} \xrightarrow{H_2O} M^{+}_{(aq)} + OH^{-}_{(aq)} $
Where 'M' represents a metal atom (like Na, K, or Ca). The arrow going only forward ($\rightarrow$) indicates the reaction goes to completion.
In contrast, a weak alkali (like ammonia solution, $NH_3$) only partially ionises. Most of the ammonia molecules remain intact, and only a small fraction react with water to produce a small number of OH⁻ ions. This is shown with a reversible arrow ($\rightleftharpoons$) in its equation: $NH_{3(aq)} + H_2O_{(l)} \rightleftharpoons NH^{+}_{4(aq)} + OH^{-}_{(aq)}$.
Common Strong Alkalis and Their Properties
Not many bases are strong alkalis. The most common ones are the hydroxides of Group 1[1] (alkali metals) and some from Group 2[2] (alkaline earth metals). Their complete ionisation gives them distinct properties.
| Name (Formula) | Common Name | Ionisation Equation | Key Property/Use |
|---|---|---|---|
| Sodium Hydroxide ($NaOH$) | Caustic Soda, Lye | $ NaOH \rightarrow Na^{+} + OH^{-} $ | Soap making, drain cleaner, paper production. |
| Potassium Hydroxide ($KOH$) | Caustic Potash | $ KOH \rightarrow K^{+} + OH^{-} $ | Making liquid soaps and batteries. |
| Calcium Hydroxide ($Ca(OH)_2$) | Slaked Lime | $ Ca(OH)_2 \rightarrow Ca^{2+} + 2OH^{-} $ | Soil treatment, mortar, water softening. |
| Barium Hydroxide ($Ba(OH)_2$) | Baryta | $ Ba(OH)_2 \rightarrow Ba^{2+} + 2OH^{-} $ | Laboratory reagent for testing sulfates. |
Shared Properties: Due to the high concentration of OH⁻ ions, all strong alkalis:
- Feel slippery or soapy on the skin (this is actually the OH⁻ ions reacting with oils in your skin to form soap!).
- Turn red litmus paper blue.
- Conduct electricity well in solution (because ions carry the current).
- React vigorously with acids in exothermic neutralisation reactions.
- Are corrosive. They can cause severe burns and damage materials like living tissue, fabrics, and metals. Safety is paramount when handling them.
The Power of Neutralisation
A quintessential reaction of strong alkalis is neutralisation with a strong acid. When a strong alkali and a strong acid react, they completely cancel each other out, forming a neutral salt and water. The key reaction is between the hydroxide ion (OH⁻) from the alkali and the hydrogen ion (H⁺) from the acid.
The ionic equation for this core reaction is:
Example: When sodium hydroxide (a strong alkali) reacts with hydrochloric acid (a strong acid), the reaction is rapid and complete:
$ NaOH_{(aq)} + HCl_{(aq)} \rightarrow NaCl_{(aq)} + H_2O_{(l)} $
If we mix exactly equal amounts of acid and alkali so that all H⁺ and OH⁻ ions are used up, the resulting solution has a pH of 7. This principle is used in titration, a lab technique to find the concentration of an unknown acid or alkali solution.
From Soap to Satellites: Applications in Action
The ability of strong alkalis to fully ionise makes them incredibly useful, though often behind the scenes. Here are some concrete examples where their high OH⁻ concentration is key:
1. Saponification – Making Soap: This is a classic chemical reaction. Animal fats or vegetable oils (which are esters) are heated with a strong alkali like $NaOH$ or $KOH$. The OH⁻ ions attack the ester molecules, breaking them down into glycerol and the sodium or potassium salts of fatty acids – which we call soap! The completeness of the alkali's ionisation ensures a high yield of soap.
2. The Kitchen Sink – Drain Cleaners: Many drain cleaners contain solid $NaOH$ pellets. When water is added, the $NaOH$ fully ionises, releasing a lot of OH⁻ ions and generating heat. This heat helps melt grease blockages, while the OH⁻ ions chemically react with fats (a process similar to saponification), turning them into soapy substances that can be washed away.
3. Food Processing: $NaOH$ solution is used to peel fruits and vegetables like tomatoes and potatoes by breaking down their skins. It's also used in processing cocoa and olives. Precise control is vital for safety.
4. Paper and Pulp Industry: Wood chips are cooked in a solution of $NaOH$ and sodium sulfide (the "kraft process"). The strong alkali helps dissolve the lignin[3] that binds wood fibers together, separating the pure cellulose needed to make paper.
5. Environmental and Chemical Synthesis: $Ca(OH)_2$ (slaked lime) is spread on acidic lakes or soils to neutralise excess acidity. In chemistry labs and industries, strong alkalis like $NaOH$ and $KOH$ are essential reagents for synthesizing many chemicals, plastics, and medicines.
Important Questions
No. "Strong" refers to the degree of ionisation (complete vs. partial). "Concentrated" refers to the amount of alkali dissolved in a given volume of water. You can have a dilute solution of a strong alkali (few $NaOH$ molecules in a lot of water, but they are 100% ionised). You can also have a concentrated solution of a weak alkali (a lot of $NH_3$ in water, but only a small percentage is ionised). Strength is a chemical property; concentration is a measure of quantity.
Their corrosiveness stems from the high concentration of reactive OH⁻ ions. These ions readily break down complex molecules. On skin, they react with fats and proteins in a process called saponification and denaturation, effectively dissolving living tissue. They also react with materials like wool, silk, and even glass over time. Always use protective gloves and goggles!
The inherent strength (complete ionisation) of a strong alkali like $NaOH$ does not change; it is a fixed property of the substance. However, its effective strength in a reaction can be altered by changing its concentration. A more concentrated $NaOH$ solution will have a higher [OH⁻] and thus a higher pH, reacting more aggressively than a dilute one. But both are still considered "strong" because the $NaOH$ in them is fully ionised.
Footnote
[1] Group 1: The first column of the periodic table, also called alkali metals. Includes lithium (Li), sodium (Na), potassium (K), etc. They are highly reactive metals that form +1 ions.
[2] Group 2: The second column of the periodic table, also called alkaline earth metals. Includes beryllium (Be), magnesium (Mg), calcium (Ca), barium (Ba), etc. They form +2 ions.
[3] Lignin: A complex organic polymer that forms key structural materials in the support tissues of most plants. It is what makes wood rigid.