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An acid is a substance that will dissociate in water to give a proton (or H+ ion) and a conjugate base. An acid is considered to be strong if dissociation nears 100%, and weak if dissociation is usually less than 1%.

In this lesson, we will learn about strong alkalis and weak alkalis.

An alkali is a soluble base that has a pH greater than 7, turns red litmus paper blue, and universal indicator solution green if it is weak and blue to purple if it is strong. But in the absence of universal indicator solution, how can we tell if an alkali is strong or weak?

In water, sodium hydroxide dissociates into a Na+ ion and a OH- ion. Since sodium hydroxide almost completely dissociates (100%), it is considered to be a strong alkali. The concentration of OH- ions in a solution containing a strong alkali will be high.

Other examples of strong alkalis include potassium hydroxide and calcium hydroxide.

These substances, when dissolved in water will turn universal indicator solution a very deep purple, and are highly corrosive.

In contrast, a weak alkali is one that does not completely dissociate in water.

A good example of a weak alkali is ammonium hydroxide (NH4OH).
In water, it does not completely dissociate into its respective ions. The concentration of OH- ions in a solution containing a weak alkali will be much lower.

These substances, when dissolved in water will turn universal indicator solution a green or light blue, and are less corrosive than strong alkalis for a given concentration.

In conclusion, a strong alkali dissociates almost completely in water to give a high concentration of hydroxide ions, whereas dissociation for a weak alkali is much less and therefore the concentration of hydroxide ions is much lower.

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