Strong and Weak Alkalis | Acids, Bases & Alkalis | Chemistry | FuseSchool

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. Our teachers and animators come together to make fun & easy-to-understand videos in Chemistry, Biology, Physics, Maths & ICT. VISIT us at, where all of our videos are carefully organised into topics and specific orders, and to see what else we have on offer. Comment, like and share with other learners. You can both ask and answer questions, and teachers will get back to you. These videos can be used in a flipped classroom model or as a revision aid. Twitter: Access a deeper Learning Experience in the FuseSchool platform and app: Friend us: This Open Educational Resource is free of charge, under a Creative Commons License: Attribution-NonCommercial CC BY-NC ( View License Deed: ). You are allowed to download the video for nonprofit, educational use. If you would like to modify the video, please contact us: Click here to see more videos:

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