 In a previous lesson, we learned about strong acids and weak acids. An acid is a substance that would dissociate in water to give a proton, or H plus ion, and a conjugate base. An acid is considered to be strong if dissociation is 100%, and weak if dissociation is usually less than 1%. You may want to watch this video on strong and weak acids if you've forgotten the differences for acids. 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? Much like their acidic counterparts, we must consider the extent of dissociation in water. So let's have a look at sodium hydroxide as an example. In water, sodium hydroxide dissociates into an Na plus ion and an OH minus ion. Since sodium hydroxide almost completely dissociates 100%, it is considered to be a strong alkali, so we use a forward arrow in the equation. So the concentration of OH minus 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 highly corrosive. The pH of these substances will be at the higher end of the scale. In contrast, a weak alkali is one that does not completely dissociate in water. A good example of a weak alkali is ammonium hydroxide. In water, it does not completely dissociate into its respective ions. Note the equilibrium arrow. The concentration of OH minus 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 a light blue and a less corrosive than strong alkalis for a given concentration. The pH of these substances will be closer to the middle of the scale. So 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.