A city acid or ethanolic acid is sort of you go to weak acid when you're talking about the what is a weak acid and the role of weak acids. This is a great one to show students thinking about equilibrium in the sense of predicting observing and explaining the changes. Dr. Arbrotis set up test follows. We've got two separate breaches, both containing an equilibrium mixture of a city acid and obviously it's conjugate base acetate as well as hydronium ions or H plus ions or whatever you want to call them. The color change you're going to see here is due to the addition of the methyl red indicator which will be sort of an orangey reddish sort of color at a low pH of about three and at above pH four it will be sort of a yellow color. If you haven't already I think it's good to maybe have the disassociation equation up on the board showing the acetate acid acid or ethanolic acid goes to the acetate ion plus H plus and I believe they're having their ion H plus relating that to be H. In the first case we are adding an extra sort of an external source of the acetate ion added as a solid. So in this case if you're looking at from an equilibrium perspective we're adding a product. In doing so we then cause the equilibrium mixture to shift to the react inside and in doing so it consumes the other product which is H plus ions. And do that? No you've still got an excess amount of acetate ions so the shadow as the principle is telling is that there's still an excess amount of acetate but you've used up H plus. You've consumed H plus to allow the reaction to shifts to the left and so the concentration of H plus has gone down and so therefore the pH has gone up. In this scenario we are adding a strong base sodium hydroxide. Now adding a base to an acid reaction we see that the pH increases and end up with a shift in color to the yellow due to this increase in pH. So what's happened here the addition of the base so you've got hydroxid ions so they have reacted with pH plus ions in solution. So that has reduced the concentration of H plus ions hence the color change but which way is the equilibrium shift as well it's actually shifted to the right. So initially we see a color change to yellow just by adding sodium hydroxide then by having the H plus which is the product of the equilibrium system consumed we see a partial offset of this yellow check. It still stays yellow because the pH is still larger than it was at the start but this is partially offset. So of course we've shifted to the right but we've, let's say the principle tells us that it is a partial compensation of the loss of H plus ions. The H plus ions lost due to the addition of the sodium hydroxide is only partially compensated by shifting your action to the left. Overall we've still got a reduction in H plus ions hence the pH is higher. It's two observations clearly connected to one equation showing the equation going shifting in different directions and yet still leading to the same pH change. So it gets students thinking about partial, why the Lachelian principle is a partial change, how we can compensate for those changes in equilibrium and the presence of the H plus ions gives us some as an observation to a greater pH change.