Acids, Bases and Salts

Activity . Take about mL of dilute NaOH solution in a test tube and add two drops of phenolphthalein solution. What is the colour of the solution? Add dilute HCl solution to the above solution drop by drop. Is there any colour change for the reaction mixture? Why did the colour of phenolphthalein change after the addition of an acid? Now add a few drops of NaOH to the above mixture. Does the pink colour of phenolphthalein reappear? Why do you think this has happened? On passing excess carbon dioxide the following reaction takes place: CaCO s Ca(HCO aq ( ) ) ( ) H O(l)+ CO (g) (Soluble in water) Limestone, chalk and marble are different forms of calcium carbonate. All metal carbonates and hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide and water. Thus, the reaction can be summarised as – In the above Activity, we have observed that the effect of a base is nullified by an acid and vice-versa. The reaction taking place is written as – NaOH(aq) + HCl(aq) → NaCl(aq) + H O(l) The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. In general, a neutralisation reaction can be written as – Base + Acid → Salt + Water