Activity 2.9

Activity . Take about 1g solid NaCl in a clean and dry test tube and set up the apparatus as shown in Fig. . . Add some concentrated sulphuric acid to the test tube. What do you observe? Is there a gas coming out of the delivery tube? Test the gas evolved successively with dry and wet blue litmus paper. In which case does the litmus paper change colour? On the basis of the above Activity, what do you infer about the acidic character of: (i) dry HCl gas (ii) HCl solution? Figure . Figure . Figure . Figure . Figure . Preparation of HCl gas This experiment suggests that hydrogen ions in HCl are produced in the presence of water. The separation of H + ion from HCl molecules cannot occur in the absence of water. HCl + H O → H O + + Cl – Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H + (aq) or hydronium ion (H O + ). H + + H O → H O + We have seen that acids give H O + or H + (aq) ion in water. Let us see what happens when a base is dissolved in water. NaOH(s) H O   Na + (aq) + OH – (aq) Note to teachers: If the climate is very humid, you will have to pass the gas produced through a guard tube (drying tube) containing calcium chloride to dry the gas. KOH(s) K (aq)+OH (aq) H O   – Mg(OH) (s) H O   Mg + (aq)+2OH – (aq) Bases generate hydroxide (OH – ) ions in water. Bases which are soluble in water are called alkalis.