Page 28 - NCERT Science Class 10 English Medium

The bulb will start glowing in the case of acids, as shown in Fig. 2.3.
But you will observe that glucose and alcohol solutions do not conduct
electricity. Glowing of the bulb indicates that there is a flow of electric
current through the solution. The electric current is carried through the
acidic solution by ions.
+
–
–
Acids contain H ion as cation and anion such as Cl in HCl, NO in
3
–
HNO , SO 2– in H SO , CH COO in CH COOH. Since the cation present in
3 4 2 4 3 3
+
acids is H , this suggests that acids produce hydrogen ions, H (aq), in
+
solution, which are responsible for their acidic properties.
Repeat the same Activity using alkalis such as sodium hydroxide, calcium
hydroxide, etc. What can you conclude from the results of this Activity?
2.2.1 What Happens to an Acid or a Base in a Water Solution?
Do acids produce ions only in aqueous solution? Let us test this.
Activity 2.9
Activity
Activity 2.9
Activity 2.92.9
Activity 2.9
n Take about 1g solid NaCl in a clean and
dry test tube and set up the apparatus as
shown in Fig. 2.4.
n Add some concentrated sulphuric acid to
the test tube.
n What do you observe? Is there a gas coming
out of the delivery tube?
n Test the gas evolved successively with dry
and wet blue litmus paper.
n In which case does the litmus paper change
colour?
n On the basis of the above Activity, what do
you infer about the acidic character of:
(i) dry HCl gas
Figure 2.4 HCl gas
Figure 2.4
Figure 2.4
Figure 2.4
Figure 2.4 Preparation of
(ii) HCl solution?
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.
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 –
2 3
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 ).
+
3
+
H + H O → H O +
2 3
We have seen that acids give H O or H (aq) ion in water. Let us see
+
+
3
what happens when a base is dissolved in water.
+
–
NaOH(s) H O Na (aq) + OH (aq)
2
→

Acids, Bases and Salts 23
2024-25