Amongst the following, the form of water with the lowest ionic conductance at 298K is:

We have to compare the ionic conductance of four different kinds of water at $$298\,\text{K}$$. Ionic conductance (often expressed through specific conductance or conductivity, $$\kappa$$) depends directly on how many ions are present in the solution and how easily these ions move. Mathematically, the relation is written as

$$\kappa = \lambda_{m}\,c$$

where $$\lambda_{m}$$ is the molar ionic conductance and $$c$$ is the molar concentration of ions. For a fixed temperature, $$\lambda_{m}$$ for common ions does not change much from one sample of water to another, so the decisive factor becomes the ionic concentration $$c$$. A higher ionic concentration means larger $$\kappa$$, and hence a higher ability to conduct electricity.

Now we identify the relative ionic concentrations in each type of water mentioned in the options.

• Sea water is famous for its high salinity. It contains significant amounts of $$\text{Na}^+$$, $$\text{Cl}^-$$, $$\text{Mg}^{2+}$$, $$\text{SO}_4^{2-}$$, and many other ions. Therefore its ionic concentration $$c_{\text{sea}}$$ is very large, giving a large $$\kappa_{\text{sea}}$$.

• Water from a well also contains dissolved minerals such as calcium, magnesium, iron, bicarbonates, sulphates, and sometimes chlorides. While the total ionic concentration $$c_{\text{well}}$$ is usually less than that of sea water, it is still appreciable, hence $$\kappa_{\text{well}}$$ is also sizeable.

• Saline water used for intravenous (IV) injection is typically a $$0.9\%$$ $$\text{NaCl}$$ solution, referred to as “normal saline.” The mass percentage $$0.9\%$$ corresponds to roughly $$0.154\,\text{mol}\,\text{L}^{-1}$$ of $$\text{NaCl}$$. The ionic concentration $$c_{\text{IV}}$$ is therefore moderate, producing a measurable conductivity $$\kappa_{\text{IV}}$$.

• Distilled water is produced by boiling water and re-condensing the steam, which removes almost all dissolved salts. The residual ionic concentration $$c_{\text{distilled}}$$ is extremely low, often in the range of $$10^{-7}\text{ to }10^{-6}\,\text{mol}\,\text{L}^{-1}$$, limited mainly by self-ionization of water itself. Substituting this tiny $$c_{\text{distilled}}$$ into $$\kappa = \lambda_{m}c$$ shows that $$\kappa_{\text{distilled}}$$ is orders of magnitude smaller than that of any of the other waters considered.

So, comparing

$$\kappa_{\text{sea}} \gg \kappa_{\text{well}} \gtrsim \kappa_{\text{IV}} \gg \kappa_{\text{distilled}}$$

we see that distilled water has the smallest specific conductance, i.e., the lowest ionic conductance of all the choices.

Hence, the correct answer is Option A.