= Solution
The <Stern layer> is the compact near-surface layer of specifically adsorbed or closely associated ions. The <diffuse layer> is the more extended, thermally distributed excess of counterions that screens the surface charge.
For water near $25^\circ\mathrm C$, the <Debye length> can be written
$$
\lambda_D\simeq\frac{0.304\ \mathrm{nm}}{\sqrt{I/(\mathrm{mol\,L^{-1}})}},\qquad
I=\frac12\sum_i c_i z_i^2.
$$
For $0.01$ M KCl, $I=0.01$ M and \b[$\lambda_D\simeq3.0$ nm]. For a fully dissociated 2:2 <electrolyte> at the same formula-unit concentration, $I=4c=0.04$ M, giving \b[$\lambda_D\simeq1.5$ nm], half as large. These are linear-screening estimates, not sharp boundaries of the <diffuse layer>.
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