Select the graph that correctly describes the adsorption isotherms at two temperatures T₁, and T₂ (T₁ > T₂) for a gas :
(x - mass of the gas absorbed ; m - mass of adsorbent ; P - pressure)

Solution

Chemistry ma’am · 3 steps

Two checks: the colder curve sits higher, and every isotherm bends over and flattens at high pressure.

1. Which curve is higher. Adsorption is exothermic, so by Le Chatelier's principle raising the temperature pushes the gas off the surface. Since T₁ > T₂, the colder T₂ curve must lie above the T₁ curve at every pressure.
2. Shape of the curve. At low pressure x/m rises almost in proportion to P. As the surface sites fill up, the rise slows and x/m levels off towards a limit. So the isotherm is a curve that flattens, not a straight line.

   x/m = kP^(1/n) (n > 1)

   Option	Shape	Curve on top	Verdict
   (a)	curves that flatten	T₂ (colder)	both checks pass
   (b)	straight lines	T₂ (colder)	wrong shape
   (c)	straight lines	T₁ (hotter)	wrong shape and wrong order
   (d)	curves that flatten	T₁ (hotter)	wrong order

3. So. Only graph (a) has curving, saturating isotherms with the lower-temperature T₂ curve on top.

The idea to keep. Adsorption releases heat, so more gas sticks at lower temperature. And x/m rises with P only until the surface fills, then it flattens out.

Answer from the NTA final answer key. Solution by PrepWiser.