Q.Why is a solution of KOH used to absorb CO2 evolved during the estimation of carbon present in an organic compound?

Carbon Dioxide Absorption — The Intuition

Imagine you're holding a glass of cold water on a hot day. After a few minutes, you notice tiny bubbles forming on the inside of the glass. Those bubbles aren't coming from the water itself — they're carbon dioxide gas from the air, dissolving into the water. That's absorption in action: a gas (CO₂) entering a liquid (water) and becoming part of it.

Now scale that up. The oceans, lakes, and even the moisture in soil are constantly doing the same thing on a massive scale. CO₂ from the atmosphere dissolves into water bodies. But here's the twist — once dissolved, CO₂ doesn't just sit there as a gas. It reacts chemically with water, turning into something else entirely.

The Precise Chemistry

When carbon dioxide gas meets water, two things happen in sequence:

Step 1 — Physical dissolution:

CO₂ (gas) simply enters the water and becomes aqueous CO₂.

Step 2 — Chemical reaction:

The dissolved CO₂ reacts with water to form carbonic acid, a weak acid.

This carbonic acid then quickly dissociates (splits) into ions:

And a tiny fraction of bicarbonate further dissociates:

Why This Matters

This isn't just a chemistry curiosity. CO₂ absorption by oceans is the Earth's largest natural carbon sink — it absorbs about 25-30% of all CO₂ humans emit. Without it, atmospheric CO₂ levels would be much higher.

But there's a catch. As we pump more CO₂ into the air, oceans absorb more, producing more carbonic acid. This ocean acidification makes seawater more acidic (lower pH), harming marine life — especially organisms with calcium carbonate shells (corals, shellfish, plankton). …