Dry Lab Challenge
Select correct drying agents for gases
What this lesson covers
Why it matters
Pour water onto quicklime and watch it hiss, swell, and give off intense heat. That hunger for water is what makes a drying agent — but a drier that attacks the gas itself is worse than useless.
The idea in plain words
Analyze the transformation: CaO + H₂O → Ca(OH)₂. Note the heat release and the formation of a new compound.
Predict first
Why is quicklime (CaO) a poor choice for drying an acidic gas such as HCl or CO₂?
A drying agent must remove water without reacting with the gas. If it reacts, you lose your product.
- It is too expensive to use in large quantities
- It reacts chemically with the gas or its impurities — correct
- It is not porous enough to absorb water
- It turns into a gas itself
What you do
Pick a drying agent that soaks up water but will NOT react with the gas itself. Acidic driers ruin basic gases, and basic driers ruin acidic gases — match each gas to its safe partner.
Check yourself
What observation confirms that slaking of lime is a chemical change?
The reaction CaO + H₂O → Ca(OH)₂ releases significant heat (exothermic) and causes the solid to swell and hiss, indicating a new substance is formed.
Why can't we use quicklime to dry acidic gases like HCl?
Drying agents must be chemically inert to the gas. CaO is a base; it would neutralize acidic gases like HCl, destroying them.
In the reaction CaO + H₂O → Ca(OH)₂, what type of reaction is this?
Two substances (CaO and H₂O) combine to form a single product (Ca(OH)₂), which defines a combination reaction.
- The solid dissolves completely
- The mixture hisses, swells, and becomes hot — correct
- The color changes from white to black
- Quicklime is not dry enough
- Quicklime is basic and will react with the acidic gas — correct
- HCl gas is too heavy to pass through lime
- Decomposition
- Combination — correct
- Displacement