A hot dissolve

The idea

Dissolving an acid or a base in water is a highly exothermic process.

Into the water it goes

A lab assistant dissolves sodium hydroxide pellets in water. She holds the beaker.

What will she feel?

• The beaker gets hot
• The beaker gets cold
• Nothing changes

Watch the thermometer

Do all three. Watch the thermometer each time.

A teacher does this in the lab, with goggles. Never try it yourself.

Dissolving gives out heat

Dissolving an acid or a base in water is highly exothermic: it gives out a lot of heat. So an acid is always added slowly to the water, with constant stirring.

Look for the warning sign on a can of concentrated sulphuric acid and on a bottle of sodium hydroxide pellets.

• Do | Never
• Add acid slowly to water | Pour water into acid
• Stir all the time | Pour it in a rush
• Teacher and goggles | Mix it at home

Notes

Dissolving an acid or a base in water is a highly exothermic process.

Check yourself

Exothermic means the process …

What happens to the temperature when NaOH dissolves in water?

How should concentrated sulphuric acid be diluted?

Why must we not pour water into concentrated acid?

• gives out heat — correct. Yes! The surroundings get warmer.
• takes in heat. That is called endothermic.
• has no heat change. Exothermic means heat comes out.
• It falls. It rises, because heat is given out.
• It rises — correct. Yes! Dissolving a base is exothermic.
• It stays the same. The thermometer showed a big rise.
• Water quickly into the acid. The heat is made in a small spot and the mixture splashes out.
• Both poured in together, fast. Slowly and with stirring is the safe way.
• Acid slowly into water, stirring — correct. Yes! The water takes up the heat gently.
• The heat can make it splash out and burn — correct. Yes! The glass can also break from the heat.
• The acid turns into a base. It stays an acid. The danger is the heat.
• Nothing happens, it is fine. A lot of heat is made at once.