Heat Up or Cool Down?

Why it matters

A small piece of sodium dropped into cold water darts about with a hiss, releasing hydrogen that can catch fire — and the water warms up.

The idea in plain words

Check the balance: 2 Na atoms and 2 H₂O molecules yield 2 NaOH and 1 H₂ molecule.

Predict first

Why does the water get hot when sodium reacts with it?

This is exothermic: the energy released when new bonds form is greater than the energy absorbed to break the old bonds, so the surroundings warm up.

• The reaction releases stored chemical energy as heat — correct
• The water was already hot before the sodium was added
• Sodium absorbs heat from the water to melt
• Friction from the sodium moving creates the heat

What you do

Drop the sodium. Watch the vigorous fizzing and the heat indicator rise.

Check yourself

The reaction 2Na + 2H₂O → 2NaOH + H₂ is classified as exothermic because...

Exothermic reactions release energy, usually as heat, causing the temperature of the surroundings to rise.

Which observation confirms that a chemical change occurred, not just physical mixing?

Formation of a new substance (H₂ gas) and a temperature change are key indicators of a chemical change.

A chemical cold pack turns icy cold when its contents mix. That reaction is…

The mixture absorbs heat from its surroundings, so the pack feels cold — the opposite of the sodium reaction: endothermic.

• it releases heat to the surroundings — correct
• it requires continuous heating to proceed
• it absorbs heat from the surroundings
• it produces a gas
• The sodium melted into a ball
• Hydrogen gas was produced, which may catch fire — correct
• The sodium floated on the water
• The water level rose slightly
• endothermic — it absorbs heat from the surroundings — correct
• exothermic — it releases heat
• not a chemical reaction, just melting