Sodium is a soft metal that bursts into flame in water. Chlorine is a green gas that was used as a weapon in World War I. Put them together and you get the salt on your fries. Many schools no longer allow this demo, because chlorine gas is too dangerous in a classroom. Here you can run it as often as you like.
Printable student worksheet (PDF) · two pages, goes with this lab
Low-write worksheet (PDF) · same questions with tick boxes, word banks and worked examples
Teacher guide: NYSSLS, SEP and CCC alignment (PDF)
These are models, so every run gives the same answer. The flask holds 1.00 L of chlorine at 20 °C and normal air pressure, which is 2.95 g (from PV = nRT). Making salt from its elements gives off 411 kJ for every 58.44 g (one mole). The energy steps and sizes are rounded textbook values: sodium atom 186 pm, Na⁺ 102 pm, chlorine atom 99 pm, Cl⁻ 181 pm. In the real demo the reaction is started with a drop of water or by warming the sodium; the flame is bright yellow because of sodium's 589 nm light (see the Atomic Spectra lab).