The Dazzling White Light

Why it matters

Hold a silvery ribbon of magnesium in a flame and look away — it burns with a light so white it can hurt your eyes. What's left behind is a soft white ash. Where did the metal go?

The idea in plain words

Balance the burn yourself: set the coefficients so magnesium and oxygen each count equal on both sides. Tip: leave the element that stands alone for last.

Predict first

After the magnesium ribbon burns completely, the white ash will weigh…

the ash is MgO: every magnesium atom plus a captured oxygen atom from the air. New partners joined, so the mass went UP.

• less than the ribbon — burning destroys some of it
• exactly the same as the ribbon
• more than the ribbon — oxygen atoms joined it — correct
• nothing is left — it all becomes light

What you do

Ignite it. Watch two elements combine into one compound on all three levels.

Check yourself

A + B → AB describes which reaction type?

Two or more substances become ONE product — a combination (synthesis) reaction. 2Mg + O₂ → 2MgO is the classic.

Magnesium oxide is white; magnesium is silvery. This colour change tells us…

MgO is a genuinely different substance: different colour, different properties, can't be undone by cooling. The hallmark of chemical change.

Which of these is also a combination reaction?

Quicklime + water → slaked lime: two substances, one product (and a lot of heat — try it in the next lessons). The others are decomposition, displacement and double displacement.

• Combination — correct
• Decomposition
• Displacement
• Double displacement
• the magnesium was painted by the flame
• a new substance with new properties has formed — a chemical change — correct
• magnesium melted and re-froze
• nothing — colour changes happen in physical changes too, always
• CaCO₃ → CaO + CO₂
• CaO + H₂O → Ca(OH)₂ — correct
• Zn + CuSO₄ → ZnSO₄ + Cu
• AgNO₃ + NaCl → AgCl + NaNO₃