chemical reaction

A chemical reaction is nature's way of rearranging atoms — taking one set of substances and rebuilding them into different ones. Think of the atoms as a fixed box of LEGO bricks: a reaction never adds or destroys bricks, it just snaps them apart and clicks them back together in a new pattern. When wood burns, iron rusts, or bread bakes, the same atoms end up wearing a different outfit.

The action happens at the bonds, the tiny links that hold atoms together. Old bonds break, new ones form, and that reshuffling is why the products look and behave so differently from what you started with — two invisible gases, hydrogen and oxygen, can become a drop of liquid water. Often you can see the change announce itself: bubbling, a color shift, heat, light, or a new smell.

Here's the rule that never bends: nothing actually vanishes. Every atom present at the start is still present at the end, so the total mass is exactly the same before and after — a principle called conservation of mass. A burning log seems to lose weight only because much of it floats off as invisible gas; weigh the gas too, and the books balance to the last atom.