An iron rod is cut with a saw, and an identical rod is left out in the rain until it rusts

Why

Karan is right that both rods end up looking different from how they started, and grouping 'looks different now' together is a natural first instinct. But cutting and rusting change the iron in fundamentally different ways underneath. Cutting the rod just rearranges the SAME iron into two smaller pieces — weigh both pieces together and you get the original rod's weight exactly, and the metal in each piece is still pure iron, unchanged. Rusting is different: the iron atoms actually combine with oxygen and water from the air to form a genuinely new substance, iron oxide, which is why rust flakes are reddish-brown, crumbly, and weigh MORE than the iron that formed them, not the same. One rearranges matter, the other creates a new substance chemically joined from old ones. Test: a cut rod's two pieces can be welded back into one rod of the same iron; rust can never be 'un-rusted' back into clean iron by any physical trick. No, they are different kinds of change: cutting is physical (same substance, new shape), rusting is chemical (a genuinely new substance forms).