Everyday Science: Why Ice Floats and Bread Rises

Most science trivia is memorised and then forgotten, because a fact without a reason has nothing to hold onto. These are the everyday ones, with the "why" attached — which is what makes them stay.

Why ice floats

Almost every substance gets denser as it freezes. Water does the opposite. As it crystallises, the molecules lock into an open, lattice-like structure that takes up more room than the liquid did, so ice is less dense than the water around it and floats.

This is not a curiosity. Because ice floats, lakes freeze from the top down, and the water underneath stays liquid — which is why fish survive winter, and arguably why life in cold climates works at all.

Why bread rises

Yeast is a living organism. Feed it sugar and it releases carbon dioxide, which gets trapped in the stretchy gluten network of the dough and inflates it like thousands of tiny balloons. The heat of the oven kills the yeast and sets the structure in place.

Every hole in a slice of bread is a bubble of gas exhaled by a fungus. Bon appétit.

Why the sky is blue

Sunlight contains every colour. When it hits the atmosphere, the shorter wavelengths — the blues — are scattered far more strongly by air molecules than the longer reds. So blue light bounces around the sky and reaches your eye from every direction.

At sunset the light travels through much more atmosphere to reach you, the blue is scattered away entirely, and what is left is red and orange. Same physics, opposite result.

Why hot water can freeze first

Under certain conditions, hot water freezes faster than cold — the Mpemba effect. It has been observed repeatedly and is still argued over. It is a good reminder that "obvious" and "true" are not the same thing.