Which litre is heaviest

A litre of water, a litre of ice, a litre of steam at the boil. Which one weighs the most?

Liquid water is densest; ice has an open crystal lattice and floats, and steam is mostly empty space. Same substance, three densities.

You drop a feather and a hammer on the Moon. What happens?

No air, no drag. An astronaut did exactly this on Apollo 15 and both hit the dust at the same instant.

Which is hotter: a candle flame or the surface of the Sun?

A candle burns at around a thousand degrees; the Sun's photosphere is several times that. The candle only feels hotter because it is closer.

Why does a spinning ice skater speed up when she pulls her arms in?

Mass closer to the axis means the same angular momentum needs a faster spin. Nothing pushes; the rule simply holds.

Which weighs more: a kilogram of feathers or a kilogram of lead, on a sensitive scale, in air?

Air gives buoyancy to anything in it, and the feathers displace far more air. The scale reads the lead very slightly heavier; in a vacuum they tie.

What colour is the Sun, seen from space?

It peaks in green but emits across the whole visible band, which our eyes read as white. The yellow is what the atmosphere leaves after scattering the blue away.

A full glass of iced water sits on a table. When the ice melts, the water level:

Floating ice displaces exactly the weight of water it will become. Archimedes settled this; the glass does not spill.

Sound travels fastest through which of these?

Stiffer material, faster wave. Steel carries sound many times faster than air, and a vacuum carries none at all.