What is the primary compositional difference that leads to the classification of Uranus and Neptune as 'Ice Giants'?

Answer

They have a much higher proportion of heavier, 'icy' elements.

Uranus and Neptune are designated 'ice' giants because, unlike Jupiter and Saturn, their composition includes a much higher concentration of heavier, 'icy' elements like oxygen, carbon, and nitrogen.

What is the primary compositional difference that leads to the classification of Uranus and Neptune as 'Ice Giants'?
waterMaterialgasicefrozen