Which of these is NOT an example of a pure substance? A a tablet of the element known as calcium B a glass of muddy water
C a glass of ice and water
D a chemical compound known as aluminium oxide

a glass of muddy water

Explanation:

Pure substances contain only one element or compound.

Calcium is one element so it is a pure substance. Ice and water have the same molecular structure, so it is a pure substance. Aluminum oxide is a chemically pure compound.

The only example that is not a pure substance is muddy water due to the multiple components of mud mixing with the components of water.

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answer: ice crystals in air

the water from the water body or another source of water gets evaporated as water vapor as an effect of heat on water. these water vapor rises in the upper layers of the earth atmosphere under high atmospheric pressure. the atmospheric pressure of these water molecules starts to decrease due to the effect of cold air. the invisible water vapor condenses into visible water droplets and ice crystals.these ice crystals and water droplets being less denser than the surrounding air floats on air as clouds.