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An element is a pure substance that is made up of similar type of atoms.

A compound is formed when more than one kind of atom combine in a definite ratio. The components of a compound cannot be separated using physical methods.

Two or more substances are present in a mixture, but no definite ratio is maintained. The components of a mixture can be separated using physical methods like distillation, filtration, sedimentation, sublimation, etc.

1. By the definition of compounds, we can say that calcium chloride is a compound as it is made by combining two different elements, calcium and chlorine, in a fixed mass ratio of 1:2.
2. As Sulfur consists of only sulfur atoms combined into octatomic molecules so it is an element.
3. As Baking powder does not have a fixed ratio of its components so it is a mixture.
4. As in cytosine atoms of different elements are bonded together so it is a compound.