When plaque becomes heavy and hardened, it is called dental calculus or

Tartar.

Explanation:

Dental plaque may be defined as the development of mass or biofilm of bacteria that grows on teeth surface. This plaque may present on teeth, between the teeth and on teeth front.

The plaque is first colorless but when it changes to dental calculus or tartar, the color of plaque becomes yellow or pale brown. This causes dental problem and different dental disease.

Thus, the answer is plaque.

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