Is limewater a pure substance?

Limewater is not a pure substance. It is a mixture.

Limewater is the aqueous solution of calcium hydroxide. In other words, limewater is the mixture of calcium hydroxide and water.

Both calcium hydroxide and water are pure substances. They are compounds.

No. Limewater refers to calcium hydroxide solution, while a solution is always a mixture. Mixture is not pure.