Question

Adding soap to hot hard water gives foam but in cold hard water is do not ? why?

Answer 1

The soap does not give foam with hard water because, the Ca⁺² ions of the cold hard water makes a scum with the soap molecules which precipitates out and prevents from making foam

But, when the hard water is heated, the hot water makes a compound CaCO₃ which is soluble and separates out and the Ca⁺² ions are not left free to react with the soap molecule and so the soap molecules make a little foam in hot hard water.