點解係d 水入面加左salt 之後electrcal resistance 會減少左呢?

there are many types of salt ( not only the salt u eat everyday , but also chemical salt ) , for simply, I take table salt( i.e. NaCl ) as example
As u study chemistry,
u will know that NaCl is actually two compounds with Na+ ions and Cl- ions
when table salt is put into water, the attract force of water to Na+ ions and Cl- ions will be greater than the attract force between Na+ ions and Cl- ions
so Na+ ions and Cl- ions will become mobile ions ( the charge in the water remain unchanged ) (mobile ions flow freely in water )
when electricity pass through the solution
the mobile ions will help in transmit electricity through the solution
so the electrical resistance will be less than only water
(that's why pure water is a bad conductor of electricity!)