why does sea water freeze below [0] zero degree celsius?

Water ordinarily freezes at 0 degrees Celsius or 32 degrees Fahrenheit. In a system at this temperature, the rate of freezing is the same as the rate of melting hence system attains equilibrium. But sea water is not regular water and contains salt in it. The salt causes the rate of freezing to occur slower than the rate of melting, causing the ice to melt and requires a lower temperature to reattain equilibrium. Hence sea water freeze below [0] zero degree Celsius.