If a substances freezes at - 58 degree Celsius, does it have to melt at the same temperature too?
The melting point and the freezing point of most substances is the same. This means that the transition from the solid state to the liquid state occurs at the same temperature as the transition from the liquid state to the solid state.
But many substances display a property called hysteresis, which changes the temperature at which they freeze and that at which they melt. For instance a substance could have a freezing point of 0 degree Celsius and a melting point of 100 degree Celsius. To melt the substance has to be heated till its temperature is 100 degree C. Once it is a liquid it continues to remain in that state till the temperature is brought all the way down to 0 degree C. If the temperature is changed to any value between 0 degree C and 100 degree C, the substance continues to stay in the state that it had earlier.
The substance you have mentioned in the question may have the same melting point as the freezing point of -58 degree Celsius, or if it displays hysteresis, the two may be different.