Every pure substance has a fixed melting point and boiling point. To predict its state at a given temperature, compare that temperature with the two values.
Below the melting point the substance is a solid. Between the melting point and the boiling point it is a liquid. Above the boiling point it is a gas. Exactly at the melting point both solid and liquid are present, and exactly at the boiling point both liquid and gas are present.
For example, ethanol melts at −114°C and boils at 78°C. At 25°C, which is above its melting point and below its boiling point, ethanol is a liquid. Oxygen melts at −219°C and boils at −183°C, so at 25°C, which is well above its boiling point, oxygen is a gas. Sodium chloride melts at 801°C and boils at 1413°C, so at 1000°C it is a liquid.
Data is often given in a table. Always write the numbers in order on a line, with the melting point and boiling point marked, and see where the temperature falls. Bromine melts at −7°C and boils at 59°C, so it is a solid at −20°C, a liquid at 20°C and a gas at 80°C.