The state of a substance at a given temperature depends on its melting point and boiling point. Below the melting point a substance is a solid. Between the melting point and the boiling point it is a liquid. Above the boiling point it is a gas. A substance whose melting point is far above room temperature, such as 1000 °C, is a solid at room temperature.
For example, suppose a substance has a melting point of -20 °C and a boiling point of 80 °C. At 25 °C it is above the melting point but below the boiling point, so it is a liquid. At 100 °C it is a gas. Remember that -50 °C is lower than -20 °C, so at -50 °C the substance is a solid. Always compare the temperature with both values.
The state symbols (s), (l) and (g) are used in equations to show the solid, liquid and gas states, and (aq) means dissolved in water. Heating a substance past its melting point and then its boiling point changes it from solid to liquid to gas.