The boiling point of any substance is also known as its saturation temperature. This property refers to point at which liquid vapour pressure equals pressure surrounding a liquid, which then changes to vapour. A liquid within a vacuum has a lower BP compared to when it is at atmospheric pressure. At high pressure, the liquid will have a higher BP than when at atmospheric pressure.
The boiling point of any liquid, therefore, varies inversely with surrounding environmental pressure. A normal boiling point refers to the BP of a liquid at pressure of 1 atmosphere. At any particular temperature, this property varies inversely with its vapour pressure. This implies that liquids having highest vapour pressures end up having normal boiling points that are very low.
For a particular chemical compound in its pure form, a normal BP may serve as an indication of its overall volatility. Such a compound only has one normal point of boiling, which in conjunction with the melting point may act as physical properties of characteristic nature. The normal boiling point varies inversely as the volatility of a given compound.
There are certain compounds that decompose at temperatures which are higher prior to reaching the normal BP, or at times even the melting point. The boiling point of any stable compound ranges between its triple and critical point, but this depends on external pressure.
The normal BP is usually higher than the MP of a particular substance, beyond the triple point. Beyond the critical point, liquid and vapour phases of a liquid merge into singular phase, which can be termed super-heated gas. If the normal BP of a specific compound is lower at a particular temperature, in general, it exists at atmospheric external pressure as gas. When the normal boiling point gets higher that substance then exists in solid or liquid form at that particular temperature, and atmospheric external pressure.
The compound exists with its vapour in equilibrium if volatile, when the vapours are contained. Certain volatile substances can gradually evaporate away when the vapours fail to be contained. This also occurs even when a compound has higher boiling point.