When a material is exposed to heat, the ultimate result—whether it transforms into a liquid or bursts into flame—is determined by a competition between two fundamental scientific processes. The outcome is not random but follows predictable rules based on the material’s internal structure and the energy required to change it. This dichotomy hinges on whether the applied thermal energy first triggers a physical rearrangement of molecules or a complete chemical transformation.
Melting A Physical Change of State
Melting is a physical process where a solid transitions into a liquid, known as a phase change. This transformation occurs when the added thermal energy increases the kinetic energy of the molecules to the point where they overcome the attractive forces between them. These attractive forces, called intermolecular forces, are relatively weak bonds like van der Waals forces or hydrogen bonds that hold molecules together in a fixed, crystalline structure.
As the material heats up, molecules vibrate more vigorously. Once the material reaches its specific Melting Point, the energy supplied is sufficient to break these intermolecular attractions, allowing the molecules to move freely past one another. Crucially, the chemical identity of the substance remains unchanged throughout this process; solid water (ice) and liquid water are both H₂O, only their state of matter is different. This transition is reversible, as removing the heat allows the molecules to slow down and reform the solid structure.
Burning A Chemical Reaction
Burning, scientifically known as combustion, is a fundamentally different process because it involves a chemical change. Combustion is a rapid oxidation reaction, meaning the material, or fuel, reacts quickly with an oxidizing agent, typically atmospheric oxygen. This reaction breaks the strong chemical bonds within the fuel molecules and forms new, more stable compounds, such as carbon dioxide and water. The process is highly exothermic, meaning it releases a significant amount of energy in the form of heat and light.
Before combustion can begin, the material requires an initial energy input, called the activation energy, to “jump-start” the reaction. Once this activation energy is overcome, the reaction becomes self-sustaining because the heat it generates can continue to activate nearby fuel molecules. This chemical transformation permanently alters the material’s composition, turning a complex substance like wood into simple ash and gases.
Comparing Critical Temperature Thresholds
The determination of whether a material melts or burns rests on a direct comparison between two thermal thresholds: the Melting Point (MP) and the Ignition Temperature (IT). The Ignition Temperature is the minimum temperature required to supply the necessary activation energy for the material to spontaneously combust without an external spark or flame. If a material’s Melting Point is lower than its Ignition Temperature (MP < IT), the material will melt before it burns. Metals, for instance, have extremely high ITs but will simply liquefy at their MP, remaining chemically intact. High-Density Polyethylene (HDPE) plastic illustrates this well, with a low MP between 129–140 °C, but an IT around 487 °C. Conversely, if the Ignition Temperature is lower than the Melting Point (IT < MP), the material will burn or decompose before it can ever form a liquid. Wood is a classic example of the burn-first scenario, as it begins to decompose and combust around 300 °C, far below the melting point of its primary component, carbon, which is over 3,500 °C. Some materials, like candle wax, exhibit both behaviors; they melt first, and this melted liquid then wicks up to the flame where it vaporizes and burns, illustrating a sequential interplay between the two thresholds.
How Chemical Composition Influences the Outcome
The inherent chemical composition of a material dictates the strength of its internal bonds, which directly determines its Melting Point and Ignition Temperature. Organic materials, such as wood, paper, and most plastics, are primarily composed of long chains of carbon and hydrogen atoms held together by covalent bonds. These carbon-rich structures are highly susceptible to oxidation, making them excellent fuels with relatively low Ignition Temperatures.
These complex organic chains often decompose, or break down into smaller gaseous molecules, before the entire structure can reach a true melting point. This decomposition process, called pyrolysis, creates the flammable gases necessary for combustion to begin.
Inorganic materials, like metals and mineral salts, are structured by much stronger metallic or ionic bonds. Breaking these powerful bonds requires far greater energy, resulting in extremely high Melting Points. Since inorganic materials also lack the readily oxidizable carbon-hydrogen framework, they are chemically resistant to combustion, giving them very high Ignition Temperatures. Therefore, materials with strong, non-carbon-based bonds will almost always melt when heated, while materials with complex, carbon-based bonds are chemically primed to burn.