Which of the following statements is true regarding non-combustible substances?

The statement that non-combustible substances may still burn under certain conditions is accurate. Non-combustible materials, by definition, are those that do not readily ignite or sustain combustion; however, this does not mean they are completely immune to burning. Under extreme conditions, such as high temperatures or the presence of strong reactive agents, non-combustible substances can experience thermal decomposition or chemical reactions that result in combustion.

For example, some non-combustible metals can melt or oxidize when subjected to intense heat, which can lead to the release of flammable gases or even the material itself burning in a controlled environment. This characteristic highlights the complexity of material behavior under varying conditions and reinforces the understanding that "non-combustible" does not equate to "non-flammable" under every circumstance.

The other options do not accurately reflect the properties of non-combustible substances. They do not always ignite easily, as this contradicts the very definition of being classified as non-combustible. Additionally, while some non-combustible substances may not evaporate under normal conditions, many can, and they certainly do not fall into the category of substances that are typically gases.