The boiling point is the temperature at which a liquid changes to a gas. At this point, the liquid’s vapor pressure equals the surrounding atmospheric pressure, allowing bubbles of vapor to form inside the liquid and rise to the surface. Below this temperature, evaporation can occur at the surface but the liquid doesn’t boil. The other terms refer to different ideas: vapor pressure is the pressure that vapor in equilibrium with the liquid exerts at a given temperature; melting point is the temperature where a solid becomes a liquid; and flash point is the lowest temperature at which a liquid can produce enough vapor to ignite in the presence of an ignition source, not the temperature at which it becomes a gas.

The term that fits the transport context is hazardous material. In transportation safety, anything that due to its properties could pose a risk to health, safety, or property when moved is labeled as a hazardous material. This designation triggers specific regulatory requirements: proper packaging, correct labeling and placarding, shipping papers, trained personnel, and emergency response information. That’s why this term is the precise, official way to describe materials that require special handling during transport. The other terms aren’t the standard regulatory language used for hazmat in transit. Shortened “Haz” isn’t a recognized label, “Dangerous Substance” isn’t the official term (the common international phrasing is “dangerous goods” or “hazardous materials”), and “Regulatory Item” doesn’t describe hazmat at all.

Specific gravity tells you how dense a liquid is compared with water. An SG of 0.8 means the liquid’s density is 0.8 times that of water. Since weight per unit volume (weight density) at the same gravity scales with density, this liquid will weigh 0.8 times as much per unit volume as water. So it weighs less than water. If SG were greater than 1, it would weigh more per unit volume; if it were 1, it would weigh the same.

Specific gravity is a way to compare how heavy a material is relative to water using the same volume. It’s the ratio of the material’s density (or weight per volume) to the density (or weight per volume) of water, typically with water’s density used as the reference at 4°C. Because it’s a ratio of two weights (or two densities) under the same gravity, the value is dimensionless. In practice, this means materials with an SG greater than 1 are heavier than water and tend to sink, while those with an SG less than 1 tend to float. This concept is captured by describing it as the ratio of the weight of a volume of the material to the weight of an equal volume of water. The other ideas—density at room temperature, rate of vaporization, or color—do not define specific gravity.