In quality indicators of raw materials, some terms are often encountered. An accurate understanding of its meaning helps to better grasp performance of raw materials. Now list some commonly used terms.

Density and relative density-Density refers to mass contained in unit volume of a substance, in short, ratio of mass to volume, and its unit is million grams/m3 (Mg/m3) or kilograms/m3 (kg/ m3) or grams/cm3 (g/cm3).
Relative density, also known as density ratio, refers to ratio of density of substance to density of reference substance under their respective specified conditions, or ratio of mass of a certain volume of material at temperature t1 to mass of an equal volume of reference material at temperature t2. Commonly used reference substance is distilled water, and it is expressed by Dt1/t2 or t1/t2, which is a dimensionless quantity.

Melting point and Freezing point——Temperature at which a substance reaches equilibrium between liquid and solid under its vapor pressure is called melting point or freezing point.
This is due to fact that regular arrangement of atoms or ions in solid is activated due to temperature rise and thermal movement becomes disorderly, forming an irregular arrangement of liquid. Opposite process is solidification. Temperature at which a liquid becomes a solid is often called freezing point or freezing point. Difference from melting point is that it emits heat instead of absorbing heat. In fact, melting point and freezing point of substance are same.

Refers to temperature range from beginning of melting of substance to full melting measured by capillary method.

Refers to phase transition temperature at which a liquid changes from a liquid to a solid during cooling process.

One of indicators indicating nature of liquid petroleum products. It means that sample is cooled to temperature at which it starts to stop flowing under standard conditions, that is, the lowest temperature at which sample can be poured when it is cooled.

Temperature at which a liquid boils when heated and becomes a gas. In other words, temperature at which liquid and its vapor are in equilibrium. Generally speaking, the lower boiling point, the greater volatility.

Under standard conditions (1013.25hPa, 0℃), distilled volume within temperature range specified by product standard.

A phenomenon in which a solid (crystalline) substance is directly transformed into a gaseous state without passing through a liquid state. Such as ice, iodine, sulfur, naphthalene, camphor, mercuric chloride, etc. can all be sublimated at different temperatures.

Evaporation refers to phenomenon of vaporization that occurs on the surface of a liquid. Evaporation rate is also known as volatilization rate. It is generally judged by boiling point of solvent. Fundamental factor that determines evaporation rate is vapor pressure of solvent at that temperature, followed by molecular weight of solvent.

Vapor pressure is abbreviation for saturated vapor pressure. At a certain temperature, liquid and its vapor reach equilibrium, equilibrium pressure at this time only changes due to nature and temperature of liquid, which is called saturated vapor pressure of liquid at that temperature.

Constant boiling point mixture formed by two (or several) liquids is called an azeotropic mixture, which refers to a mixed solution when composition of gas phase and liquid phase are exactly same in an equilibrium state. Corresponding temperature is called azeotropic temperature or azeotropic point.

Refractive index is a physical quantity that represents ratio of speed of light in two different (isotropic) media. Speed of light varies from medium to medium. When light enters another transparent medium with a different density from one transparent medium, it changes in direction of progress due to change in speed, so it is called refraction.
Ratio of sine of incident angle of light to sine of refraction angle, or ratio of speed of light when passing through a vacuum and when passing through a medium, is refractive index. Generally expressed refractive index n refers to value of light entering any medium from air. Refractive index usually referred to is measured by sodium yellow light (D line) at tC, so it is expressed by ntD. If measured at 20℃, it is n20D.

Flash point, also known as ignition flash point, represents one of indicators of properties of flammable liquids. It refers to the lowest temperature at which flammable liquid is heated to minimum temperature when mixture of vapor pressure and air on liquid surface contacts flame to cause a flash fire. Flash is usually a light blue spark, which will go out at a flash and cannot continue to burn.
Flashover is often harbinger of a fire. There are open cup method and closed cup method for measuring flash point. Generally, the former is used to measure high flash point liquids, and the latter is used to measure low flash point liquids.

Ignition point is also known as ignition point, which represents one of indicators of properties of flammable liquids. It refers to the lowest temperature at which flammable liquid is heated to surface of vapor and air mixture, flame immediately catches fire and can continue to burn. Ignition point of flammable liquid is 1～5℃ higher than flash point. The lower flash point, the smaller difference between ignition point and flash point.

The lowest temperature at which a combustible substance can cause a fire without contact with an open flame is called spontaneous ignition point. The lower self-ignition point, the greater risk of fire. Spontaneous ignition point of same substance varies with pressure, concentration, heat dissipation, other conditions and test methods.

Combustible gas, vapor of combustible liquid or dust of combustible solid mixed with air or oxygen to reach a certain concentration range at a certain temperature and pressure, it will explode when it encounters a fire source. This certain concentration range is called explosion limit or combustion limit. If composition of mixture is not within this certain range, no matter how much energy is supplied, it will not catch fire.
Minimum concentration at which vapor or dust mixes with air and reaches a certain concentration range, will burn or explode when encountering a fire source is called lower explosion limit; the highest concentration is called upper explosion limit. Explosion limit is usually expressed by volume percentage of vapor in the mixture, that is,% (vol); dust is expressed by concentration of mg/m3.
If concentration is lower than lower explosion limit, although open flame will not explode or burn, because air accounts for a large proportion at this time, concentration of combustible vapor and dust is not high; if concentration is higher than upper explosion limit, there will be a lot of combustible substances, but there is a lack of oxygen to support combustion. Without air supplement, it will not explode even if it encounters an open flame. Flammable solvents have a certain explosion range, and the wider explosion range, the greater danger.

Viscosity is internal frictional resistance of a fluid (liquid or gas) in flow, its size is determined by factors such as substance type, temperature, and concentration. It is generally abbreviation for dynamic viscosity, and its unit is Pa·s (Pa·s) or milliPa·s (mPa·s).