Properties of chemicals
A chemical substance has properties that determine its behaviour in different situations. This paragraph explains the properties used in this manual and relevant for chemical spill response and risk assessment.
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Physical state
A chemical substance can either be solid, liquid or gaseous. This is called the physical state of the substance. The physical state can be changed reversibly by increasing or decreasing the temperature or pressure.
At temperatures above the boiling point of a substance, it exists in gas form. Below the freezing point, a substance is solid. Between these temperatures it is liquid.
In this manual, only the physical state of a substance at normal (ambient) temperature and pressure is considered, since a spilled substance will after release be subject to these conditions.
Vapour pressure/volatility
As mentioned before, above its boiling point a liquid becomes a gas or vapour. However at lower temperature some of the liquid will vaporise by what is called evaporation. The speed of evaporation depends on the vapour pressure of the liquid as set out in the following list:
| Vapour pressure | Evaporation speed |
| |
Evaporates very slow |
| |
Evaporates |
| > 3.000 Pa | Evaporates fast |
| > 100.000 Pa | Gas |
We consider substances with a vapour pressure >3.000 Pa at ambient temperatures to be very volatile. Substances with lower vapour pressures will evaporate slower and thus are less volatile. The vapour pressure of a substance depends on its temperature. Warm liquid has a higher vapour pressure and thus evaporates faster.
For instance, at 20 °C the vapour pressure of methanol is just below 13.000 Pa, for water it is less than 2.400 Pa. Thus methanol evaporates quickly fast, and water evaporates slowly, the vapour pressure of hydrocarbons normally decreasing or increasing by a factor 1.5 for each 10 degrees Celsius.
Density
The density of a substance is the mass per unit volume. It is expressed in kg/m3. Substances with a higher density than water will sink and substances with a lower density than water will float. The density of seawater depends on the salinity and temperature. For the North Sea a density of 1.023 kg/m3 is mostly used. A substance like palm oil (923 kg/m3) will float while phosphoric acid (1.829 kg/m3) will sink, though it will dissolve.
The density of a gas is referred to as vapour density. For a solid the correct term is bulk density.
Density is slightly temperature dependent.
Viscosity
The viscosity of a substance is a measure for its resistance to flow. Substances with a high viscosity are syrup like, and flow slowly. Substances with a low viscosity flow easily like water. Viscosity is temperature dependant.
Honey does not flow easily and is therefore a substance with a high viscosity. If it it is warmed, it flows more easily meaning the viscosity has been lowered. The viscosity of thixotropic substances also depends on agitation. Thus, stirring such substances will lower the viscosity, as will wave action. If such substance are collected from the water surface and put in a tank their viscosity could increase drastically to the detriment of transfer pumping.
Solubility and miscibility
A solid or gas (solutes) can be dissolved in water (solvent) forming a solution. If more and more substance is added the solution will at some point be saturated, meaning it will not dissolve any more solute.
The amount of a substance that can be dissolved in water (or another solvent) is called the solubility.
Solubility can either be expressed in grams of solute per grams of solvent, or in grams of solute per volume of solvent.
When dealing with chemical spills at sea, the latter is more relevant since the amount of solvent is infinitive.
When solubility is expressed in kg/m3, a solute that completely dissolves has solubility equal to its density.
When mixing two liquids, the correct term is miscibility. For instance, ethanol is fully miscible with water. But water and oil are immiscible. Some substances are partly miscible.
pH
The pH scale is a scale between 0 and 14, expressing the acidity of a solution. A pH value of 7 means the solution is neutral. pH values less than 7 are increasingly acidic. pH values above 7 are increasingly basic.
The pH value of seawater is around 8 (between 7,5 and 8,4). Substances that are acidic are such as sulphuric acid and substances that are basic are such as sodium hydroxide (caustic soda).
Flammability
The flammability of a substance is a qualitative term describing the tendency of a substance to burn. It is expressed in a quantitative way as flash point, fire point, or auto-ignition temperature.
Flash point
The flash point of a liquid is the temperature at which it gives off enough vapour to create a flammable mixture with air. In the transport sector a distinction is made in substances with a flash point lower than 61 ˚C and above. For substances with a flash point lower than 61 ˚C special precautions need to be taken as such substances have a high flammability and/or explosion risk
Fire point
The fire point is the temperature at which the heat resulting from a fire is high enough to generate a vapour that is flammable. i.e. At temperatures above the fire point, a burning vapour is sustained because of the vapours created by the heat of burning. Normally the fire point of a hydrocarbon is about 10 degrees higher than the flash point.
Auto-ignition temperature
The auto-ignition temperature of a substance is the temperature at which it will ignite spontaneously. For hydrocarbons this temperature is normally above 200 ˚C.
Explosive limits
Flammable gas or vapour has to be in the right mixture with air (oxygen) to be able to ignite. The lower explosive limit (L.E.L.) and the upper explosive limit (U.E.L.) identify the concentrations between which the gas or vapour is capable of ignition.
Concentrations above the U.E.L. are too rich; Concentrations below the L.E.L. are too poor to ignite.
