Gram per litre
Encyclopedia
A gram per liter or litre (g/L) is a unit of measurement of mass concentration that shows how many gram
Gram
The gram is a metric system unit of mass....

s of a certain substance
Chemical substance
In chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. They can be solids, liquids or gases.Chemical substances are...

 are present in one litre
Litre
pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...

 of a usually liquid or gaseous mixture. It is not an SI unit because it contains the non-SI unit litre. The SI unit of mass concentration is kilogram per cubic meter, which is numerically equivalent (1 g/L = 1 kg/m³).

SI-style prefixes are often applied, giving units like milligrams per litre (mg/L) or micrograms per decilitre (μg/dL or mcg/dL). When measuring concentration in water, parts per million is an older expression of mg/L, since one liter of water under standard conditions
Standard conditions for temperature and pressure
Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...

 weighs one kilogram or one million milligrams.

Milligrams per litre are often used in medicine
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

 and in medical prescription
Medical prescription
A prescription is a health-care program implemented by a physician or other medical practitioner in the form of instructions that govern the plan of care for an individual patient. Prescriptions may include orders to be performed by a patient, caretaker, nurse, pharmacist or other therapist....

s. For example, a description of a solution that involves two substances
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

, where one of the substances involves adding water, would state: "10 mg/L water and substance"
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