Filter (chemistry)
In chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 and common usage, a filter is a device (usually a membrane or layer) that is designed to physically block certain objects or substances while letting others through. Filters are often used to remove solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...

 substances suspended
Suspension (chemistry)
In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...

 in fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

s, for example to remove air pollution
Air pollution
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....

, to make water drinkable, and to prepare coffee
Coffee is a brewed beverage with a dark,init brooo acidic flavor prepared from the roasted seeds of the coffee plant, colloquially called coffee beans. The beans are found in coffee cherries, which grow on trees cultivated in over 70 countries, primarily in equatorial Latin America, Southeast Asia,...

. Some devices that are called filters may also carry out other processes, such as waste treatment, (e.g. biofilter
Biofiltration is a pollution control technique using living material to capture and biologically degrade process pollutants. Common uses include processing waste water, capturing harmful chemicals or silt from surface runoff, and microbiotic oxidation of contaminants in air...

). Several types of filters are used in chemistry in order to facilitate separation, thereby purifying a liquid (or gas). Many filters use gravity, or gravity enhanced by vacuum (suction) in order to create this separation, often through a funnel-shaped device
Funnels (laboratory)
Laboratory funnels are funnels that have been made for use in the chemical laboratory. There are many different kinds of funnels that have been adapted for these specialized applications. Filter funnels, thistle funnels , and dropping funnels have stopcocks which allow the fluids to be added to a...

. Filter efficiency can be improved in a number of ways, such as with the use of fluted filter paper. Other types of materials may be used to effect separations based on size, similar to filters, such as molecular sieves.

The process of passing a mixture through a filter is called filtration
Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...

. The liquid produced after filtering a suspension of a solid in a liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 is called filtrate, while the solid remaining in the filter is called retentate, residue, or filtrand.

Filter media

There are two types of filter media — a surface filter, a solid sieve which traps the solid particles, with or without the aid of filter paper
Filter paper
Filter paper is a semi-permeable paper barrier placed perpendicular to a liquid or air flow. It is used to separate fine solids from liquids or air.-Properties:Filter paper comes in various porosities and grades depending on the applications it is meant for...

 (e.g. Büchner funnel
Büchner funnel
A Büchner funnel is a piece of laboratory equipment used in suction filtration. It is traditionally made of porcelain, but glass and plastic funnels are also available. On top of the funnel-shaped part there is a cylinder with a perforated plate separating it from the funnel. The Hirsch funnel has...

, Belt press
Belt press
Belt printing is a method for printing. A belt printer uses huge screens that cover the entire front and back of the garment. The method is best for all-over pattern printing, but works best on designs with limited colors.It was largely popular in the 1970s....

, Rotary vacuum-drum filter
Rotary vacuum-drum filter
Rotary vacuum filter drum consists of a drum rotating in a tub of liquid to be filtered.The technique is well suited to high solids liquids that would blind or block other forms of filter. The drum is pre-coated with a filter aid, typically of diatomaceous earth or Perlite. After pre-coat has...

, Crossflow filters), and a depth filter, a bed of granular material which retains the solid particles as it passes (e.g. sand filter
Sand filter
Sand filters are used for water purification. There are three main types;# rapid sand filters# upflow sand filters# slow sand filtersAll three methods are used extensively in the water industry throughout the world...

). The first type allows the solid particles, i.e. the residue, to be collected intact; the second type does not permit this. However, depth filters are less prone to clogging due to the greater surface area where the particles can be trapped. Also, when the solid particles are very fine, it is often cheaper and easier to discard the contaminated granules than to clean the solid sieve.

Filter media can be cleaned by rinsing with solvents or detergents. Alternatively, in engineering applications, such as swimming pool water treatment plants, they may be cleaned by backwashing.


For certain situations, such as separating an aqueous substance from a solid one, filtering may be replaced with decanting or centrifugation
A centrifuge is a piece of equipment, generally driven by an electric motor , that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis...

. Magnets can be used to remove magnetic contaminants from lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...

 oils etc. Separation based on differences in physical and/or chemical properties can be achieved through a variety of techniques, including chromatography
Chromatography is the collective term for a set of laboratory techniques for the separation of mixtures....


See also

  • Tubular fluid
    Tubular fluid
    Tubular fluid is the fluid in the tubules of the kidney. It starts as a renal ultrafiltrate in the glomerulus, changes composition through the nephron, and ends up as urine leaving through the ureters.-Composition table:...

    , the renal glomerulus
    A glomerulus is a capillary tuft that is involved in the first step of filtering blood to form urine.A glomerulus is surrounded by Bowman's capsule, the beginning component of nephrons in the vertebrate kidney. A glomerulus receives its blood supply from an afferent arteriole of the renal...


Filters for various substances:
  • Water filter
  • Air filter
    Air filter
    A particulate air filter is a device composed of fibrous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air. A chemical air filter consists of an absorbent or catalyst for the removal of airborne molecular contaminants such as volatile organic compounds...

  • Oil filter

Filters for specific applications:
  • Pneumatic filter
    Pneumatic filter
    A pneumatic filter is a device which removes contaminants from a compressed air stream. This can be done using a number of different techniques, from using a "media" type that traps particulates, but allows air to pass through to a venturi, to a membrane that only allows air to pass through.-...

  • Coffee filter
    Coffee filter
    A coffee filter is a coffee-brewing utensil, usually made of disposable paper. A stainless steel filter is used to prepare Indian filter coffee, the form of coffee common in India....

  • Filter mask

Filters by various methods:
  • Disk filter
    Disk filter
    A disk filter is a type of water filter used primarily in irrigation, similar to a screen filter, except that the filter cartridge is made of a number of disks stacked on top of each other like a pile of poker chips. The water passes through the small grooves in between and the impurities are...

  • Paper filter
    Filter paper
    Filter paper is a semi-permeable paper barrier placed perpendicular to a liquid or air flow. It is used to separate fine solids from liquids or air.-Properties:Filter paper comes in various porosities and grades depending on the applications it is meant for...

  • Media filter
    Media filter
    A media filter is a type of filter that uses a bed of sand, crushed granite or other material to filter water for drinking, swimming pools, aquaculture, irrigation, stormwater management and other applications.-Filter Design:...

  • Membrane filter
    Membrane technology
    The membrane technology covers all process engineering measures for the transport of substances between two fractions with the help of permeable membranes...

  • Cross-flow filtration
    Cross-flow filtration
    In chemical engineering, biochemical engineering and protein purification, crossflow filtration is a type of filtration...

  • Sand separator
    Sand separator
    A hydrocyclone is a device to classify, separate or sort particles in a liquid suspension based on the ratio of their centripetal force to fluid resistance. This ratio is high for dense and coarse particles, and low for light and fine particles...

  • Screen filter
    Screen filter
    A screen filter is a type of filter using a rigid or flexible screen to separate sand and other fine particles out of water for irrigation or industrial applications...

  • Sand filter
    Sand filter
    Sand filters are used for water purification. There are three main types;# rapid sand filters# upflow sand filters# slow sand filtersAll three methods are used extensively in the water industry throughout the world...

Related devices and concepts:
  • Activated carbon
    Activated carbon
    Activated carbon, also called activated charcoal, activated coal or carbo activatus, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.The word activated in the name is sometimes replaced...

  • Artificial membrane
    Artificial membrane
    An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of twentieth century. A...

  • Non-Filter
    Non-Filter or Non Filter is a category of air purification system created by Microgenix Technology Ltd., a UK-based company that invented the technology....

     (trade name).
  • Gas mask
    Gas mask
    A gas mask is a mask put on over the face to protect the wearer from inhaling airborne pollutants and toxic gases. The mask forms a sealed cover over the nose and mouth, but may also cover the eyes and other vulnerable soft tissues of the face. Some gas masks are also respirators, though the word...

  • Sieve
    A sieve, or sifter, separates wanted elements from unwanted material using a woven screen such as a mesh or net. However, in cooking, especially with flour, a sifter is used to aerate the substance, among other things. A strainer is a type of sieve typically used to separate a solid from a liquid...

  • Colander
    A colander is a bowl-shaped kitchen utensil with holes in it used for draining food such as pasta or rice.The perforated nature of the colander allows liquid to drain through while retaining the solids inside...

  • Reverse osmosis
    Reverse osmosis
    Reverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...

  • Filter feeder
    Filter feeder
    Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...

  • Filter binding assay
    Filter binding assay
    In biochemistry or chemistry, one of the ways to learn about an interaction between two molecules is to determine the binding constant, which is a number that describes the ratio of unbound and bound molecules...

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