CHEMISTRY: PROPELLANTS
Overview- A propellant is a material that is used to propel an object
- The object is usually expelled by the pressure created by a gas
- This pressure may be created by a compressed gas or by a gas produced by a chemical reaction
- Propellants may be solids, liquids, gases or plasmas
- Common chemical propellants consist of a fuel and an oxidiser
Types of propellants
- Aerosol sprays
- Aerosol spray is a dispensing system that creates an aerosol (fine) mist of liquid particles
- In aerosol sprays, the propellant is simply a pressurised gas in equilibrium with its liquid form
- As some gas escapes to expel the payload, more liquid evaporates thereby maintaining an even pressure
- The aerosol spray can was invented by Erik Rotheim (Norway) in 1927
- Aerosol sprays are typically used to dispense insecticides, deodorants and paints
- Propellants used for propulsion
- Rockets typically use bipropellants, which contain a combination of a fuel and an oxidiser. Tripropellants, which are not used commonly, use liquid hydrogen as a third component to provide additional efficiency
- Propellants are usually made from low explosives, which deflagrate (burn) rather than detonate (explode)
- The controlled burning of the propellants produces thrust by gas pressure which is then used to accelerate a rocket, projectile or other vehicles
- Propellants are commonly used in rockets, firearms and artillery
Solid propellants
- Solid propellants are used for rockets, firearms and artillery
- Examples of solid propellants include gunpowder (sulphur + charcoal + potassium nitrate), nitrocellulose and cordite
- Single based propellants: They have nitrocellulose as its chief ingredient. Stabilizers and other chemicals may be added for chemical stability
- Double based propellants: they contain nitrocellulose with nitroglycerin or other liquid nitrate explosives added. Nitroglycerin reduces smoke and increases energy output. Used in small arms, cannons, mortars and rockets
- Triple based propellants: consist of nitrocellulose, nitroquanidine, and nitroglycerin or other nitrate explosives. Used in cannons
- Composite propellants: consist of a fuel such as metallic aluminium, a binder such as synthetic rubber and an oxidiser such as ammonium perchlorate. Used in large rocket motors such as spacecraft
- Solid propellants have been used since the 11th century to power rockets based on gunpowder
- Solid fuel rockets offer ease of handling, reliability and long storage periods
- Solid fuel rockets are used for missiles due to their long storage periods and reliability of launch on short notice
- Currently, solid fuel rockets are not used for space explorations, but are commonly used as booster rockets to launch spacecraft
Liquid propellants
- Liquid propellants are usually used in combinations of fuel and oxidiser
- Common liquid propellant combinations include
- Liquid oxygen and liquid hydrogen
- Liquid oxygen and kerosene
- Nitrogen tetraoxide and kerosene
- Liquid fuel rockets are desirable because they offer higher energy output, they can be throttled and shut down and can be reused
- Liquid fuel rockets are used to power space shuttles
- A variant of liquid fuel engine is cryogenic fuel engine – these are engines that use gases which are super-cooled into their liquid forms
Propellants used in the PSLV
- The Polar Satellite Launch Vehicle (PSLV) has a four stage propulsion system, using solid and liquid propellants alternately
- First stage: solid – Hydroxyl terminated polybutadiene (HTPB)
- Second stage: liquid – unsymmetrical di-methyl hydrazine (UDMH) as fuel and nitrogen tetraoxide as oxidiser
- Third stage: solid – HTPB
- Fourth stage: solid – mono methyl hydrazine as fuel and mixed oxides of nitrogen as oxidiser
Propellants used in the GSLV
- The Geosynchronous Satellite Launch Vehicle (GSLV) is a three stage launch vehicle using solid, liquid and cryogenic propellants
- First stage – solid – HTPB
- Second stage – liquid – UDMH as fuel and nitrogen tetraoxide as oxidiser
- Third stage – cryogenic – liquid hydrogen and liquid oxygen
CHEMISTRY: EXPLOSIVES
Overview
- An explosive is a substance that contains a great deal of stored energy that can produce an explosion, usually accompanied by the production of light, heat and pressure
- The energy stored in an explosive material may be
- Chemical energy such as nitroglycerine
- Pressurised compressed gas such as a gas cylinder or aerosol can
- Nuclear energy such as Uranium and plutonium
CHEMICAL REACTIONS IN EXPLOSIVES
- Deflagration
- Deflagration is a term that describes subsonic combustion that propagates through thermal conductivity
- Deflagration is easier to control and so is used when the goal is to move an object with the force of expanding gas
- Examples of deflagration include gas stove, internal combustion engine, gunpowder, pyrotechnics etc
- Detonation
- Detonation is a combustion process in which a supersonic shock wave through the body of a material
- In detonation, a supersonic shock wave originating at the point of ignition compresses the surrounding material, thus increasing its temperature to the point of ignition
- Because detonations generate high pressures, they are much more destructive than deflagrations
- Detonations are difficult to control and are used primarily for demolition and in warfare.
- Examples of detonation includes high explosives, oxygen-methane mixture
CLASSIFICATION OF EXPLOSIVES
- High explosives
- Materials that explode faster than the speed of sound are called high explosives
- This type of explosion is known as detonation
- Used in mining, demolition and military applications
- Low explosives
- Materials that explode slower than the speed of sound are called low explosives.
- This type of explosion is known as deflagration
- Used as propellants, gun powder, pyrotechnics (such as flares and fireworks)
- Primary explosives
- A primary explosive is an explosive that is extremely sensitive to stimuli. These stimuli include impact, friction, heat, static electricity and electromagnetic radiation
- For primary explosives, a relatively small amount of energy is required for initiation of explosion
- In general, primary explosives are considered to be those explosives that are more sensitive than PETN
- Used in detonators to trigger larger charges of more stable secondary explosives
- E.g.: Mercury fulminate, Nitrogen trichloride, acetone peroxide, ammonium permanganate
- Secondary explosives
- Secondary explosives are less sensitive than primary explosives and require more energy to be initiated
- They are safer to handle and store
- In general, secondary explosives are considered to be those explosives that are less sensitive than PETN
- Secondary explosives are usually used in large quantities and are initiated by small amounts of primary explosives
- E.g.: TNT, RDX
- Secondary explosives are less sensitive than primary explosives and require more energy to be initiated
SOME COMMON EXPLOSIVES
- Trinitrotoluene (TNT)
- TNT is a useful explosive material with convenient handling properties. TNT is sometimes also used as a reagent in chemical synthesis
- TNT was first prepared by Joseph Wilbrand (GermanY) in 1863
- The explosive yield of TNT is considered to be the standard measure of strength of bombs and other explosives
- Sulphitation is a process used in the manufacture of TNT, specifically to stabilize the explosive
- TNT is one of the most commonly used explosives for industrial and military applications
- It is insensitive to shock and friction, reducing the occurrence of accidental detonation. TNT melts without exploding (allowing it to be combined with other explosives), does not absorb or dissolve in water (allowing use in wet environments) and is stable compared to other explosive
- TNT contains energy of 4.6 Mega Joules per kilogram (MJ/kg). By comparison gun powder contains 3 MJ/kg, dynamite contains 7.5 MJ/kg and gasoline contains 47.2 MJ/kg
- TNT is used as a reference for other explosives. Nuclear weapons have energy content measured in kilotonnes (kT) or megatonnes (MT) of TNT equivalent.
- TNT is usually used in mixture with other substances. E.g.: Amatol (TNT + ammonium nitrate)
- RDX
- RDX, chemically cyclotrimethylnetrinitramine, is also known as cyclonite and T4
- RDX is usually used in mixture with other explosives and plasticizers
- RDX is stable in storage and is considered one of the most powerful of military explosives
- RDX was discovered in 1898 by Goerg Friedrich Henning (Germany)
- Pentaerythritol tetranitrate (PETN)
- PETN is one of the most powerful high explosives known
- It is more difficult to detonate than primary explosives, but less stable than secondary explosives
- It is more sensitive than other high explosives, and is rarely used alone
- Usually used in small calibre ammunition, detonators of land mines
- PETN is an effective underwater explosive
- PETN is a major ingredient of Semtex (plastic explosive)
- PETN was first synthesised by Bernhard Tollens (Germany) in 1891
- PETN is one of the most powerful high explosives known
- Dynamite
- Dynamite is based on nitroglycerine
- It was invented by Alfred Nobel (Sweden) in 1867
- Used mainly for mining, quarrying, construction
- Dynamite was the first safely manageable explosive stronger than black powder
- Dynamite is based on nitroglycerine
- Plastic explosive
- Plastic explosives are explosives that are soft and can be moulded by hand
- Common plastic explosives include Semtex (Czech Republic) and C-4 (USA)
- Used mainly for demolition, also used by terrorists
- The first plastic explosive was Gelignite, invented by Alfred Nobel (Sweden) in 1875
- C-4 (composition 4) is made of RDX while Semtex is made from RDX and PETN
- Semtex became notoriously popular with terrorists because it is difficult to detect. Semtex was invented by Stanislav Berbera (Czech R.) in the 1950s
CHEMISTY: CERAMICS
Overview
- A ceramic is an inorganic, non-metallic solid prepared by the action of heating and subsequent cooling
- The earliest ceramic materials were pottery made from clay
- Ceramics are resistant to chemical erosion and high temperatures (up to 1600C)
PROPERTIES OF CERAMICS
- Mechanical properties
- Ceramic materials are usually formed by ionic or covalent bonds
- These materials tend to not be elastic and fracture easily
- Ceramics are also porous
- In general ceramics have poor toughness and have low tensile strength
- Electrical properties
- Some ceramics are semiconductors
- Semiconducting ceramics are made using zinc oxide
- Under extremely low temperatures, some ceramics exhibit superconductivity
- Most ceramics exhibit piezoelectricity i.e. the conversion of mechanical stress to electrical signals. This effect is commonly used in quartz watches
- Optical properties
- Ceramics (esp. those based on aluminium oxide) can be made translucent
- This has immediate applications in sodium-vapour lamps and dental restorations
- Ceramics can be made transparent with applications in laser technology
TYPES OF CERAMICS
- Structural ceramics such as bricks, pipes, floor, roof tiles etc
- Refractory ceramics such as kiln lining, steel and glass making crucibles
- Whitewares such as tableware, wall tiles, pottery, sanitary products
- Technical ceramics such as jet engine turbine blades, ballistic protection etc
- Milling
- Process by which materials are reduced in size
- Involves breaking of cemented material or pulverization
- Techniques used include ball mill, roll crusher, jaw crusher, wet attrition mills
- Batching
- Is the process of weighing the oxides according to recipes and preparing them for further processing
- Mixing
- Involves mixing the various components in the appropriate proportions
- Uses ribbon mixers, Mueller mixers and pug mills
- Forming
- This is the process of the making the mixed materials into desired shapes such as toilet bowls, spark plugs etc
- Forming techniques include extrusion, pressing and slip casting
- Drying
- Controlled heat is applied to dry the materials and obtain rigid shape
- Firing
- Dried parts are processed through a controlled heating process and oxides are chemically changed to cause sintering and bonding
BIO-CERAMICS
- Bacteria, plants and animals exhibit a tendency to form crystalline materials composed of silicon
- These bioceramics show exceptional physical properties such as strength, fracture resistance etc
- Bio-ceramics are usually made of proteins such as keratin, elastin, chitin and collagen
- The mother-of-pearl portion of marine shells exhibit the strongest mechanical strength and fracture toughness of any non-metallic substance known
APPLICATIONS OF CERAMICS
| Application | Ceramic components | Notes |
| Armoured vests | Alumina, boron carbide | Protects against high-calibre rifle fire |
| Dental implants, synthetic bone | Artificial hydroxyapatite (natural mineral of bone) | |
| Ball bearings | Silicon nitride | Harder, more resistant to heat than metal bearings |
| Earthenware | Kaolin, boll, flint | Opaque Used to make cups, saucers etc |
| Chinaware | Leached granite (to remove quartz and mica) | Translucent Resists scratching |
| Porcelain | Kaolin, feldspar, quartz | White, semi-opaque Highly resistant to scratching Stronger than glass |
| Stoneware | Kaolin, feldspar, quartz | Similar to porcelain but from poor grade raw materials Hard, infusible |
| Space shuttles | Extremely pure Silica | Used on the outer surface of shuttles to withstand heating during atmospheric re-entry Space shuttle Colombia burnt up on re-entry due to damage to ceramic tiles |
CHEMISTRY: POLYMERS
Overview- A polymer is a large molecule consisting of repeating structural units
- The repeating units are usually connected by covalent chemical bonds
- Polymers can be of two types
- Natural polymers: shellac, amber, rubber, proteins etc
- Synthetic polymers: nylon, polyethylene, neoprene, synthetic rubber etc
- Synthetic polymers are commonly referred to as plastics
- The first plastic based on a synthetic polymer to be created was Bakelite, by Leo Baekeland(Belgium/USA) in 1906
- Vulcanization of rubber was invented by Charles Goodyear (USA) in 1839. Vulcanization is the process of making rubber more durable by addition of sulphur
- The first plastic to be created was Parkesine (aka celluloid) invented by Alexander Parkes (England) in 1855
Synthesis of polymers
- The synthesis of polymers – both natural and synthetic – involves the step called polymerization
- Polymerization is the process of combining many small molecules (monomers) into a covalently bonded chain (polymer)
- Synthetic polymers are created using of two techniques
- Step growth polymerization: chains of monomers are combined directly
- Chain growth polymerization: monomers are added to the chain one at a time
- Natural polymers are usually created by enzyme-mediated processes, such as the synthesis of proteins from amino acids using DNA and RNA
Categories of polymers
- Organic polymers are polymers that are based on the element carbon. Eg: polyethylene, cellulose etc
- Inorganic polymers are polymers that are not based on carbon. Eg: silicone, which uses silicon and oxygen
- Copolymer is one that is derived from two or more monomeric units. Eg: ABS plastic
- Fluoropolymers are polymers based on fluorocarbons. They have high resistance to solvents, acids and bases. Eg: teflon
TYPES OF BIOPOLYMERS 
DNA as a biopolymer
DNA as a biopolymer
- Structural proteins
- Structural proteins are proteins that provide structural support to tissues
- They are usually used to construct connective tissues, tendons, bone matrix, muscle fibre
- Examples include collagen, keratin, elastin
- Functional proteins
- Proteins that perform a chemical function in organisms
- Usually used for initiate or sustain chemical reactions
- Examples include hormones, enzymes
- Structural polysaccharides
- They are carbohydrates that provide structural support to cells and tissues
- Examples include cellulose, chitin
- Storage polysaccharides
- Carbohydrates that are used for storing energy
- Eg: starch, glycogen
- Nucleic acids
- Nucleic acids are macromolecules composed of chains of nucleotides
- Nucleic acids are universal in living beings, as they are found in all plant and animal cells
- Eg: DNA, RNA
TYPES OF SYNTHETIC POLYMERS
- Thermoplastics
- Thermoplastics are plastics that turn into liquids upon heating
- Also known as thermosoftening plastic
- Thermoplastics can be remelted and remoulded
- Eg: polyethylene, Teflon, nylon
- Recyclable bottles (such as Coke/Pepsi) are made from thermoplastics
- Thermosetting plastics
- Thermosettings plastics are plastics that do not turn into liquid upon heating
- Thermosetting plastics, once cured, cannot be remoulded
- They are stronger, more suitable for high-temperature applications, but cannot be easily recycled
- Eg: vulcanized rubber, bakelite, Kevlar
- Elastomers
- Elastomers are polymers that are elastic
- Elastomers are relatively soft and deformable
- Eg: natural rubber, synthetic polyisoprene
IMPORTANT NATURAL POLYMERS AND THEIR APPLICATIONS
| Polymer | Application | Notes | |
| Collagen | Connective tissue Gelatine (food) | Most abundant protein in mammals | |
| Keratin | Hair, nails, claw etc | ||
| Enzymes | Catalysis | ||
| Hormones | Cell signalling | ||
| Cellulose | Cell wall of plants Cardboard, paper | Most common organic compound on Earth | |
| Chitin | Cell wall of fungi, insects | ||
| Starch | Energy storage in plants | Most important carbohydrate in human diet | |
| Glycogen | Energy storage in animals | ||
| DNA | Genetic information | ||
| RNA | Protein synthesis | ||
IMPORTANT SYNTHETIC POLYMERS AND THEIR APPLICATIONS
| Polymer | Developed by | Constituent elements | Application | Notes |
| Parkesine | Alexander Parkes (Britain, 1855) | Cellulose | Plastic moulding | First man-made polymer |
| Bakelite | Leo Baekeland (USA, 1906) | Phenol and formaldehyde | Radios, telephones, clocks | First polymer made completely synthetically |
| Polyvinylchloride (PVC) | Henri Regnault (France, 1835) | Vinyl groups and chlorine | Construction material | Third most widely used plastic |
| Styrofoam | Ray McIntre (USA, 1941) | Phenyl group | Thermal insulation | Brand name for polystyrene |
| Nylon | Wallace Carothers (USA, 1935) | Amides | Fabric, toothbrush, rope etc | Family of polyamides First commercially successful synthetic polymer |
| Synthetic rubber | Fritz Hoffman (Germany, 1909) | Isoprene | Tyres, textile printing, rocket fuel | |
| Vulcanized rubber | Charles Goodyear (USA, 1839) | Rubber, sulphur | Tyres | Vulcanized rubber is much stronger than natural rubber |
| Polypropylene | Karl Rehn and Guilio Natta (Italy, 1954) | Propene | Textiles, stationary, automotive components | Second most widely used synthetic polymer |
| Polyethylene | Hans von Pechmann (Germany, 1898) | Ethylene | Packaging (shopping bags) | Most widely used synthetic polymer |
| Teflon | Roy Plunkett (USA, 1938) | Ethylene | Cookware, construction, lubricant | Brand name for polytetrafluoroehtylene (PTFE) Very low friction, non-reactive |
DEGRADATION OF POLYMERS 
Ozone cracking in natural rubber tubing
Ozone cracking in natural rubber tubing
- Degradation of polymers can be desirable as well undesirable: desirable when looking for biological degradation, undesirable when faced with loss of strength, colour etc
- Polymer degradation usually occurs due to hydrolysis of covalent bonds connecting the polymer chain
- Polymer degradation can happen because of heat, light, chemicals and galvanic action
- Ozone cracking is the cracking effect of ozone on rubber products such as tyres, seals, fuel lines etc. Usually prevented by adding antiozonants to the rubber before vulcanization
- Chlorine can cause degradation of plastic as well, especially plumbing
- Resin Identification Code is the system of labelling plastic bottles on the basis of their constituent polymers. This Code helps in the sorting and recycling of plastic bottles
- Degradation of plastics can take hundreds to thousands of years
Biodegradable plastics
- Biodegradable plastics are plastics than can break down upon exposure to sunlight (especially UV), water, bacteria etc
- Biopol is a biodegradable polymer synthesized by genetically engineered bacteria
- Ecoflex is a fully biodegradable synthetic polymer for food packaging
Bioplastics
- They are organic plastics derived from renewable biomass sources such as vegetable oil, corn, starch etc
Oxy-biodegradable plastics
- Plastics to which a small amount of metals salts have been added
- As long as the plastic has access to oxygen the metal salts speed up process of degradation
- Degradation process is shortened from hundreds of years to months
BIOLOGY: GENETIC DISORDERS
About genetic disorders
Huntington's disease is inherited in the autosomal dominant fashion
- Genetic disorders are disorders that are passed on from generation to generation
- They are caused by abnormalities in genes or chromosomes
- Some genetic disorders may also be influenced by non-genetic environmental factors. Eg: cancer
- Most genetic disorders are relatively rare and only affect one person in thousands or millions
- To recollect, males have XY chromosome pairs while females have XX pairs
Single Gene Disorders
- Single gene disorders result from the mutation of a single gene
- They can be passed onto subsequent generations in multiple ways
- Single gene disorders include sickle cell disease, cystic fibrosis Huntington disease
Multiple gene disorders
- Multiple gene disorders result from mutation on multiple genes in combination with environmental factors
- They do not have a clear pattern of inheritance, which makes it difficult to assess risk of inheriting a particular disease
- Examples include heart disease, diabetes, hypertension, obesity, autism
TYPES OF SINGLE GENE GENETIC DISORDERS
- Autosomal dominant
Sickle cell disease is inherited in the autosomal recessive pattern
- Only one mutated copy of the gene is necessary for inheritance of the mutation
- Each affected person usually has one affected parent
- There is a 50% chance that the child will inherit the mutated gene
- Autosomal dominant disorders usually have low penetrance i.e. although only one mutated copy is needed, only a small portion of those who inherit that mutation will develop the disorder
- Eg: Huntington’s disease, Marfan syndrome
- Only one mutated copy of the gene is necessary for inheritance of the mutation
- Autosomal recessive
- Two copies of the gene must be mutated for a person to be affected
- An affected person usually has unaffected parents who each have one mutated gene
- There is a 25% chance that the child will inherit the mutated gene
- Eg: Cystic fibrosis, sickle cell disease, Tay-Sachs disease, dry earwax, Niemann-Pick disease
- Two copies of the gene must be mutated for a person to be affected
- X-linked dominant
- X-linked dominant disorders are caused by mutations on the X chromosome
- Males and females are both affected by such disorders. However, males are affected more severely
- For a man with a X-linked dominant disorder, his sons will all be unaffected (since they receive their father’s Y chromosome) while his daughters will all be affected (since they receive his X chromosome)
- A woman with a X-linked dominant disorder has a 50% chance of passing it on to progeny
- Eg: Hypophosphatemic rickets, Rett syndrome, Aicardi syndrome
- X-linked recessive
X-linked recessive with a carrier mother
- Caused by mutations on the X-chromosome
- Males are affected more frequently than females
- The sons of a man affected by a X-linked recessive disorder will not be affected, while his daughters will carry one copy of the mutated gene
- The sons of a woman affected by a X-linked recessive disorder will have have a 50% chance of being affected by the disorder, while the daughters of the woman have a 50% chance of becoming carriers of the disorder
- Eg: colour blindness, muscular dystrophy, hemophilia A
- Y-linked disorders
- Caused by mutations on the Y chromosome
- Y chromosomes are present only in males
- The sons of a man with Y-linked disorders will inherit his Y chromosome and will always be affected while the daughters will inherit his X chromosome and will never be affected
- Eg: male infertility
- Mitochondrial disorders
- These disorders are caused by mutations in the mitochondrial DNA
- Only mothers can pass on mitochondrial disorders to children, since only egg cells (from the mother) contribute mitochondria to the developing embryo
- Eg: Leber’s Heriditary Optic Neuropathy
CHEMISTRY: FERTILIZERS AND PESTICIDES
FERTILIZERS
About fertilizers
- Fertilizers are soil amendments applied to promote plant growth
- Can be applied to soil or directly to leaves
- Main nutrients in a fertilizer are nitrogen, phosphorous and potassium
Synthetic fertilizers
- Synthetic fertilizers are manufactured using the Haber-Bosch process to produce ammonia, which is then used to manufacture other nitrogen fertilizers
- Urea is the most commonly used fertilizer. It has the highest nitrogen content
- Synthetic fertilizers do not replace trace minerals in the soil (eg Zinc, copper, magnesium etc)
- Production of synthetic fertilizers is highly energy intensive. The production of synthetic ammonia currently accounts for 5% of global natural gas consumption
- Excess and unregulated use of synthetic fertilizers can cause Fertilizer Burn, in which plant tissues die due to excess nitrogenous salts
Biofertilizers
- Include naturally occurring minerals such as manure, worm castings, compost, etc.
- Primary sources of biofertilizers are
- Bacteria: Rhibozium, Azopirillum
- Fungi: Mycorrhiza
- Fern: Azolla
- Cover crops can also be used to enrich soil between plantings of the main crop. Cover crops work through the principle of nitrogen fixation: i.e. convert atmospheric nitrogen into a plant-accessible form
- Minerals such as limestone, rock phosphate and sulphate of potash can also be used
- Biofertilizers release their nutrients much slowly compared to synthetic fertilizers and thereby prevent Fertilizer Burn
- In addition to improving crop yields, biofertilizers also improve the health and long-term productivity of soil
Environmental effects of fertilizer use
- Oxygen depletion: Nitrogen compounds in fertilizer run-off are primarily responsible for serious oxygen depletion in oceans and lakes. This lack of dissolved oxygen causes serious damage to aquatic life in lakes and along coastal areas. Also leads to discolouration of water (green, yellow, red, brown)
- Soil acidification: Nitrogen containing synthetic fertilizers cause soil acidification
- Heavy metal accumulation: Synthetic fertilizers, especially those based on phosphates, can contain significant amounts of cadmium, uranium, zinc, lead and radioactive polonium, all of which can be stored in plant tissues, and later enter the food chain in the form of produce
- Greenhouse gases: Due to the large scale use of fertilizers, nitrous oxide has now become the third most important greenhouse gas (after carbon dioxide and methane).
PESTICIDES
Overview- A pesticide is a substance that is used to kill pests
- Pests can include insects, molluscs, birds, weeds etc
- In addition to preventing crop losses due to pests, pesticides can kill disease-spreading mosquitoes, allergy inducing bees or wasps, and also to control algae levels in lakes
- Due to its negative effects on birds, DDT has been banned as a pesticide for agricultural use under the 2001 Stockholm Convention. However, it is still used in developing countries for malaria prevention and other vector control
Commonly used pesticides
| Pesticide | Used to control | Example |
| Algaecide | Algae | Copper sulphate, barley straw |
| Avicide | Birds | Strychnine, DRC1339, parathion (in diesel oil) |
| Bactericides | Bacteria | Chlorine, iodine, oxygen, alchohol, phenol |
| Fungicide | Fungi, oomycete (water molds) | Sulphur, neem oil, tea tree oil, rosemary oil, milk |
| Herbicide | Weeds | Dichlorophenoxyacetic acid (2,4D), atrazine, glyphosate |
| Insecticide | Insects | Organochlorine, organophosphates, carbamates, pyrethroids |
| Miticide | Mites | Methoprene, permethrin, dicofol |
| Molluscicides | Molluscs (slugs and snails) | Metal salts (iron phosphate, aluminium sulphate), metaldehyde |
| Nematicide | Nematodes (worms) | Nematophagus fungi, neem cake |
| Rodenticide | Rodents | Anticoagulants, metal phosphides, hypercalcemia |
Environmental effects of pesticides
- Over 98% of insecticides and 95% of herbicides reach a destination other than their target species
- Pesticides contaminate land and water when they run-off from fields, when discarded, sprayed etc
- Air pollution: pesticide drift occurs when pesticides suspended in the air get carried away to other areas. Pesticides also react with other chemicals to produce ozone, accounting for about 6% of total ozone production
- Water pollution: run-off and eroding soil lead to pesticide pollution of water. This affects water solubility, and also the pesticides enter the food chain through water. Some pesticides are toxic to fish, kill off zooplankton (the main food source for fish). Harmful to amphibians such as tadpoles and frogs
- Soil contamination: nitrogen fixation is affected by pesticides in soil. Pesticides also kill bees and are responsible for pollinator decline, leading to decreased crop yields. Widespread use of pesticides eliminates animals’ food sources causing them to change food habits or starve. Pesticide poisoning also travels up the food chain
Pest resistance and rebound
- Pests may evolve to become resistant to pesticides
- Managed through pesticide rotation
- Mixture of pesticides may also be used
- Certain pests sometimes themselves act as pesticides in the sense that they control other pests. In this case pesticides that target one pest species may lead to a secondary pest outbreak due to the other species
- Also, sometimes use of pesticides may affect natural enemies of the pest more than the pest itself. In this case, the pesticide may lead to temporary decrease in pest populations, but in the long-term the pest population may increase due to the absence of its natural enemies (especially for mosquitoes). This is called pest rebound.
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