Leather preparation process

The leather production process

The tanning industry enables a by-product of the food industry to be recovered and made into something special and noble.

Leather tanning is without a doubt one of the oldest human activities.  In the beginning, skins obtained from hunting and livestock breeding could be used for clothing as a protection from the atmospheric elements.

The tanning process has undergone many changes from prehistory to today, especially in the twentieth century,  when industrialization and new machines have allowed development in the research for specific  and less polluting chemicals and  new methods of tanning and finishing.

Leather is produced traditionally even today. The skin, discard  of the food industry, is "recycled" from the tanneries and processed  with advanced machinery and vast research, in such a way  to make it a  “noble” and fashionable material. There is a large number of tanneries, but the uniqueness of the result is given by the ability and the skill of experienced  craftsmen.

Immediately after killing the animal, in order to avoid degradation processes in the tissues, the skin is salted, dried or refrigerated before the production process of tanning starts.  Skins are salted with common marine salt, which penetrates very quickly into the fibers, helping to  a partial removal of water. This is a very efficient and economical process , easy to apply and widespread. The drying system is to eliminate  as much water as possible from the skin,  so to avoid the development of micro-organisms and bacteria. The  drying system is more suitable  for sheep and goat skins,  while less to preserve cowhides.

Defrost is another method , but  it is not common in Italian slaughterhouses:  this system cools the skin at very low temperatures

The leather manufacturing process can be basically divided into three major phases: tanning, re-tanning and finishing.

All  chemical tanning operations are used to stabilize the skin making it unputrescible and are carried out with the use of water in the drums.
The drum is the typical machine for tannery : it consists in a cylinder rotating around its own axis , filled  with water, chemical reagents and skins.

The drum used to be a simple wooden cylinder  and  has now transformed into a complex computer-controlled machine that regulates the input of water, the temperature, the speed of rotation, and the release of chemical reagents, thus avoiding manual errors.

THE LEATHER PREPARATION PROCESS

The preparation for tanning, starting from preserved raw materials,  may be divided as follows:

Leather soaking: Once cured, the skins are then soaked in water for several hours to several days. The water and  surfactants help in the removal of salt, dirt, debris, blood and excess animal fats. Rehydration is  also reintroduced.

Leather  fleshing: subcutaneous material is removed. 

Leather unharing: the majority of hair is removed. 

Liming  is used to loosen the fibers allowing the skin to  absorb chemicals that will be used later in the tanning processes. Limed hides appear swollen and with an increased thickness,  therefore can be easily  split into two or more layers. The  splitting is necessary for the heavy  cow hides, which are too  thick to be used in the production of manufactured items. The process is carried out with the splitting machine in which the limed skin is pushed by two cylinders against a band blade which cuts the skin in two layers parallel to its surface.
The top layer  is  the grain (typical design of the skin of the animal, the external part) this is the full grain part , while the lower layers  are the flesh split. In general, the thinner skins (sheep, goats and calves) are skived but  not split.

Deliming:  this process  brings to removal of alkali from the  pelt with the consequent deswelling of the fibers and helps lowering of the  pH to the values used in the bating process. it Is carried out with slightly acidic chemicals.

Bating :  is an operation to complete the deliming  process, by eliminating residues of other substances and  loosen the fibers of the skin,  in order to smooth the grain  and achieve  a soft and flexible leather.

Degreasing: is carried out to remove and to reduce the natural fatty acids from the skin, which   could lead to difficulties in the absorption of chemicals. The degreasing process  removes excess fat and distribute it evenly .

Tanning is the process which converts the protein of the raw hide or skin into a stable material which will not putrefy and it is suitable for a wide variety of end applications,  the leather.

There are several types of tanning: chrome tanning is the most widespread. The duration of the  Chromium tanning is around 2 or 3 hours for small and thin skins,  up to a maximum of 24 hours for thicker  ones. At the end of the tanning  the skins appear blue-green. This is called wet-blue and at this stage it can be sold.

The vegetable tanning is the oldest, made with the use of tannins which give the vegetable tanned leather shades of brown, more or less intense.

There are two types of vegetable tanning:

- slow  tanning  in which the skins are immersed in tanks containing solutions of increasing concentrations of tannins. This method takes about 30 days and is meant to produce leather for soles,  very thick and stiff leathers.
- rapid tanning , made ??in the drums which , due to the rotation, creates a more flexible leather suitable for any kind of  productions. This process  lasts about 36 to 48 hours.

Re tanning:

The tanned leather is not yet usable to produce articles.

It is still wet and  even if  dry , it would be too rigid and  colored with the typical color of tanning used.

To turn it into a marketable product  the leather must be further treated with  chemical and mechanical  processes in the drums.

Drying process:  is a mechanical operation  which  eliminates most of the water soaked by the tanned leather,  by pressing  the skins between two cylinders , where  the top one  is covered with pewter.

Shaving  : this operation smoothens the thickness of the whole surface of the skin and eliminates the residues of fleshing. This operation is carried out  with  a cylinder machine where the top one is provided with helical blades.  

Splitting - the leather is split into one or more horizontal layers. This can be done after liming or after chrome tanning. The choice depends on the product we want to achieve.

Re tanning - additional tanning agents are added to impart properties and change the features of the main tanning : to achieve a thick and stiff leather, skins need to be re tanned with vegetable tannins- For a soft and flexible leather , re tanning needs to be with chrome .

Dyeing is the process which gives the leather  the requested color.  It Is made in the drums with dyes.

Stuffing - fats/oils and waxes are added between the fibers,  thus giving the leather the flexibility and the softness needed for the various products. 

Drying :  all above mentioned processes are carried out with water so leather needs to be dried.  This can be done to various moisture levels,  by simple exposure to air ,  by temperature controlling or  by vacuum. All  drying systems aim to achieve a uniform drying and constant timing. The choice of the drying system depends not only on  economic factors but also on the end-use material.

FINISHING:

It 's the final stage and  the most complex process, which  includes all operations to be carried out on dried skins,   to change the surface effect,  both for aesthetic and functional aims. Finishing can be mechanical or chemical.

Mechanical finishing operations may include:

POLISHING : to create a shiny surface by rubbing it with a velvety wheel

IRONING and PLATING: to obtain a flat and smooth surface

EMBOSSING :to obtain a three-dimensional print

TUMBLING by rotating the drum quickly to create a more evident grain and a smooth   surface

Chemical Finishing involves the application of a film, natural or synthetic by using curtain coaters, roller coatings and spraying.

Leather Tanning(Processing)

Tanning and Leather Finishing

Tanning is the chemical process that converts animal hides and skins into leather. The term hide is used for the skin of large animals (e.g., cows or horses), while skin is used for that of small animals (e.g., sheep). Hides and skins are mostly by-products of slaughterhouses, although they may also come from animals that have died naturally or been hunted or trapped. Tanning industries are usually located near stock-raising regions; however, hides and skins may be preserved and transported prior to tanning, so the industry is widespread.
The tanning process consists in strengthening the hide’s protein structure by creating a bond between the peptide chains. The hide is composed of three layers: epidermis, dermis and subcutaneous layer. The dermis consists of about 30 to 35% protein, which is mostly collagen, with the remainder being water and fat. The dermis is used to make leather after the other layers have been removed using chemical and mechanical means. The tanning process uses acids, alkalis, salts, enzymes and tanning agents to dissolve fats and non-fibrous proteins and chemically bond the collagen fibres together.
Tanning has been practised since prehistoric times. The oldest system of tanning relies on the chemical action of vegetable material containing tannin (tannic acid). Extracts are taken from the parts of plants that are rich in tannin and processed into tanning liquors. The hides are soaked in pits or vats of increasingly strong liquors until they are tanned, which may take weeks or months. This process is used in countries with low levels of technology. This process is also used in developed countries to produce firmer, thicker leather for shoe soles, bags, cases and straps, although process changes have been introduced to shorten the time needed for tanning. Chemical tanning using mineral salts such as chromium sulphate was introduced during the late 19th century and has become the primary process to produce softer, thinner leather for goods such as handbags, gloves, garments, upholstery and shoe uppers. Tanning may also be accomplished using fish oils or synthetic tannins.
There is great variation in the scale and types of tanning facilities. Some tanneries are highly mechanized and use closed automatic systems and many chemicals, whereas others still use largely manual work and natural tanning substances with techniques essentially unchanged over the centuries (see figure 1). The type of product required (e.g., heavy-duty leather or fine flexible leathers) influences the choice of tanning agents and the finishing required.
Figure 1. Manual working methods in an Afghanistan tannery

Process Description
Leather production can be divided into three stages: preparation of the hide for tanning, which includes processes such as the removal of hair and adherent flesh; the tanning process; and the finishing process. Finishing includes mechanical processes to shape and smooth the leather and chemical treatments to colour, lubricate, soften and apply a surface finish to the leather (see figure 2). All of these processes may take place in one facility, although it is common for leather finishing to be conducted at locations different from tanning in order to take advantage of transportation costs and local markets. The implication is that it affects the likelihood of cross-contamination among the processes.
Figure 2. Typical processes for leather tanning & finishing

Curing and shipment. Because raw hides and skins decay rapidly, they are preserved and disinfected prior to shipment to the tannery. The hide or skin is flayed from the carcass and then preserved by curing. Curing can be accomplished by a variety of means. Curing by drying is suitable in regions where hot and dry climatic conditions prevail. Drying consists of stretching the hides on frames or spreading them on the ground in the sun. Dry-salting, another method of curing hides, consists of rubbing the fleshy side of the hide with salt. Brine curing, or brining, consists of submerging the hides in a solution of sodium chloride to which naphthalene may have been added. Brining is the most common form of preservation in developed countries.
Prior to shipment, hides are generally treated with DDT, zinc chloride, mercury chloride, chlorophenols or other agents for disinfection. These substances may represent hazards both at the site of curing and on receipt at the tannery.
Preparation. The cured hides and skins are prepared for tanning by several operations, collectively referred to as beamhouse operations. First the hides are sorted, trimmed and then washed in vats or drums. Disinfectants such as bleaching powder, chlorine and sodium acid fluoride in the water prevent putrefaction of hides. Chemicals such as caustic soda, sodium sulphide and surfactants are added to the water to accelerate soaking of dry-salted or dried hides.
The soaked hides and skins are then limed by immersing in milk of lime to loosen the epidermis and hair roots and to remove other unwanted soluble proteins and fats. In another method, a depilatory paste of lime, sulphide and salt is applied to the flesh side of the skins in order to save hair and wool. The limed hides are unhaired to remove the loosened hairs and defleshed. Epidermal debris and fine hair roots are mechanically removed by the scudding operation.
These operations are followed by deliming and bating with buffering salts, such as ammonium sulphate or ammonium chloride, and the action of proteolytic enzymes neutralizes the high alkalinity of limed hides. In pickling, hides are placed in an acid environment consisting of sodium chloride and sulphuric acid. The acid is necessary because chrome-tanning agents are not soluble under alkaline conditions. Vegetable-tanned hides do not need to be pickled.
Many of the beamhouse operations are carried out by processing the hides in solutions using large pits, vats or drums. Solutions are piped or poured into the containers and later emptied through pipes or into open drainage channels in the work area. The chemicals may be added to the containers by pipes or manually by workers. Good ventilation and personal protective equipment are needed to prevent respiratory and dermal exposure.
Tanyard. Various substances may be used for tanning, but the main distinction is between vegetable and chrome tanning. Vegetable tanning may be carried out either in pits or in rotating drums. Rapid tanning, in which high concentrations of tannins are used, is carried out in rotating drums. The chrome-tanning process most often used is the one-bath method, in which the hides are milled in a colloidal solution of chromium (III) sulphate until tanning is complete. A two-bath chrome-tanning process was used in the past, but this process involved potential exposure to hexavalent chromium salts and required more manual handling of the hides. The two-bath process is now considered obsolete and is rarely used.
Once tanned, the hide is further processed to shape and condition the leather. The hide is removed from the solution and excess water is removed by wringing. Chrome leather must be neutralized after being tanned. Splitting is the longitudinal division of wet or dry leather that is too thick, for articles such as shoe uppers and leather goods. Roll machines with cutting blades are used to further reduce the leather to the thickness required. A large amount of dust may be released when the leather is split or shaved while dry.
Re-tanning, colouring and fat-liquoring. After tanning, most leathers except sole leathers undergo colouring (dyeing). Generally, colouring is performed in a batch mode; and re-tanning, colouring and fat liquoring operations are all performed in sequence in the same drum with intermediate steps of washing and drying. Three major types of dyes are used: acid, basic and direct. Blends of dyes are used in order to obtain the exact shade desired, so the composition is not always known except by the supplier. The purpose of fat-liquoring is to lubricate leather to give it strength and flexibility. Oils, natural fats, their transformation products, mineral oils and several synthetic fats are used.
Finishing. After drying, vegetable tanned leather is subjected to mechanical operations (setting and rolling) and given a final polish. The finishing process for chrome leather includes a series of mechanical operations and, normally, the application of a covering layer to the leather surface. Staking is a mechanical beating operation used to make the leather soft. To improve the final appearance, the grain side of the leather is buffed using a sanding drum. This process generates a tremendous amount of dust.
A final surface finish is applied, which may contain solvents, plasticizers, binders and pigments. These solutions are applied by pads, flow coating or spraying. Some tanneries employ hand labour to apply the finish using pads, but this is usually carried out by machines. In flow coating, the solution is pumped into a reservoir above the conveyor carrying the leather and flows down onto it. In most cases, painted or sprayed leathers are not dried in ovens, but on trays on shelves. This practice provides a wide evaporating surface and contributes to air pollution.
Hazards and Their Prevention
Infectious hazards. In the early stages of the beamhouse operations, there may be some risk of infection due to zoonoses from the raw hides. Anthrax was a recognized hazard among workers engaged in handling hides and skins, particularly dry and dry-salted hides. This hazard has been virtually eliminated in tanneries due to disinfection of hides prior to shipment to the facilities. Colonies of fungi may develop on leathers and on the surface of the liquors.
Injuries. Slippery, wet and greasy floors form a serious hazard in all parts of a tannery. All floors should be of impervious material, have an even surface and be well drained. Good maintenance and housekeeping are essential. Mechanized transfer of hides and skins from one operation to another and proper drainage of liquors from vats and drums will help to reduce spillage and manual-handling ergonomic problems. Open pits and vats should be fenced to prevent injuries due to drowning and scalds.
There are many hazards connected with the operating parts of the machines—for example, injuries caused by revolving drums, in-running rollers and knives. Efficient guarding should be provided. All transmission machinery, belts, pulleys and gear wheels should be guarded.
Several operations involve manual lifting of the hides and leather, which represents an ergonomic hazard. Noise associated with the machinery is another potential hazard.
Dust. Dust is produced in a variety of tanning operations. Chemical dust can be produced during the loading of hide-processing drums. Leather dust is produced during mechanical operations. Buffing is the major source of dust. The dust in tanneries may be impregnated with chemicals, as well as fragments of hair, mould and excrement. Effective ventilation is needed for dust removal.
Chemical hazards. The large variety of acids, alkalis, tannins, solvents, disinfectants and other chemicals can be respiratory and skin irritants. Dusts of vegetable tanning materials, lime and leather and chemical mists and vapours arising in the various processes may be responsible for causing chronic bronchitis. Several chemicals may cause contact dermatitis. Chrome ulceration may occur in chrome tanning, especially on the hands. Exposures in the beamhouse operations are mainly to sulphur compounds such as sulphides and sulphates. Since these are alkaline substances, there is a potential to generate hydrogen sulphide gas if these substances contact acids.
Potential cancer-causing agents used in leather tanning and finishing include hexavalent chromium salts (in the past), aniline and azo dyes, vegetable tannins, organic solvents, formaldehyde and chlorophenols. The International Agency for Research on Cancer (IARC) evaluated the leather tanning industry in the early 1980s and concluded that there was no evidence to suggest an association between leather tanning and nasal cancer (IARC 1981). Case reports and epidemiological studies since the IARC evaluation have indicated increased risk for cancers among leather tanning and finishing workers—including lung cancer, sinonasal cancer and pancreatic cancer associated with leather dust and tanning (Mikoczy et al. 1996) and bladder cancer and testicular cancer associated with dyes or solvents in the finishing process (Stern et al. 1987). None of these associations is clearly established at this time.

The leather production process

The leather production process
The tanning industry enables a by-product of the food industry to be recovered and made into something special and noble.

Leather tanning is without a doubt one of the oldest human activities. In the beginning, skins obtained from hunting and livestock breeding could be used for clothing as a protection from the atmospheric elements.

The tanning process has undergone many changes from prehistory to today, especially in the twentieth century, when industrialization and new machines have allowed development in the research for specific and less polluting chemicals and new methods of tanning and finishing.

Leather is produced traditionally even today. The skin, discard of the food industry, is "recycled" from the tanneries and processed with advanced machinery and vast research, in such a way to make it a “noble” and fashionable material. There is a large number of tanneries, but the uniqueness of the result is given by the ability and the skill of experienced craftsmen.

Immediately after killing the animal, in order to avoid degradation processes in the tissues, the skin is salted, dried or refrigerated before the production process of tanning starts. Skins are salted with common marine salt, which penetrates very quickly into the fibers, helping to a partial removal of water. This is a very efficient and economical process , easy to apply and widespread. The drying system is to eliminate as much water as possible from the skin, so to avoid the development of micro-organisms and bacteria. The drying system is more suitable for sheep and goat skins, while less to preserve cowhides.

Defrost is another method , but it is not common in Italian slaughterhouses: this system cools the skin at very low temperatures

The leather manufacturing process can be basically divided into three major phases: tanning, retanning and finishing.

All chemical tanning operations are used to stabilize the skin making it unputrescible and are carried out with the use of water in the drums.
The drum is the typical machine for tannery : it consists in a cylinder rotating around its own axis , filled with water, chemical reagents and skins.

The drum used to be a simple wooden cylinder and has now transformed into a complex computer-controlled machine that regulates the input of water, the temperature, the speed of rotation, and the release of chemical reagents, thus avoiding manual errors.
THE LEATHER PREPARATION PROCESS

The preparation for tanning, starting from preserved raw materials, may be divided as follows:

Leather soaking: Once cured, the skins are then soaked in water for several hours to several days. The water and surfactants help in the removal of salt, dirt, debris, blood and excess animal fats. Rehydration is also reintroduced.

Leather fleshing: subcutaneous material is removed.

Leather unharing: the majority of hair is removed.

Liming is used to loosen the fibers allowing the skin to absorb chemicals that will be used later in the tanning processes. Limed hides appear swollen and with an increased thickness, therefore can be easily split into two or more layers. The splitting is necessary for the heavy cow hides, which are too thick to be used in the production of manufactured items. The process is carried out with the splitting machine in which the limed skin is pushed by two cylinders against a band blade which cuts the skin in two layers parallel to its surface.
The top layer is the grain (typical design of the skin of the animal, the external part) this is the full grain part , while the lower layers are the flesh split. In general, the thinner skins (sheep, goats and calves) are skived but not split.

Deliming: this process brings to removal of alkali from the pelt with the consequent deswelling of the fibers and helps lowering of the pH to the values used in the bating process. it Is carried out with slightly acidic chemicals.

Bating :Bating is an operation to complete the deliming process, by eliminating residues of other substances and loosen the fibers of the skin, in order to smooth the grain and achieve a soft and flexible leather.

Degreasing: is carried out to remove and to reduce the natural fatty acids from the skin, which could lead to difficulties in the absorption of chemicals. The degreasing process removes excess fat and distribute it evenly .

Tanning is the process which converts the protein of the raw hide or skin into a stable material which will not putrefy and it is suitable for a wide variety of end applications, the leather.

There are several types of tanning: chrome tanning is the most widespread. The duration of the Chromium tanning is around 2 or 3 hours for small and thin skins, up to a maximum of 24 hours for thicker ones. At the end of the tanning the skins appear blue-green. This is called wet-blue and at this stage it can be sold.

The vegetable tanning is the oldest, made with the use of tannins which give the vegetable tanned leather shades of brown, more or less intense.



There are two types of vegetable tanning:

- slow tanning in which the skins are immersed in tanks containing solutions of increasing concentrations of tannins. This method takes about 30 days and is meant to produce leather for soles, very thick and stiff leathers.
- rapid tanning , made ??in the drums which , due to the rotation, creates a more flexible leather suitable for any kind of productions. This process lasts about 36 to 48 hours.



Re tanning:

The tanned leather is not yet usable to produce articles.

It is still wet and even if dry , it would be too rigid and colored with the typical color of tanning used.

To turn it into a marketable product the leather must be further treated with chemical and mechanical processes in the drums.

Drying process: is a mechanical operation which eliminates most of the water soaked by the tanned leather, by pressing the skins between two cylinders , where the top one is covered with pewter.

Shaving : this operation smoothens the thickness of the whole surface of the skin and eliminates the residues of fleshing. This operation is carried out with a cylinder machine where the top one is provided with helical blades.

Splitting - the leather is split into one or more horizontal layers. This can be done after liming or after chrome tanning. The choice depends on the product we want to achieve.

Re tanning - additional tanning agents are added to impart properties and change the features of the main tanning : to achieve a thick and stiff leather, skins need to be re tanned with vegetable tannins- For a soft and flexible leather , re tanning needs to be with chrome .

Dyeing is the process which gives the leather the requested color. It Is made in the drums with dyes.

Stuffing - fats/oils and waxes are added between the fibers, thus giving the leather the flexibility and the softness needed for the various products.

Drying : all above mentioned processes are carried out with water so leather needs to be dried. This can be done to various moisture levels, by simple exposure to air , by temperature controlling or by vacuum. All drying systems aim to achieve a uniform drying and constant timing. The choice of the drying system depends not only on economic factors but also on the end-use material.



FINISHING:

It 's the final stage and the most complex process, which includes all operations to be carried out on dried skins, to change the surface effect, both for aesthetic and functional aims. Finishing can be mechanical or chemical.

Mechanical finishing operations may include:

POLISHING : to create a shiny surface by rubbing it with a velvety wheel

IRONING and PLATING: to obtain a flat and smooth surface

EMBOSSING :to obtain a three-dimensional print

TUMBLING by rotating the drum quickly to create a more evident grain and a smooth surface

Chemical Finishing involves the application of a film, natural or synthetic by using curtain coaters, roller coatings and spraying.

How Soap Cleans?

How Soap Cleans?

Soaps are sodium or potassium fatty acids salts, produced from the hydrolysis of fats in a ch ith a carboxylate 'head'. In water, the sodium or potassium ions float free, leaving a negatively-charged head.

 Soap is an excellent cleanser because of its ability to act as an emulsifying agent. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed. 

head that isin contact

with the water and a center

of hydrophobic tails,

which can be used

to isolate grime.

The organic part of a natural soap is a negatively-charged, polar molecule. Its hydrophilic (water-loving) carboxylate group (-CO2) interacts with water molecules via ion-dipole interactions and hydrogen bonding. The hydrophobic (water-fearing) part of a soap molecule, its long, nonpolar hydrocarbon chain, does not interact with water molecules. The hydrocarbon chains are attracted to each other by dispersion forces and cluster together, forming structures called micelles. In these micelles, the carboxylate groups form a negatively-charged spherical surface, with the hydrocarbon chains inside the sphere. Because they are negatively charged, soap micelles repel each other and remain dispersed in water.

Grease and oil are nonpolar and insoluble in water. When soap and soiling oils are mixed, the nonpolar hydrocarbon portion of the micelles break up the nonpolar oil molecules. A different type of micelle then forms, with nonpolar soiling molecules in the center. Thus, grease and oil and the 'dirt' attached to them are caught inside the micelle and can be rinsed away.

Although soaps are excellent cleansers, they do have disadvantages. As salts of weak acids, they are converted by mineral acids into free fatty acids:

CH3(CH2)16CO2-Na+ + HCl → CH3(CH2)16CO2H + Na+ + Cl-

These fatty acids are less soluble than the sodium or potassium salts and form a precipitate or soap scum. Because of this, soaps are ineffective in acidic water. Also, soaps form insoluble salts in hard water, such as water containing magnesium, calcium, or iron.

2 CH3(CH2)16CO2-Na+ + Mg2+ → [CH3(CH2)16CO2-]2Mg2+ + 2 Na+

The insoluble salts form bathtub rings, leave films that reduce hair luster, and gray/roughen textiles after repeated washings. Synthetic detergents, however, may be soluble in both acidic and alkaline solutions and don't form insoluble precipitates in hard water. But that is a different story...

Why Does Ice Float?

Question: Why Does Ice Float?
There are two parts to the answer for this question. First, let's take a look at why anything floats. Then, let's examine why ice floats on top of liquid water, instead of sinking to the bottom.
Answer: A substance floats if it is less dense, or has less mass per unit volume, than other components in a mixture. For example, if you toss a handful of rocks into a bucket of water, the rocks, which are dense compared to the water, will sink. The water, which is less dense than the rocks, will float. Basically, the rocks push the water out of the way, or displace it. For an object to be able to float, it has to displace a weight of fluid equal to its own weight.

Iceberg west of Ilulissat
inlet, Greenland

Water reaches its maximum density at 4°C (40°F). As it cools further and freezes into ice, it actually becomes less dense. On the other hand, most substances are most dense in their solid (frozen) state than in their liquid state. Water is different because of hydrogen bonding.

A water molecule is made from one oxygen atom and two hydrogen atoms, strongly joined to each other with covalent bonds. Water molecules are also attracted to each other by weaker chemical bonds (hydrogen bonds) between the positively-charged hydrogen atoms and the negatively-charged oxygen atoms of neighboring water molecules. As water cools below 4°C, the hydrogen bonds adjust to hold the negatively charged oxygen atoms apart. This produces a crystal lattice, which is commonly known as 'ice'.

Ice floats because it is about 9% less dense than liquid water. In other words, ice takes up about 9% more space than water, so a liter of ice weighs less than a liter water. The heavier water displaces the lighter ice, so ice floats to the top. One consequence of this is that lakes and rivers freeze from top to bottom, allowing fish to survive even when the surface of a lake has frozen over. If ice sank, the water would be displaced to the top and exposed to the colder temperature, forcing rivers and lakes to fill with ice and freeze solid.

Why Do Onions Make You Cry?

No more tears? Try chilling
your onion before cutting it.

Question: Why Do Onions Make You Cry?

Answer: Unless you've avoided cooking, you've probably cut up an onion and experienced the burning and tearing you get from the vapors. When you cut an onion, you break cells, releasing their contents. Amino acid sulfoxides form sulfenic acids. Enzymes that were kept separate now are free to mid your eyes. This gas reacts with the water in your tears to form sulfuric acid. The sulfuric acid burns, stimulating your eyes to release more tears to wash the irritant away. 

Cooking the onion inactivates the enzyme, so while the smell of cooked onions may be strong, it doesn't burn your eyes. Aside from wearing safety goggles or running a fan, you can keep from crying by refrigerating your onion before cutting it (slows reactions and changes the chemistry inside the onion) or by cutting the onion under water.

The Future of Chemistry

The Future of Chemistry

Chemistry for the Future

As chemists, biologists, physicists, and other scientists continue to unveil nature's secrets, a flood of facts accumulates with stunning momentum. Each answer is a new beginning— material for new experiments. Many researchers assert that there's never been a more exciting time to be a scientist. After much effort was spent in the last century finding individual puzzle pieces, scientists can now revel in the process of fitting the pieces together.

Not that everything's been figured out— not by a long shot. The science of today beckons researchers to think big—to integrate singular items, and even single pathways—into the grander scheme of what it is that makes entire organisms tick with such precision.

Perhaps ironically, as science grows larger in scope and broader in focus, some of the most promising tools to synthesize the hows, whats, and wheres of human biology are exceedingly tiny. Micromachines, tiny biosensors, and miniature molecular reaction vessels will undoubtedly be standard items in a chemist's toolbox in 10 or 20 years.

Unraveling—and making sense of—the genetic instructions that spell life for organisms as diverse as flies, plants, worms, and people has sparked a most exciting revolution. Every minute of every day, scientists all over the world work feverishly, weaving a compelling tale of the chemistry that underlies our health.

It's all very exciting, but the progress mandates still more work. Much more work! Among the questions still awaiting answers are these:

How do the 6-foot long stretches of DNA in every cell in our bodies know how to keep our biochemical factories running smoothly?

Who will find a way to outwit resistant bacteria?

When will someone figure out how to fight disease by manipulating the intricate sugar coatings on our cells?

Who will invent the tools that will revolutionize chemistry labs of the future?

What unexpected places hold treasure troves for new medicines?