WHAT MAKES GLASS TRANSPARENT - Glass is a special kind of solid known as an amorphous solid. This is a state of matter in which the atoms and molecules are locked into place, but instead of forming neat, orderly crystals, they arrange themselves randomly. As a result, glasses are mechanically rigid like solids, yet have the disordered arrangement of molecules like liquids. Amorphous solids form when a solid substance is melted at high temperatures and then cooled rapidly -- a process known as quenching. In many ways, glasses are like ceramics and have all of their properties: durability, strength and brittleness, high electrical and thermal resistance, and lack of chemical reactivity.

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What makes glass transparent?

BY WILLIAM HARRIS

Ever watch a house being built?

Carpenters first erect the basic skeleton of the structure using two-by-four studs. Then they nail sheathing, usually plywood, to the studs to make walls.

Most walls include a window opening, which holds a sheet of glass situated within a frame.

Windows make a home feel bright, warm and welcoming because they let light enter.

But why should a glass window be any more transparent than the wood that surrounds it?

After all, both materials are solid, and both keep out rain, snow and wind. Yet wood is opaque and blocks light completely, while glass is transparent and lets sunshine stream through unimpeded.

You may have heard some people -- even some science textbooks -- try to explain this by saying that wood is a true solid and that glass is a highly viscous liquid.

They then go on to argue that the atoms in glass are spread farther apart and that these gaps let light squeeze through.

They may even point to the windows of centuries-old houses, which often look wavy and unevenly thick, as evidence that the windows have "flowed" over the years like the slow crawl of molasses on a cold day.

In reality, glass isn't a liquid at all. It's a special kind of solid known as an amorphous solid.

This is a state of matter in which the atoms and molecules are locked into place, but instead of forming neat, orderly crystals, they arrange themselves randomly.

As a result, glasses are mechanically rigid like solids, yet have the disordered arrangement of molecules like liquids.

Amorphous solids form when a solid substance is melted at high temperatures and then cooled rapidly -- a process known as quenching.

In many ways, glasses are like ceramics and have all of their properties: durability, strength and brittleness, high electrical and thermal resistance, and lack of chemical reactivity.

Oxide glass, like the commercial glass you find in sheet and plate glass, containers and light bulbs, has another important property: It's transparent to a range of wavelengths known as visible light.

To understand why, we must take a closer look at the atomic structure of glass and understand what happens when photons -- the smallest particles of light -- interact with that structure.
Electron to Photon: You Don't Excite Me

First, recall that electrons surround the nucleus of an atom, occupying different energy levels. To move from a lower to a higher energy level, an electron must gain energy.

Oppositely, to move from a higher to a lower energy level, an electron must give up energy. In either case, the electron can only gain or release energy in discrete bundles.

Now let's consider a photon moving toward and interacting with a solid substance. One of three things can happen:

1. The substance absorbs the photon. This occurs when the photon gives up its energy to an electron located in the material. Armed with this extra energy, the electron is able to move to a higher energy level, while the photon disappears.

2. The substance reflects the photon. To do this, the photon gives up its energy to the material, but a photon of identical energy is emitted.

3. The substance allows the photon to pass through unchanged. Known as transmission, this happens because the photon doesn't interact with any electron and continues its journey until it interacts with another object.

Glass, of course, falls into this last category. Photons pass through the material because they don't have sufficient energy to excite a glass electron to a higher energy level. 

Physicists sometimes talk about this in terms of band theory, which says energy levels exist together in regions known as energy bands.

In between these bands are regions, known as band gaps, where energy levels for electrons don't exist at all. Some materials have larger band gaps than others.

Glass is one of those materials, which means its electrons require much more energy before they can skip from one energy band to another and back again.

Photons of visible light -- light with wavelengths of 400 to 700 nanometers, corresponding to the colors violet, indigo, blue, green, yellow, orange and red -- simply don't have enough energy to cause this skipping.

Consequently, photons of visible light travel through glass instead of being absorbed or reflected, making glass transparent.

At wavelengths smaller than visible light, photons begin to have enough energy to move glass electrons from one energy band to another.

For example, ultraviolet light, which has a wavelength ranging from 10 to 400 nanometers, can't pass through most oxide glasses, such as the glass in a window pane.

This makes a window, including the window in our hypothetical house under construction, as opaque to ultraviolet light as wood is to visible light.

https://science.howstuffworks.com/question404.htm

PVC PLASTICS: POLYVINYL CHLORIDE - PVC is predominant in the construction industry due to its low production cost, malleability, and light weight. It is used as a replacement for metal in many applications where corrosion can compromise functionality and escalate maintenance costs. Many of the world's pipes are made from PVC and these are used in industrial and municipal applications. It is also used to make pipe fitting and pipe conduits. It does not have to be welded and can be connected with the use of joints, solvent cement and special glues -- key points that highlight its installation flexibility.

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PVC Plastics: Polyvinyl Chloride

An Introduction to Polyvinyl Chloride

by Todd Johnson 

Polyvinyl chloride (PVC) is a popular thermoplastic that contains high levels of chlorine which can reach up to 57%.

Carbon, which is derived from oil or gas is also used in its fabrication.

It is an odorless and solid plastic that is white, brittle and can also be found on the market in the form of pellets or white powder.

PVC resin is often supplied in the powder forms and its high resistance to oxidation and degradation make it possible to store the material for long periods.

Some authors/activists that oppose the manufacturers of PVC often refer to it as the "Poison Plastic" due to the toxic pollutants it might release.

When plasticizers are added it becomes softer and more flexible.

Uses of PVC

PVC is predominant in the construction industry due to its low production cost, malleability, and light weight.

It is used as a replacement for metal in many applications where corrosion can compromise functionality and escalate maintenance costs.

Many of the world's pipes are made from PVC and these are used in industrial and municipal applications.

It is also used to make pipe fitting and pipe conduits.

It does not have to be welded and can be connected with the use of joints, solvent cement and special glues -- key points that highlight its installation flexibility.

The material is also present in the electrical components such as electrical insulation, wires, and cable coatings.

In the healthcare industry, it is used to make feeding tubes, blood bags, intravenous (IV) bags, parts of dialysis devices and many other items.

This is only possible when phthalates are added to it.

Phthalates are used as plasticizers to produce flexible grades of PVC (and other plastics), thus making it better suited for the aforementioned applications due to improved performance characteristics.

Common consumer products such as raincoats, plastic bags, toys, credit cards, hoses, doors and window frames and shower curtains are also made from PVC.

This is not an exhaustive list of the many products that can be found around the household with PVC as its main constituent.

Advantages of PVC

As mentioned earlier, PVC is a low-cost material that is lightweight and as such, is easy to handle and install.

Compared to other types of polymers, its manufacturing process is not limited to the use of crude oil or natural gas.

Some use this point to argue that it a sustainable plastic since these forms of energy are known to be nonrenewable.

PVC is also a durable material and is not affected by corrosion or other forms of degradation. 

It can easily be converted into different forms making its use across various industries an evident advantage.

Being a thermoplastic it can be recycled and converted into new products for different industries, but this is not an easy process due to the many formulations used to manufacture PVC.

It also presents chemical stability which is an important factor when PVC products are applied in environments with different types of chemicals.

This characteristic guarantees that it maintains its properties without undergoing significant changes when chemicals are added. 

Other advantages include:

·               Biocompatible

·               Clarity and transparency

·               Resistance to chemical stress cracking

·               Low thermal conductivity

·               Requires little to no maintenance

Disadvantages of PVC

PVC is often referred to as the "Poison Plastic" and this is due to the toxins it can release during manufacture, when exposed to fire, or decomposed in landfills.

These toxins have been linked to health problems that include, but are not limited to cancer, birth development problems, endocrine disruption, asthma, and lung problems.

While many PVC manufacturers point to its high content of salt as being a major advantage, it is this main ingredient along with the possible release of dioxin and phthalate that are possible contributing factors to the hazards it might pose to human health and the environment.

The health concerns of PVC plastics, if any, are still highly debatable.

Future of PVC Plastics

PVC plastics account for a lot of plastics that are used in the world today. This material is ranked as the third most used plastic falling behind polyethylene and polypropylene.

The concerns regarding its threat to human health have prompted research around the use of sugarcane ethanol as the feedstock for PVC instead of naphtha.

Additional research is also being conducted on bio-based plasticizers as a solution for phthalate-free plasticizers.

These experiments are still in their initial stages, but the hope is to develop more sustainable forms of PVC that do not affect human health or threaten the environment during the manufacture, use and disposal stages.

With the many excellent characteristics that PVC presents, it continues to be a widely used plastic across various industries.

Todd Johnson

Regional Sales Manager for Composites One, a distributor of composite materials.

B.S. in Business Management from University of Colorado Boulder's Leeds School of Business

Business Development Manager for Ebert Composites Corporation

Experience

Todd Johnson is a former writer for ThoughtCo, who wrote about plastics and composite materials for 2-1/2 years between 2010 and 2013. He is a Regional Sales Manager at Composites One, a composite materials distributor in San Diego, CA. Johnson provides support to the Greater San Diego manufacturers of fiber reinforced and polymer products. He regularly attends composite industry trade shows including JEC, ACMA, SME, and SAMPE. In 2008 he presented at the Global Pultrusion Conference in Baltimore, MD. Previously, Todd spent six years as the Business Development Manager for Ebert Composites Corporation. 

Education

B.S., Business, Management, Marketing, and Related Support Services - the University of Colorado-Boulder's Leeds School of Business; attended Griffith University in Queensland, Australia. 

 Todd Johnson

​ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

For more than 20 years, Dotdash brands have been helping people find answers, solve problems, and get inspired. We are one of the top-20 largest content publishers on the Internet according to comScore, and reach more than 30% of the U.S. population monthly. Our brands collectively have won more than 20 industry awards in the last year alone, and recently Dotdash was named Publisher of the Year by Digiday, a leading industry publication.

https://www.thoughtco.com/what-is-pvc-plastics-820366

WATER POLLUTION DUE TO EXCESS NUTRIENTS - The overabundance of nitrates cause environmental problems. Nutrients generally consist of phosphorus and nitrogen which algae and aquatic plants use to grow and proliferate. Nitrogen is present in abundance in the atmosphere, but not in a form that is available to most living things. When nitrogen is in the form of ammonia, nitrite, or nitrate, however, it can be used by many bacteria, algae, and plants. Excess nitrates and phosphorus encourage the growth of aquatic plants and algae. Some of the algae making up the blooms produce toxins that are dangerous to fish, wildlife, and humans. The blooms eventually die off, and their decomposition consumes a lot of dissolved oxygen, leaving waters with low oxygen concentrations.

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Water Pollution: Nutrients

by Frederic Beaudry 

According to the Environmental Protection Agency, over half of the nation’s streams and rivers are polluted, and of those, 19% are impaired by the presence of excess nutrients.

What Is Nutrient Pollution?

The term nutrient refers to sources of nourishment supporting organism growth.

In the context of water pollution, nutrients generally consist of phosphorus and nitrogen which algae and aquatic plants use to grow and proliferate.

Nitrogen is present in abundance in the atmosphere, but not in a form that is available to most living things.

When nitrogen is in the form of ammonia, nitrite, or nitrate, however, it can be used by many bacteria, algae, and plants (here's a nitrogen cycle refresher).

Generally, it is the overabundance of nitrates that causes environmental problems.

What Causes Nutrient Pollution?

·      Some common agricultural practices lead to excess nutrients in water bodies.

Phosphorus and nitrates are important components of the fertilizers used in agricultural fields – they are present in both synthetic fertilizers and natural ones like manures.

If the crops do not pick up all of the fertilizer applied, or if rain has a chance to wash it away before they are absorbed by plants, the excess fertilizer is flushed into streams.

Another major source of nutrients also comes from the way agricultural fields are only used seasonally. 

Most crops are present in the fields over a relatively short growing season, and the rest of the year the soil is left exposed to the elements.

Meanwhile soil bacteria is feasting on decaying roots and plant debris, releasing nitrates.

Not only do bare fields cause sediment pollution, but this practice allows the massive release and washing away of nitrates.

·      Sewage can carry nutrients to streams and water.

Septic systems, especially if older or improperly maintained, can leak into streams or lakes.

Households connected to municipal sewer systems also contribute to nutrient pollution.

Wastewater treatment plants sometimes function improperly, and are periodically overwhelmed during heavy rain events and release sewage into rivers.

·      Stormwater. Rain falling in urban or suburban areas picks up nutrients from lawn fertilizer, pet waste, and various detergents (for example, the soap used to wash one’s car in the driveway).

The stormwater is then canalized into municipal drainage systems and released into streams and rivers, loaded with phosphorus and nitrogen.

·     Burning fossil fuels release nitrogen oxides and ammonia into the air, and when those are deposited in water, they can contribute significantly to the excess nutrient problem.

Most problematic are coal-fired power plants and gas- or diesel-powered vehicles.

What Environmental Effects Do Excess Nutrients Have?

Excess nitrates and phosphorus encourage the growth of aquatic plants and algae.

Nutrient-boosted algae growth leads to massive algae blooms, visible as a bright green, foul smelling sheen on the water’s surface.

Some of the algae making up the blooms produce toxins that are dangerous to fish, wildlife, and humans.

The blooms eventually die off, and their decomposition consumes a lot of dissolved oxygen, leaving waters with low oxygen concentrations.

Invertebrates and fish are killed when oxygen levels dip too low. Some areas, called dead zones, are so low in oxygen that they become empty of most life.

A notorious dead zone forms in the Gulf of Mexico every year due to agricultural runoff in the Mississippi River watershed.

Human health can be affected directly, as nitrates in drinking water are toxic, especially to infants.

People and pets can also become quite ill from exposure to toxic algae.

Water treatment does not necessarily solve the problem, and can in fact create dangerous conditions when chlorine interacts with the algae and produces carcinogenic compounds.  

Some Helpful Practices

·      Cover crops and no-till farming protect agricultural fields and mobilize nutrients. The cover plants die out in winter, and the following growing season they give back those nutrients to the new crop.

·      Maintaining well vegetated buffers around farm fields and next to streams allows plants to filter out nutrients before they enter the water.

·      Keep septic systems in good working order, and conduct regular inspections.

·      Consider your nutrient inputs from soaps and detergents, and reduce their use whenever possible.

·      In your yard, slow the water runoff and allow it to be filtered by plants and soil. To accomplish this, establish rain gardens, keep drainage ditches well vegetated, and use rain barrels to harvest roof runoff.

·     Consider using pervious pavement in your driveway. These surfaces are designed to let water percolate into the soil below, preventing runoff.

Frederic Beaudry

Associate professor of environmental science at Alfred University in New York

Ph.D. in wildlife ecology from the University of Maine

Experience

Dr. Frederic Beaudry is a former writer for ThoughtCo who contributed articles on pollution, global warming, and climate science for three years. He is an associate professor of environmental science at Alfred University in New York. Prior to teaching, he worked as a wildlife biologist, focusing on the ecology and conservation of birds and turtles. Beaudry has authored several scientific papers on land use and conservation and has conducted research examining land use changes and their effects on bird and amphibian communities.

Education

Beaudry has a B.S. in biology from Université du Québec à Rimouski and an M.A. in natural resources from Humboldt State University. He earned a Ph.D. in wildlife ecology at the University of Maine. Beaudry completed postdoctoral research at the University of Wisconsin-Madison.

ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

For more than 20 years, Dotdash brands have been helping people find answers, solve problems, and get inspired. We are one of the top-20 largest content publishers on the Internet according to comScore, and reach more than 30% of the U.S. population monthly. Our brands collectively have won more than 20 industry awards in the last year alone, and recently Dotdash was named Publisher of the Year by Digiday, a leading industry publication.

https://www.thoughtco.com/water-pollution-nutrients-1204127

SALINITY - Seawater has an average of 35 parts of dissolved salt per thousand parts of water, or 35 ppt. By comparison, tap water has a salinity level of 100 parts per million (ppm). Salinity levels can affect the movement of ocean currents. They can also affect marine life, which may need to regulate its intake of saltwater. Normal seawater has an average of 35 parts of dissolved salt per thousand parts of water, or 35 ppt. By comparison, fresh water has just 100 parts of salt per million parts of water, or 100 ppm. Salinity can affect the density of ocean water —water that has higher salinity is denser and heavier and will sink underneath less saline, warmer water.

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Salinity

Definition and Importance to Marine Life

A body of water's salinity affects its density

by Jennifer Kennedy 

The simplest salinity definition is that it is a measure of dissolved salts in a concentration of water.

Salts in seawater include not just sodium chloride (table salt) but other elements such as calcium, magnesium, and potassium.

These substances get into the ocean through complex processes including volcanic eruptions and hydrothermal vents as well as less complex ways such as the wind and rocks on land, which dissolve into sand and then salt.

Key Takeaways: Defining Salinity

·         Seawater has an average of 35 parts of dissolved salt per thousand parts of water, or 35 ppt. By comparison, tap water has a salinity level of 100 parts per million (ppm).

·        Salinity levels can affect the movement of ocean currents. They can also affect marine life, which may need to regulate its intake of saltwater.

·       The Dead Sea, located between Israel and Jordan, is the saltiest body of water in the world with a salinity level or 330,000 ppm, or 330 ppt, making it nearly 10 times saltier than the world's oceans.

Salinity Definition

Salinity in seawater is measured in parts per thousand (ppt) or practical salinity units (psu).

Normal seawater has an average of 35 parts of dissolved salt per thousand parts of water, or 35 ppt.

That equates to 35 grams of dissolved salt per kilogram of seawater, or 35,000 parts per million (35,000 ppm), or 3.5 percent salinity, but it can range from 30,000 ppm to 50,000 ppm.

By comparison, fresh water has just 100 parts of salt per million parts of water, or 100 ppm.

The water supply in the U.S. is restricted to a salinity level of 500 ppm, and the official salt concentration limit in U.S. drinking water is 1,000 ppm, while water for irrigation in the U.S. is limited to 2,000 ppm, according to The Engineering Toolbox.

History

Throughout Earth's history, geological processes, such as the weathering of rocks, have helped make the oceans salty, says NASA.

Evaporation and the formation of sea ice caused the salinity of the world's oceans to rise. These "salinity rising" factors were counterbalanced by the inflow of water from rivers as well as rain and snow, NASA adds.

Studying the salinity of the oceans has been difficult throughout human history due to limited sampling of ocean waters by ships, buoys, and moorings, NASA explains.

Still, as far back as the years 300 to 600 "awareness of changes in salinity, temperature, and smell helped Polynesians explore the southern Pacific Ocean," says NASA.

Much later, in the 1870s, scientists on a ship named the H.M.S. Challenger measured salinity, temperature, and water density in the world's oceans. 

Since then, techniques and methods for measuring salinity have changed drastically.

Why Is Salinity Important?

Salinity can affect the density of ocean water —water that has higher salinity is denser and heavier and will sink underneath less saline, warmer water.

This can affect the movement of ocean currents. It can also affect marine life, which may need to regulate its intake of saltwater.

Seabirds can drink saltwater, and they release the extra salt via the salt glands in their nasal cavities.

Whales can't drink much saltwater; instead, the water they need comes from whatever is stored in their prey.

They do have kidneys that can process extra salt, however. Sea otters can drink salt water because their kidneys are adapted to process the salt.

Deeper ocean water may be more saline, as is ocean water in regions with a warm climate, little rainfall, and plenty of evaporation.

In areas close to shore where there is more flow from rivers and streams, or in polar regions where there is melting ice, the water may be less saline.

Even so, according to the U.S. Geological Survey, there is enough salt in the world's oceans that if you removed it and spread it evenly over the Earth's surface, it would create a layer about 500 feet thick.

In 2011, NASA launched Aquarius, the agency's first satellite instrument designed to study the salinity of the world's oceans and predict future climate conditions. NASA says the instrument, launched aboard Argentine spacecraft Aquarius/Satélite de Aplicaciones Científicas, measures the salinity in the surface — about the top inch — of the world's oceans.

Saltiest Sea and Bodies of Water in the World

The Mediterranean Sea has a high level of salinity because it is mostly closed off from the rest of the ocean.

It also has warm temperatures that result in frequent humidity and evaporation.

Once the water evaporates, the salt remains, and the cycle begins all over again.

In 2011, the salinity of the Dead Sea, which is situated between Israel and Jordan, was measured at 34.2 percent, though its average salinity is 31.5 percent.

If the salinity in a body of water changes, it can affect the water's density. The higher the saline levels, the denser the water.

For example, visitors are often astonished that they can simply float on their backs, without any effort, on the surface of the Dead Sea, due to its high salinity, which creates high water density.

Even cold water with high salinity, such as that found in the northern Atlantic Ocean, is denser than warm, fresh water.

Jennifer Kennedy

Scientist, educator, and naturalist with more than two decades of experience studying and educating the public about the oceans

The executive director of the ​Blue Ocean Society for Marine Conservation

Whale watch naturalist who conducts educational programs about the marine environment

Experience

Jennifer Kennedy is a former writer for ThoughtCo, where she contributed 260 stories about marine life during her more than seven-year tenure.  Jennifer worked in marine research and education for over 20 years. She is the executive director of the Blue Ocean Society for Marine Conservation, based in Portsmouth, New Hampshire. With this organization, Jennifer works as a whale watch naturalist, directs educational programs about the marine environment, coordinates local aquatic conservation activities, and manages the organization's operations. 

Jennifer is a scientist, educator, and naturalist with more than two decades of experience studying and educating the public about the oceans. She was a naturalist and human resources manager with the Isles of Shoals Steamship Company before joining Blue Ocean. Her master's thesis involved studying cooperative research between fishers and scientists. Jennifer is a member of the Gulf of Maine Marine Education Association and the New Hampshire Sea Grant Policy Advisory Committee.

Education

Jennifer Kennedy earned a Master Science (M.S.) in Resource Administration and Management from the University of New Hampshire in 2003. She also holds a Bachelor Science (B.S.) in Natural Resources from Cornell University.

ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

For more than 20 years, Dotdash brands have been helping people find answers, solve problems, and get inspired. We are one of the top-20 largest content publishers on the Internet according to comScore, and reach more than 30% of the U.S. population monthly. Our brands collectively have won more than 20 industry awards in the last year alone, and recently Dotdash was named Publisher of the Year by Digiday, a leading industry publication.

https://www.thoughtco.com/salinity-definition-2291679