Physical States and Properties

Substances: Pure substances and mixtures

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Did you know that you encounter and interact with a variety of substances on a daily basis? Substances are everywhere and can vary significantly, from the water you drink to your favorite breakfast meal. Given that there are so many, how do scientists keep all of these substances straight? Let’s find out.

Think of all the amazing substances you use during the first hours of your day. You take a shower with soap and water. You eat your favorite breakfast cereal and brush your teeth with toothpaste. On your way to class, you may grab a drink in an aluminum can from a vending machine. 

What are these substances made from? What makes them similar or different from one another? Let’s find out, beginning with a substance you use so often that you probably take it for granted: water.

Water as a substance

Have you ever thought about what you are drinking when you drink a glass of water? Take a look at the two glasses in Figure 1. What do they contain? They contain a clear liquid that looks like water. But is the water identical in the two glasses? 

Figure 1: Two glasses of water, Glass A and Glass B, that appear the same. Are they the same?

image © CC BY-SA 3.0 S Nova

You can't really tell any differences between the water in the two glasses using your naked eye. However, if you were able to view them at the molecular level, you might see something as shown in Figure 2. 

Figure 2: A model of the water in Glass A and in Glass B as viewed at the molecular level. Glass A contains only one type of molecule, while glass B contains a mixture of elements and molecules.

In what ways are the models of water in Glass A and B similar? In what ways are they different?

First, consider Glass A. Glass A contains water “molecules” that all look the same. Molecules are formed by the chemical bonding of two or more “atoms,” and atoms are the smallest particles of “elements.” An exception are diatomic molecules formed by some elements when existing alone in nature, such as nitrogen gas existing as N2.

Do you know the chemical formula for water? It is H2O, meaning that two atoms of the element hydrogen are bonded to one atom of the element oxygen to form each molecule of water. In Figure 2, the white spheres represent hydrogen atoms, and the red spheres represent oxygen atoms. These molecules collectively make up the “compound,” water. 

The depiction of water in Glass A (Figure 2) is considered a “pure substance” because only water molecules are present. Pure substances can be made up of atoms or molecules. Atoms are the smallest parts of elements - pure substances found on the periodic table. They cannot be chemically made or separated. Pure substances made up of molecules are called compounds. Compounds are chemically made and can only be chemically separated. 

Now look at Glass B. Glass B contains molecules of water mixed with atoms of elements. This water is not pure but part of a mixture. The blue and green spheres in the model represent atoms or ions (atoms that have lost or gained electrons to become charged) of other elements. To figure out the elements, you should know that Glass B contains tap water, which can come from a lake, river, or groundwater source. Tap water is filtered at a water treatment plant to remove many particles from the mixture. Then it is disinfected with the addition of compounds that release chloride ions. Additionally, fluoride ions are often similarly added to tap water to reduce tooth decay. The blue and green spheres in Glass B represent chloride and fluoride ions. Because the particles in the model do not all look the same, Glass B is a model of a mixture. Mixtures can be made up of atoms, molecules, or a combination of both. Additionally, mixtures are physically mixed and can be physically separated. The atoms, molecules, or a combination of the two mixed together do not change but retain their unique characteristics and simply exist in a shared space.

Comprehension Checkpoint
What is another name for a pure substance made up of one type of atom?

You learned that water is considered a pure substance when it consists of only water molecules and nothing else. While you can drink this water, you may not like its taste. It does not have any flavor because there are no minerals in it. This type of water can be made through “distillation” or other purification techniques. In the distillation process, water is heated to form steam through evaporation. The impurities remain behind. Then steam is cooled to form distilled water through condensation.

Figure 3 shows the distillation process in the laboratory setting. Water is boiled in the round-bottom flask on the left. The steam travels upward and down the narrow tube on the right into the condenser. Cold water is forced through the outer part of the condenser tube to cool the steam in the narrow tube. Distilled water drips from the narrow tube into the flask on the right. 

Figure 3: The distillation process to purify water. Water, in the flask on the left, is boiled and condenses in the long tube leaving impurities behind.

image © CC BY-SA 3.0 William Crochot

Which water do we drink?

You also learned that water is a mixture when it consists of water molecules and other atoms or ions, like chloride and fluoride. The water we drink is a mixture. Remember that water can come from a lake, river, or groundwater source. It is mixed with lots of substances: dust, chemicals, parasites, bacteria, viruses, and more. It must be filtered and disinfected at a water treatment plant to make it safe for your consumption. But when did humans figure out the need to filter and disinfect water?

Ancient Greek, Egyptian, and Sanskrit writings indicate that water was filtered as early as 2000 BCE. Historical records and archaeological sites have indicated that water was treated in ancient Egypt, which is located in the Sahara Desert. Ancient Egyptians harvested rainwater. Rainwater was naturally purified and safe for drinking if it was stored in clean containers. 

Rainwater is a “homogeneous mixture” or “solution.” The word “homogeneous” is derived from the Greek roots homos (meaning the same) and genos (meaning kind). When you look at rainwater, it looks the same throughout but dissolved gases from the atmosphere are mixed in. The atmospheric gases are uniformly mixed or evenly distributed in the water, so rainwater is a solution

Ancient Egyptians also collected water from the Nile River. The river water was a “heterogeneous mixture,” meaning you could see sediments and tiny organisms mixed in the water. The word “heterogeneous” is derived from the Greek roots heteros (meaning different) and genos (meaning kind). 

The ancient Egyptians could see that the Nile River’s water was unclean. They were the first to add the chemical alum, a naturally occurring mineral, to river water because they found that it caused particles to settle at the bottom of the water container. Ancient Egyptians used the cleaner water at the top of the container and discarded the settled matter at the bottom of the container. 

The ancient Egyptians also used ceramic filters to clean the river water. They added herbs, seeds, and stones to improve the water quality and taste. Other ancient cultures used methods such as gravel filters, sand filters, and boiling to improve water quality. Regardless of the method, these examples show how river water (a heterogeneous mixture) could be physically separated to provide cleaner water for humans.

Comprehension Checkpoint
The term “heterogeneous” refers to rainwater because it is a mixture of water and dissolved gases.

The history of water disinfection

Ancient civilizations focused on physically separating water from the other particles they could see in the heterogeneous mixture. What was not seen were the disease-causing organisms from animal waste or other sources that can contaminate water. But the causes of these diseases were unknown in ancient times. In fact, many people believed that diseases were caused by humans upsetting the gods. 

The disinfection of water began centuries later after a cholera outbreak in London in 1854. Cholera is an intestinal disease caused by bacteria that leads to vomiting and diarrhea. In the mid-1800s, Dr. John Snow, a British medical doctor, mapped the location of each cholera death in the Soho area of London to look for a pattern, as shown in Figure 4.

Figure 4: A representation of the 1854 map of cholera deaths by Snow. The map revealed a pattern of cases occurring near the Broad Street water pump, as shown in red.

image ©Public Domain

At the time, indoor plumbing was rare. Instead, most people used one of several central pumping stations to get their water or wash household items. By mapping the locations of cholera deaths, Snow discovered something interesting—many of the people who developed cholera lived near and used London’s Broad Street pump. 

Snow studied the conditions at the Broad Street pump. He learned that many residents washed their clothes near the pump since it was their only water source. One resident, a mother whose baby suffered from cholera, washed contaminated diapers three feet from the pump. Snow realized the contaminated wash water could leak into the water supply. 

Snow appealed to the local governing body, the Board of Guardians, who took action by preventing residents from accessing the Broad Street water supply. With access to the Broad Street pump closed, the number of cholera cases in the city declined quickly. Snow had used his maps, a form of a scientific model, to understand London’s cholera outbreak and take action to control it. By the late 1800s, the British took advantage of another scientific discovery—that chloride ions could be used to control outbreaks of infectious diseases. The British began adding compounds with chloride ions to drinking water to kill the bacteria that caused cholera and other diseases. 

The practice of disinfecting water supplies with chloride ions began in the United States shortly after that. Over time, treatment methods and technologies have improved. Today our water supply is constantly monitored to ensure it is safe for human consumption. The Environmental Protection Agency (EPA) sets national standards for safe drinking water in the United States.  

How can you find out if the tap water you drink is safe? Communities release consumer confidence reports annually explaining the substances in the water supply. You can find information from your local community about what substances might be present in the homogeneous solution that is your tap water. While water testing has made our drinking water much cleaner and safer than in the 1800s, there are occasional breakdowns in the system, as infamously happened in Flint, Michigan, in the mid-2010s. 

Comprehension Checkpoint
What did the British add to their drinking water to kill harmful bacteria that cause cholera and other diseases?

Water crisis in Flint, Michigan

Dr. Mona Hanna-Attisha knows all too well what can happen if governments do not take on the responsibility of treating drinking water. Hanna-Attisha immigrated to the United States as a child refugee from Iraq. She received college degrees in environmental studies, sustainability, and public health before attending medical school. In 2015, Hanna-Attisha was practicing pediatrics in Flint, Michigan, when she visited an old high school friend after work one day. She learned from her friend, a water quality expert, that the water in Flint was not being treated properly and probably contained lead. Hanna-Attisha was alarmed.

She knew that the city water supply had changed over the previous year and a half to save money. Rather than receiving treated water from Detroit, water from the Flint River was pumped to a water treatment plant to provide tap water for residents. However, what was unknown was that the water was not treated correctly—the treatment did not account for the old pipes in the city’s infrastructure made of iron, copper, and lead. High levels of chloride ions were added to kill pathogens in the river water. However, the high levels of chloride ions also caused lead from the pipes to dissolve into the water forming an invisible solution

Hanna-Attisha knew that children exposed to lead might physically and behaviorally be affected. The exposure could impair children's nervous systems, hearing, growth, and blood cell function and formation. Further, exposed children may have learning problems, including hyperactivity. 

Hanna-Attisha knew that she needed data to immediately sound the alarm on the presence of high levels of lead in the tap water. So, she looked at medical records to see if blood lead levels in children across the city had changed since the shift in the water supply. What she found was alarming: Blood lead levels had doubled and even tripled for some children. 

Hanna-Attisha knew that publishing her findings would take months for peer review, but this problem was immediate. So, because of her concern for children’s health, Hanna-Attisha risked her career by publicly announcing her findings in 2015 before publishing them. Although she initially faced criticism, ultimately, she was praised for exposing the Flint water crisis. Figure 5 shows the results of Dr. Hanna-Attisha’s study that was published in 2016 (Hanna-Attisha et al. 2016). This bar graph shows the percent of children with elevated blood lead levels outside of Flint (left) and in Flint (right). The blue bars show the percentages before the change in the water source, and the red bars show the percentages after. The percentage of children with elevated blood lead levels increased significantly in all of Flint after the change in the water source.

Figure 5: A bar graph showing the results of Hanna-Attisha’s study. The graph shows that a higher percentage of children in the city of Flint, Michigan had elevated blood lead levels compared to those who lived outside of Flint.

image © data from Hanna-Attisha et al. 2016

Although residents had previously complained of the bad taste and smell of the water, city officials ignored the complaints. High levels of E. coli, an indicator of animal or sewage waste contamination, had even prompted a boil-water order several months after the shift in the water supply. Still, the city did not change its supply. The third-largest outbreak of Legionnaires disease in U.S. history occurred in 2015 because of the tainted water supply, killing 12 people and causing more than 87 people to report being ill.

In Flint, Michigan, government safeguards had failed. Part of the problem was that the city overlooked many complaints because of the demographics. Nearly 41% of the city’s population lived below the poverty line. Demographically, about 57% of the population was Black, 37% white, 4% Latino, and 4% mixed race. 

Advocates for the victims say that the citizens of Flint experienced environmental racism. These events, and Hanna-Attisha’s research, led to a group of citizens and activists suing the city and state. As a result, the city gave bottled water to residents, improved water testing significantly, and eventually replaced lead-containing pipes. 

Comprehension Checkpoint
How did they identify lead in Flint drinking water?

Classification of substances

Let’s take what you have learned about water as a substance and use it to build a model for the classification of substances. The terms and examples you learned about water as a substance are used in the model.

Figure 6: This is the classification system used by scientists to describe substances. The system categorizes substances as either pure substances, which are made up of a single type of element or compound, or mixtures of more than one.

Figure 6 shows a classification system that most scientists use to describe substances as pure or mixtures. Pure substances include elements made up of the same atoms and compounds made up of the same molecules. Mixtures are either homogeneous or heterogeneous. They can be made up of atoms, molecules, or a combination of the two. 

Remember the description of water treatment in ancient Egypt? We learned that rainwater is a homogeneous mixture (or solution). If you look at a sample of rainwater closely, it appears to look the same throughout, although there are gases dissolved within. River water is a heterogeneous mixture. If you look at a sample of river water closely, you will see sediments and tiny organisms. It does not look the same throughout. Both types of mixtures can be physically separated through distillation or filtration, as described earlier.

Let’s look at some of the substances you use daily and use the model in Figure 6 to classify them. When you use liquid soap in the shower, you use a mixture of water, detergent, oil, fragrance, and color compounds. It is most likely homogeneous because it is the same throughout the container. Toothpaste is fun because it can be either a homogeneous or heterogeneous mixture. Some toothpaste is the same throughout the container—often a chalky white color. These are homogeneous mixtures. However, some types of toothpaste have different colored stripes or sparkles running through the tube. In this case, they are heterogeneous mixtures because they look different throughout. Toothpaste contains fluoride, abrasives, flavoring, glycerol, and detergent compounds. 

What about breakfast foods and drinks? They are usually mixtures of compounds, elements, or a combination of both. For example, many breakfast cereals contain whole grains and compounds like sugar, flavoring, vitamins, and minerals. What is sugar by itself when sprinkled on your cereal? It is a compound or pure substance. If you have an energy drink, you are having a homogeneous mixture or solution of mostly the compounds water, sugar, and caffeine. However, the aluminum can that it comes in is a pure substance because aluminum is an element. And the coins you used to buy the drink from a vending machine? They are likely a mixture of copper and nickel—even solid substances can be mixtures. 

Let’s see if you recognize the classification of substances based on models. Look at Figure 7. 

Figure 7: Models of two different substances at the molecular level. How would you classify these substances?

In Figure 7, the model on the left includes molecules that are all the same. This is a pure substance containing molecules of the same compound. What about the model on the right? There are molecules of two different compounds. One compound contains two atoms of different elements, represented by the blue and orange colors. The other compound has two atoms of the same element, as shown in green—a type of compound we call “diatomic.” We can’t tell whether this substance is heterogeneous or homogeneous on this scale. Sometimes it is difficult to classify a substance without more information. 

Pure substances can either be elements or compounds. Remember that compounds are chemically made and can be chemically separated; mixtures are physically mixed and can be physically separated. Let’s see if the properties of each substance can help.

Comprehension Checkpoint
Mixtures may include different combinations of elements and compounds.

Pure substances have different characteristics than mixtures. Let’s learn what these characteristics are. Table 1 compares typical properties of pure substances and mixtures to help you recognize some patterns.

Table 1: A comparison table of the properties of pure substances versus mixtures. What patterns do you see between pure substances and mixtures in the table?

Properties of substances

Pure substances are just that—pure. They are made up of a single type of compound or element and thus have a chemical formula defined by that compound or element. Pure substances cannot be separated into parts, except in the case of compounds, which can be chemically broken down into their elements.  

Mixtures are combinations of two or more pure substances made by physical means (pouring them together, for example). They can be separated by physical means (like filtering), though sometimes this can take a lot of work. Mixtures have no single chemical formulas because they do not have definite compositions—they are made up of different amounts of elements or compounds. 

For example, you may use a recipe to stir a chocolate chip cookie mixture, but the recipe varies according to individual tastes. You physically mix up chocolate chip cookies, but how could you physically separate them? Do you ever pick the chocolate chips out of the dough and eat them? The dough could also be separated by dissolving it, evaporating the water, and sorting the ingredients. The ingredients retain their unique properties in the dough. Chocolate chip cookie dough is a heterogeneous mixture because you can see the particles that make it up.

Brief recap

You have learned about many amazing substances that we use every day. In its purest form, water is a pure substance made up of only molecules containing hydrogen and oxygen. But as this same water travels to your home, it becomes a mixture (or solution). As it goes through a local river or lake, the water picks up particles and organisms that make it a heterogeneous mixture. That heterogeneous mixture is physically separated by filtration at a water treatment plant. Then it is mixed with chloride and fluoride ions, forming a homogeneous mixture that comes out of our faucets and you can drink from a glass. Mixtures and pure substances are all around us, and we depend on them for life!

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