- Define a mixture and understand why mixtures are different than pure substances.
- Classify mixtures as homogeneous or heterogeneous.
- Describe several ways to separate mixtures.
- Distinguish between elements and compounds.
- chemical change
- chemical formula
- chemical symbol
- heterogeneous mixture
- homogeneous mixture
Check Your Understanding
Recalling Prior Knowledge
- What is a pure substance?
- What are some examples of physical changes, and what happens to the identity of a substance when it undergoes a physical change?
In the previous lesson, “Properties of Matter,” we learned about the physical properties of pure substances and about the physical changes that matter can undergo. However, there are many examples of matter that cannot be classified as pure substances because they have a composition that is variable rather than fixed.
Ordinary table salt is called sodium chloride. It is considered a pure substance because it has a uniform and definite composition. All samples of sodium chloride are chemically identical. Water is also a pure substance. Salt easily dissolves in water, but salt water cannot be classified as a pure substance because its composition can vary. You may dissolve a small amount of salt or a large amount into a given sample of water. A mixture is a physical blend of two or more components, each of which retains its own identity and properties. Only the form of the salt is changed when it is dissolved into water. It retains its composition and chemical properties.
An activity that makes it easy to understand the differences between elements, compounds, and mixtures can be found at www.bbc.co.uk/schools/ks3bitesize/science/chemical_material_behaviour/compounds_mixtures/activity.shtml.
A homogeneous mixture is a mixture in which the composition is uniform throughout the mixture. The salt water described above is homogeneous because the dissolved salt is evenly distributed throughout the entire salt water sample. It is often easy to confuse a homogeneous mixture with a pure substance because they are both uniform. The difference is that the composition of the pure substance is always the same. Air is a mixture because the different gases that make up air may be present in varying amounts. A troublesome aspect of modern society is that the burning of fossil fuels in large amounts has increased the amount of carbon dioxide in our atmosphere compared to the levels that were present hundreds of years ago before the industrial revolution. Carbon dioxide acts as a greenhouse gas, trapping heat in our atmosphere. The increase in carbon dioxide levels has led to a corresponding increase in average global temperatures (Figure below).
Carbon dioxide levels in Earth’s atmosphere (measured in parts per million) have increased dramatically since 1900. The average global temperature has also seen an increase, as seen on the left-hand axis.
A heterogeneous mixture is a mixture in which the composition is not uniform throughout the mixture. Vegetable soup is a heterogeneous mixture. Any given spoonful of soup will contain varying amounts of the different vegetables and other components of the soup.
A phase is any part of a sample that has a uniform composition and properties. By definition, a pure substance or a homogeneous mixture consists of a single phase. A heterogeneous mixture consists of two or more phases. When oil and water are combined, they do not mix evenly (Figure below). Instead, they form two separate layers, each of which is considered a phase.
Oil and water do not mix. Instead, they form two distinct layers called phases. The oil phase is less dense than the water phase, so the oil floats on top of the water.
Separation of Mixtures
A pizza is a heterogeneous mixture. If you have a pizza slice with green peppers on it and you don’t like green peppers, you could easily pick them off before eating the slice. Other mixtures may not be as easy to separate. The oil and water mixture can be separated by carefully pouring off the oil layer in a process called decanting. The components of all mixtures can be separated by some physical means.
Sand does not dissolve in water. In order to separate sand from water, the mixture can be poured through a filter. The tiny holes of a filter, called pores, allow the water to pass through while trapping the sand grains behind. Filtration is a technique that separates solids from liquids in a heterogeneous mixture (Figure below).
This filtering apparatus uses suction to pull the water through a fine glass filter, leaving behind a yellow-colored solid.
Distillation is a method of separating a mixture based on differences in boiling points. A distillation apparatus is pictured in the Figure below.
A small-scale laboratory distillation apparatus is used to separate substances based on differences in their boiling points.
For example, the impure liquid could be a mixture of alcohol and water. As the mixture is heated, the alcohol begins to boil before the water does because of its lower boiling point. The alcohol vapors rise up in the flask and into the long cylindrical tube, which is called a condenser. Cold water is continually pumped into the outside of the condenser, which causes the alcohol vapors to condense back into the liquid state. The purified alcohol then drips into the flask labeled “distilled liquid,” while the water is left behind in the original flask.
Distillation is commonly used to purify water by separating it from the various dissolved minerals that are found in natural water sources.
You can view the distillation of methanol to separate it from a dye at www.youtube.com/watch?v=74rOJ1GFbC0 (1:16).
As described in the lesson, “Properties of Matter,” pure substances have a definite and fixed composition. Pure substances can further be classified as either elements or compounds.
An element is the simplest form of matter that has a unique set of properties. Examples of well-known elements include oxygen, iron, and gold (Figure below). Elements cannot be broken down into simpler substances. Additionally, one element cannot be chemically converted into a different element.
Chemical elements are the simplest of pure substances. (A) An oxygen tank like this is used by people with a need for breathing assistance. (B) A simple skillet can be made from cast iron. (C) Gold bars are formed and used for monetary purposes.
A compound is a pure substance containing two or more elements that have been chemically combined in a fixed proportion. The elements carbon and hydrogen combine to form many different compounds. One of the simplest is called methane, which always has four times as many hydrogen particles as carbon particles. Methane is a pure substance because it always has the same composition. However, it is not an element because it can be broken down into simpler substances (the elements carbon and hydrogen). The process of breaking down a compound into its elements is sometimes called decomposition.
Recall that the components of a mixture can be separated from one another by physical means. This is not true for a compound. Table salt is a compound consisting of equal parts of the elements sodium and chlorine. Salt cannot be separated into its two elements by filtering, distillation, or any other physical process. Salt and other compounds can only be decomposed into their elements by a chemical process. A chemical change is a change that produces matter with a different composition. Many compounds can be decomposed into their elements by heating. When sugar is heated, it decomposes to carbon and water. Water is also a compound, but it cannot be broken down into hydrogen and oxygen by heating. However, the passage of an electrical current through water will produce hydrogen and oxygen gases.
The properties of compounds are generally very different than the properties of the elements from which the compound is formed. Sodium is an extremely reactive soft metal that cannot be exposed to air or water. Chlorine is a deadly gas. The compound sodium chloride is a mostly unreactive white solid that is essential for all living things (Figure below).
(A) Sodium is so reactive that it must be stored under oil. (B) Chlorine is a poisonous yellow-green gas. (C) Salt crystals are a compound of sodium and chlorine.
Distinguishing Between Pure Substances and Mixtures
The flowchart pictured in the Figure below summarizes the types of matter discussed in this chapter.
The flowchart describes the classification of matter.
Matter can be divided into two categories: pure substances and mixtures. Pure substances can be either elements or compounds. Mixtures can be either homogeneous or heterogeneous. Another term for a homogeneous mixture is a solution. Sometimes it can be difficult to tell the difference between a compound and a homogeneous mixture based solely on appearance. Consider gasoline. It is certainly homogeneous and seems a likely candidate for a compound, meaning that it would have a fixed composition. However, as you may know, gasoline can be purchased at a station in a variety of grades, such as regular or premium grade. Premium grades have a higher octane rating and cost more than regular grades. Gasoline is thus a blend of different compounds, so it is a mixture.
Chemical Symbols and Formulas
Chemists use symbols and formulas as an abbreviated way of writing chemical reactions and the elements and compounds involved in them. A chemical symbol is a unique one- or two-letter designation of an element. Some examples of chemical symbols are O for oxygen, Zn for zinc, and Fe for iron. The first letter of a symbol is always capitalized. If the symbol contains two letters, the second letter is lower case. The majority of elements have symbols that are based on their English names. However, some of the elements that have been known since ancient times have maintained symbols that are based on their Latin names (Table below).
Symbols and Latin Names for Various Elements
As stated earlier, compounds are combinations of two or more elements. A chemical formula is an expression that shows the elements in a compound and the relative proportions of those elements. For example, water is composed of hydrogen and oxygen in a two to one ratio. The chemical formula for water is H2O. Sulfuric acid is one of the most widely produced chemicals in the Unites States and is composed of the elements hydrogen, sulfur, and oxygen. The chemical formula for sulfuric acid is H2SO4.
- Mixtures are physical blends of two or more pure substances and can either be homogeneous or heterogeneous. Mixtures can be separated into their components by physical means such as filtration or distillation.
- Pure substances may be either elements or compounds. Elements are the simplest types of matter, while a compound is a chemical combination of two or more elements. A compound may only be decomposed into its elements by a chemical process.
- Chemical symbols and formulas are used to represent elements and compounds.
Lesson Review Questions
- Why can’t a mixture be classified as a pure substance?
- Classify each of the following as a homogeneous or heterogeneous mixture.
- iced tea
- cooking oil
- When is filtration used to separate a mixture? When is distillation used?
- What is wrong with the following explanation of an experiment? “When it was heated, the sample of carbon decomposed.”
- Identify the following as either a chemical symbol or a chemical formula.
- Describe a way to separate a mixture of fine sand and table salt without having to directly sort tiny crystals using tweezers.
- Name the elements found in each of the following compounds.
- ammonia (NH3)
- saltpeter (KNO3)
- glucose (C6H12O6)
- millerite (NiS)
- Identify each of the following items as an element, compound, or mixture. If it is a mixture, further classify it as homogeneous or heterogeneous.
- carbon dioxide
Further Reading / Supplemental Links
- Another method for separating a mixture is chromatography. You can see how bomb specialists use chromatography to identify the chemical components that were used in the making of the bomb. There is also a chromatography activity that you can do at home! Go to www.pbs.org/wgbh/nova/bombingactivity/chromatog.html.
Points to Consider
In a chemical reaction, one or more substances are changed into different substances.
- How are chemical changes different from physical changes?
- What are the visual clues that a chemical reaction is taking place?