In this explainer, we will learn how to determine the purity of a substance based on its proportion by mass and physical properties.
In everyday life, we use the word pure to describe many things. For example, we might describe the clear sound from an instrument as a pure note. Some fabrics are labeled pure cotton or cotton because no synthetic fabric fiber has been mixed with the cotton to make the cloth.
We sometimes also call certain foods pure. For example, some foods have labels on them that say pure apple juice, pure olive oil, or pure honey.
When a product such as apple juice claims to be pure, it usually means the juice is made only from apples. Pure olive oil is made from olives alone, and pure honey is honey direct from the comb, with nothing added. From this, we can see the term pure is used in many ways, with slightly different meanings.
In chemistry, the term pure has a very specific meaning. A chemically pure substance is a substance that contains only one type of element or compound, with only one chemical composition.
Definition: Pure Substance
It is a substance that contains only one type of element or compound.
Apple juice is not a pure substance, even if it is made only from apples. Apple juice contains various different substances, including water, sucrose, fructose, glucose, starch, tannin, pectins, vitamin C, and minerals. Olive oil contains several different substances and so does honey. Olive oil and honey are not chemically pure substances.
The table below shows some chemically pure substances.
Pure crystals of table sugar contain only sucrose () molecules. Pure copper wire contains only atoms of the element copper. The silver gas canister in the last photograph contains pure liquid helium.
All matter can be categorized into one of two categories, pure substances and mixtures. Any mixture consists of more than one substance, and so mixtures can be considered impure substances. The substances in a mixture are not chemically bonded to each other; therefore, the components can be separated by physical means.
Definition: Impure Substance
It is a combination or mixture of two or more different substances not chemically bonded together.
Example 1: Using the Everyday and Scientific Meanings of the Term Pure
The image below shows a labeled bottle of orange juice.
- Why might the company claim the orange juice is pure?
- The solution is completely orange in color.
- It contains no added or artificial products.
- It is made up of any one type of compound.
- It has a pH of 7, making it neutral.
- The oranges were organically grown.
- Why might a chemist say the orange juice is not pure?
- It does not contain just one substance.
- It is slightly acidic.
- The particles are free to move around each other.
- It only contains an artificial substance.
- The solution is not colorless.
The label says that the orange juice is made from orange concentrate. Orange concentrate is orange juice that is extracted from oranges, some of the water is removed to concentrate the juice, then later water is added back to reconstitute it. Orange juice, in both the concentrated and reconstituted forms, contains many compounds, including water, sucrose, glucose, fructose, ascorbic acid, citric acid, other vitamins, and minerals. Statement C, “it is made up of any one type of compound,” is not correct.
We cannot determine whether the oranges were organically grown or not from the information provided on the label. Organically grown does not mean pure in a scientific sense. Organically grown means grown without the use of pesticides and herbicides. Statement E, “the oranges were organically grown,” is not correct.
The color of orange juice, as well as that of other mixtures, does not indicate its purity. Often, liquids are uniform in color but, like orange juice, may be mixtures of many substances. Statement A, “the solution is completely orange in color,” does not indicate purity.
The pH of a solution is related to the acidity or basicity of a solution. Orange juice contains several different acids and it, therefore, has a slightly acidic, or slightly low, pH. It does not have a pH of 7, and it is not neutral. The pH of orange juice does not indicate purity but merely that it is an acidic solution. Statement D, “it has a pH of 7, making it neutral,” is untrue.
The label says pure orange juice, so we can assume from this that the manufacturer means the juice is only made from oranges, with no other added or artificial substances. The correct answer is B, “it contains no added or artificial products.”
The scientific meaning of the term pure is a substance that contains only one type of element or compound. Orange juice does not contain only one substance; it contains many different substances. Statement A, “it does not contain just one substance,” is an accurate description a chemist might make to explain why orange juice is not pure. The correct answer is A.
Even if a substance contains the tiniest amount of another substance, it is considered impure. In other words, if a substance is not pure, consisting of that substance alone, it is considered impure.
How pure a substance is can be called its purity.
It is a measure of the extent to which a substance is free from impurities.
Consider a beaker containing only ethanol molecules, several beakers containing water and ethanol in different proportions (, , and ), and a beaker containing water molecules only. We will assume there are no other substances, such as gas molecules, mixed in the beakers. The diagram below shows this information graphically.
We can measure the purity of water in each beaker to any value between and . When only ethanol is present, water has a percentage purity of . When only water is present, it has a percentage purity of . When of the liquid is composed of water, the percentage purity of water is , and so on.
It is very difficult and costly for a chemist to purify a chemical to . Usually, the purest a chemist can make a substance is approximately pure, and a substance above pure is considered to be of high purity.
Ultrahigh purity (purity above ) is important and necessary in some applications, such as in medicines. Very small percentages of an impurity in a medicine could potentially be harmful, depending on what the identity of the impurity is.
However, we do not always require ultrapure substances. For example, many cleaning products are of low purity, but this does not matter, as they can still perform the task they are needed for.
It is an undesired substance that is mixed in with a desired substance, making it impure.
Impurities can occur in many forms. An impurity might be in the same phase as or in a different phase than the desired substance or in a large or a small proportion. Impurities can sometimes be easy to spot because they have a different color from the desired product, but often they have the same color, making them more difficult to notice.
The presence of impurities is not always a disadvantage. The presence of atoms of other elements in metals can create alloys with much greater physical properties than the pure metal. On its own, pure iron is very soft, which does not make it useful for many applications. However, the addition of carbon as an impurity creates the alloy steel. Here, the impurity is beneficial, as it improves the hardness of the metal, making it much more useful. In addition to carbon, some chromium can also be added to steel to create stainless steel. The benefit of the chromium impurity is to slow down, or even prevent, the formation of rust.
Gemstones such as sapphires and rubies are minerals of aluminum oxide that contain impurities such as chromium, iron, or titanium.
Example 2: Describing What an Impurity Is
Which of the following statements best describes what an impurity is?
- An unwanted substance mixed within a desired substance
- A wanted substance mixed within a desired substance
- A substance finely dispersed in another substance
- A substance dissolved in another substance
- A substance that can be separated from another substance
An impurity is a substance that contaminates a desired substance, making it impure. The impurity can be in the same phase as or in a different phase than the desired substance, can be in a large or a small proportion, and can be the same color or a different color. The important thing is that it is not wanted. The correct answer is A: an unwanted substance mixed within a desired substance.
The formula for calculating the percentage purity of a sample is or
We can refine this formula by replacing the word amount with either mass, volume, or moles. Therefore, the percentage purity expression for mass is
A 19 g sample of sulfur powder and iron filings is found to contain 0.25 g of iron filings. We can calculate the percentage purity of the sulfur in this mixture.
The total mass of the impure sample is 19 g, while the mass of the iron impurity is 0.25 g. Using this information, we can calculate what percentage of the entire sample is sulfur and, therefore, the percentage purity of sulfur.
We start by calculating the mass of sulfur. To do this, the mass of iron is subtracted from the total mass of the impure sample:
Then, we can use the formula for percentage purity, and substitute in 18.75 g of pure sulfur and 19 g for the total mass of the impure sample. The equation thus becomes
The two gram units cancel each other out, and we get
From the calculation, we can see that of the sulfur–iron sample consists of sulfur. A percentage purity of means that the sample of sulfur is of high purity.
Example 3: Calculating the Purity of a Sample of Magnesium Chloride
An impure sample of magnesium chloride has a mass of 50 g. After perfect purification, 45 g of magnesium chloride is recovered. What is the percentage purity of the original sample?
The percentage purity of a sample refers to the percentage of a specific desired compound in the sample. In others words, the percentage of the sample that is composed of a particular substance. The formula for percentage purity by mass is
We are told that a mass of 45 g of the pure compound magnesium chloride is obtained from a total mass of 50 g of an impure sample of magnesium chloride. We can substitute these values into the equation and solve:
We get a percentage purity of magnesium chloride of . This means that of the original impure sample was composed of the compound magnesium chloride.
A chemical can appear to look or feel pure, but often this is not the case. Consider the following examples:
- Air appears pure and looks pure but is actually composed of many different substances, including gases like , , , , , and as well as tiny droplets of water as humidity.
- Tap water can appear clear, colorless, and therefore pure. However, tap water contains several dissolved ionic substances (minerals), dissolved gas particles, some microscopic dust particles, and very small quantities of other chemical impurities from water treatment plants.
- Common household substances, such as salt, look pure but have a small amount of impurities, such as anticaking agents that prevent the salt from clumping.
Factors like atmospheric conditions, as well as how a substance is synthesized and purified, will determine whether residual impurities remain in the substance or not. A seemingly pure sample of sugar may contain tiny amounts of water from humidity in the air.
We cannot confirm if a substance is pure just by the way it looks or feels. We need to test other properties to confirm purity. We can test the boiling point for liquids and the melting point for solids to confirm whether they are pure or not.
Definition: Boiling point
The boiling point is the temperature at which a liquid is converted to a gas.
Definition: Melting Point
The melting point is the temperature at which a solid is converted to a liquid.
Consider two clear colorless liquids. One is pure water, and the other is salt water. But we do not identify them by inspection because they both have similar appearances.
Besides tasting the liquids, which we should not do in the laboratory, we could distinguish between the two liquids by performing a boiling point test, as shown in the diagram below.
At 1 atmosphere of pressure, pure water boils at . This is a characteristic property of pure water. However, when water contains some dissolved impurity (for example, table salt), it boils at a slightly higher temperature (around ). The boiling point is usually higher if the liquid is impure.
For a solid substance, we can determine whether it is pure by testing its melting point. The melting point of a pure substance occurs at a specific temperature unique to that substance. Table sugar, or sucrose, melts at . This melting point is characteristic of sucrose, as shown in the diagram below.
However, when sucrose is contaminated with an impurity, the melting point is slightly lower and the substance melts over a small range of temperatures instead of sharply at just one value.
Usually, the melting point of an impure solid is slightly lower and occurs over a range of temperatures, compared with a pure solid. A common test to characterize the purity of a substance is to determine whether its melting point is precise or occurs over a range of temperatures.
To remove an impurity from a sample, we can perform purification processes. These include the following:
- Distillation to purify an impure liquid: the desired liquid is boiled off and the vapors are recondensed as the pure liquid.
- Filtration to separate insoluble solids from a liquid: the impure sample is poured into a funnel containing filter paper. The insoluble substance is trapped on the filter paper, while the soluble substance moves through the funnel in the solution as the filtrate. The desired substance may be the insoluble solid, which can be collected from the filter paper, or it may be a dissolved solid in the filtrate solution, which can then be evaporated to remove the solvent.
- Recrystallization to purify a solid: it is the process of cooling a saturated solution to cause crystallization of the desired substance, which can then be separated by filtration.
Example 4: Relating Melting and Boiling Point Values to the Purity of a Substance
A student obtains a sample of water and wants to test its purity by determining its melting and boiling points.
- What results should the student expect to find if the sample is pure?
- The sample boils and freezes over a large temperature range.
- The sample boils between and and freezes between and .
- The sample boils at exactly and freezes at exactly .
- The sample boils over a large temperature range but freezes at exactly 0°C.
- How can the student obtain pure water if the sample is found to contain soluble impurities?
Pure substances have unique characteristic properties that help us identify them. Pure water boils at a temperature of at a pressure of 1 atmosphere. Pure ice melts at . The student would expect the sample of water to boil at exactly and melt at exactly if the water was pure. The correct answer is C. The sample boils at exactly and freezes at exactly .
One way to obtain a pure liquid from an impure liquid is distillation. The student could boil the water containing the dissolved impurities. The water vapor that comes off the boiling liquid can be collected, cooled, and recondensed as pure liquid water. The correct answer is D: distillation.
Let us summarize what has been learned in this explainer.
- In chemistry, a pure substance is a chemical that contains only one type of particle, with only one chemical composition.
- An impure substance is a combination or mixture of two or more different substances that are not chemically bonded together.
- Purity is how pure a substance is or the degree to which a substance is free from contamination.
- An impurity is a substance that is mixed with, and contaminates, a desired substance.
- Percentage purity can be calculated using the equation or
- The appearance of a chemical alone is not enough to determine whether it is pure or not.
- We can use a melting point test for solids and a boiling point test for liquids to determine whether they are pure.
- The melting point of a pure solid is precise, but that of an impure solid usually decreases and occurs over a broad range of temperatures.
- The boiling point of a pure liquid is precise, but that of an impure liquid usually increases and occurs over a broad range of temperatures.
- Methods of purification include distillation, filtration, and recrystallization.