Impurity: A foreign substance mixed with a pure substance
Understanding Purity and Mixtures
At its heart, an impurity is a type of mixture. To understand it, let's start with pure substances. A pure substance has a fixed composition and definite properties. For example, pure water (H2O) boils at exactly 100°C at sea level. When we add salt (sodium chloride, NaCl) to it, the salt becomes an impurity. The water is no longer pure; it is a mixture where salt is the impurity from water's perspective.
It's a matter of perspective. If you wanted pure salt, then the water would be the impurity! An impurity is defined relative to what you consider the main, desired substance. This mixture can be:
- Homogeneous: The impurity is evenly mixed at the molecular level, like sugar dissolved in water. You cannot see the separate parts.
- Heterogeneous: The impurity is not evenly distributed, like sand mixed into a bag of rice. You can often see and separate the parts physically.
Sources and Types of Impurities
Impurities get into materials in many ways. They can come from the raw materials used, the equipment used to process them, or the environment (like air or water). Scientists categorize impurities to better understand and control them.
| Type of Impurity | Description | Everyday Example |
|---|---|---|
| Natural | Occur in raw materials from nature. | Minerals in well water; nitrogen gas in a sample of argon from the air. |
| Added (Intentional) | Put in on purpose to change properties. | Adding boron to silicon to make semiconductors; adding chlorine to water to kill germs. |
| Process-Induced | Introduced during manufacturing or handling. | Metal shavings from machinery ending up in food; dust settling on a clean surface. |
| Chemical Byproduct | Formed as an unwanted result of a chemical reaction. | Sulfur dioxide (SO2) as an impurity in industrial chemical production. |
The Double-Edged Sword: Harmful vs. Helpful Impurities
Impurities are not always bad. Their impact depends entirely on the situation.
Harmful Effects: Often, impurities degrade quality or cause problems.
- Medicine: Impurities in a drug can reduce its effectiveness or cause dangerous side effects[1]. This is why pharmaceutical companies purify substances to very high levels.
- Metallurgy: A tiny amount of sulfur or phosphorus in iron makes the resulting steel brittle and weak. Removing these impurities is crucial for making strong steel for buildings and cars.
- Electronics: For a silicon crystal to work in a computer chip, it must be extremely pure—often 99.9999999% pure ("nine nines" purity). A few unwanted atoms can disrupt the flow of electricity.
Helpful Effects: Sometimes, we intentionally add impurities to get desired properties.
- Alloys: Steel itself is an alloy—iron with intentionally added carbon and other metal impurities. These "impurities" make iron stronger, harder, and more resistant to rust.
- Colored Gemstones: Pure corundum (Al2O3) is clear and colorless. Add a little chromium impurity, and it becomes a beautiful red ruby. Add iron and titanium, and it becomes a blue sapphire.
- Food and Water: Iodine is added to table salt (as an impurity) to prevent iodine deficiency in people. Fluoride is added to drinking water to help prevent tooth decay.
Purification: The Quest for Purity
When impurities are unwanted, we use purification methods to remove them. The choice of method depends on the nature of the mixture.
| Method | How It Works | Used To Separate... |
|---|---|---|
| Filtration | Passes a mixture through a barrier with tiny holes. Larger particles (impurities) get trapped. | Sand from water; solid impurities from a liquid. |
| Distillation | Heats a liquid mixture. The substance with the lower boiling point evaporates first, is cooled, and collected as a pure liquid. | Pure water from saltwater; ethanol from water. |
| Chromatography | Uses a medium (like paper) to separate mixtures based on how quickly different substances move through it. | Different dyes in ink; complex biological mixtures. |
| Crystallization | Dissolves a mixture in a hot solvent, then cools it slowly. The desired substance forms pure crystals as it becomes less soluble, leaving impurities in the liquid. | Purifying sugar; obtaining pure salts from a solution. |
The level of purity needed is described in different ways. In a lab, you might see labels like "ACS Reagent Grade" (very pure for chemistry) or "USP Grade"[3] (pure enough for medicine). In industry, purity is often expressed as a percentage, like 99.5% pure, or in "nines," like "five nines" (99.999%) pure.
From Kitchen to Laboratory: Practical Examples
Let's connect these ideas to real-world scenarios you might encounter.
Example 1: Making Ice Cream Saltier to Make Ice Cream Colder. When making ice cream at home, you mix rock salt with ice in the outer chamber of the ice cream maker. The salt dissolves in the thin layer of water on the ice, creating a brine solution. This solution has a lower freezing point than pure water. The impurity (salt) disrupts the water's ability to form ice crystals, causing it to absorb more heat from the ice cream mixture inside. This extra heat absorption freezes the ice cream. Here, the impurity is used intentionally to change a physical property (freezing point).
Example 2: The Story of a Silicon Wafer. This journey shows both the removal and addition of impurities.
- Starting Material: Silicon comes from sand (silicon dioxide, SiO2). Sand is melted and reacted to produce metallurgical-grade silicon, which is only about 98% pure—full of impurities.
- Purification: Through a complex chemical process, the silicon is converted to a gas (silane or trichlorosilane), distilled to remove gaseous impurities, and then deposited as hyper-pure silicon crystals. This is ultrapurification.
- Intentional "Impurity" Addition (Doping): The pure silicon crystal is then "doped" with a precise, tiny amount of an element like boron or phosphorus. These atoms become part of the crystal structure and give silicon the electrical properties needed to make transistors—the building blocks of all computers and phones.
Important Questions
In theory, yes, but in practice, achieving 100% purity is incredibly difficult and often unnecessary. There will almost always be a few atoms or molecules of something else present. For most applications, we define a practical standard of purity that is "good enough." For example, distilled water for a car battery doesn't need to be as pure as water used in a sensitive chemistry experiment.
Absolutely! This is one of the most visible effects of impurities. As mentioned with rubies and sapphires, trace metal ions can cause vibrant colors. Similarly, common glass is made from silica sand, which often has iron impurities that give it a greenish tint. "Clear" glass for windows or labware is made from sand with very low iron content.
Scientists use analytical instruments. For example, Spectroscopy measures how a substance interacts with light to identify even tiny amounts of different elements. Chromatography (like in the purification table) is also used as an analytical tool to separate and identify the components of a mixture, showing what impurities are present and how much.
Footnote
[1] Side Effects: Unintended and often undesirable physical or mental effects caused by a medicine or treatment.
[2] Doping: In semiconductor physics, the intentional introduction of impurities into an extremely pure semiconductor to change its electrical properties.
[3] USP Grade: United States Pharmacopeia Grade. A purity standard set by the USP for chemicals used in medicines and food, ensuring they are safe and of high quality.