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Introduction

A mixture is a combination of different substances. Those substances can be solids, liquids, or gases. For example, soup is a mixture of liquid broth and solid vegetables.

The substances in a mixture keep their physical properties, such as color, density, and taste. For example, salt water is a mixture of salt and water. The water keeps its properties, such as being a liquid and being transparent. The salt also keeps its properties, such as tasting salty. The salt may get broken into smaller particles that are too small to see, but it is still the same substance, with the same properties.

In science there are different names for mixtures, depending on the size of the parts of the mixture and whether the mixture is the same throughout. Some mixtures have large parts that can be seen and can be separated easily. A bowl of cereal with milk is a mixture. In other mixtures the substances that are combined are too small to see.

There are four main types of mixtures:

• mixtures with large particles
• suspensions: mixtures with medium-small parts
• colloids: mixtures with very small particles
• solutions: mixtures with tiny particles that are too small to see separately

• air: a mixture of multiple gases
• muddy water: a mixture of solids (dirt) and liquid water
• sand and gravel: a mixture of solids of different sizes
• soup: a mixture of solids and liquids
• brass: a mixture of multiple kinds of metal
• soda: a mixture of air bubbles in liquid

Suspensions

In a suspension, particles of a solid are mixed into a liquid or gas. The solid particles are too large to stay mixed for very long. For example, if sand is stirred into water, the sand will stay up, or suspended, in the water for a little while. However, once the stirring stops, much of the sand will settle to the bottom within minutes. Many liquid medicines are also suspensions. When the mixture sits on a shelf, the medicine settles to the bottom of the bottle. Before taking the medicine, the bottle must be shaken to mix the solid, powdered medicine back into the liquid. Other examples of suspensions are salad dressing, mud, and juice with pulp.

Colloids

The particles of colloids are small enough to stay mixed, but the particles can make the mixture look cloudy or opaque. They stay the same over time, but it is difficult or impossible to see through them. Colloids can be made of solids, liquids, or gases. Foam rubber is a colloid of a solid with air bubbles—a gas—mixed throughout. Milk and ink are colloids of liquids. Fog is a colloid of a gas and a liquid. The gas is air, and the liquid is water droplets. Other examples of colloids are perfume, smoke, butter, and cheese.

Solutions

One common type of mixture is a solution. A solution is a mixture of two or more substances that stay evenly mixed. In some solutions it is clear what the parts are, even though we cannot see them, such as sugar water. When a spoonful of sugar is stirred into a glass of water, it will dissolve and form a solution. You cannot see the sugar any more because the tiny sugar particles, called molecules, spread apart evenly throughout the water. Other solutions may not seem like they are made up of different parts. For example, steel is actually a mix of iron and carbon. The parts get mixed when the iron is so hot that it melts and can be stirred. Then it hardens, with the parts mixed together.

Solutes and Solvents

A solution forms when a substance dissolves, or breaks apart, when mixed into another substance. The substance that dissolves is called a solute. The substance that a solute dissolves into is called a solvent.

In a sugar-water solution sugar is the solute and water is the solvent. The sugar dissolves into the water. Another example of a substance that dissolves in water is table salt. Salt is a solute that dissolves in water to form a saltwater solution.

Solid, Liquid, and Gas Solutions

Solutions are commonly thought of as liquids. However, solutions may be gases, liquids, or solids. The solvent and solute parts may also be gases, liquids, or solids.

Air is a solution. It consists mainly of nitrogen and oxygen gases, along with small amounts of several other gases. There is more nitrogen than any other gas in air, so nitrogen is considered the solvent. All the other gases in air are the solutes.

Brass is a solid solution made up of copper and zinc. These metals can be mixed together after they have been heated to a very high temperature. The brass solution that forms stays evenly mixed even after it has cooled and hardened. Brass has more copper in it than zinc. Therefore, copper is the solvent and zinc is the solute.

Vinegar is an example of a solution made up of two liquids. Water is the solvent, and acetic acid is the solute. In a solution made up of liquid substances, the liquid substance of greater amount is usually considered the solvent.

Physical Changes and Solutions

Physical changes occur when solutions are formed. The solute and solvent substances do not turn into new substances, or undergo chemical changes. However, the individual particles of the different substances in the solution affect one another.

The solute will break up into molecules or atoms that arrange themselves uniformly throughout the solvent. This changes the physical properties of the solvent. For example, the temperature at which pure liquid water turns to ice is 32 °F (0 °C). This is called its freezing point. When table salt is dissolved in pure water, the freezing point of the solution is lower than 32 °F. The more salt that is added to the water, the more the freezing point will decrease. This is why salt is often spread on roads and sidewalks during snowy, cold weather.

Another physical property that changes when a solution is formed is the boiling point—or the temperature at which a liquid turns to gas. For example, the boiling point of pure water is 212 °F (100 °C). However, salt water has a higher boiling point than pure water. The more salt that is added to water, the more the boiling point will increase.

Separating Mixtures

All mixtures can be separated. However, some are easier to separate than others. Some mixtures with large particles can be taken apart by hand. For other mixtures, other methods are necessary.

Sedimentation and Decantation

InteractiveMixtures with larger particles, including suspensions such as sand and water, can be separated by a process called sedimentation. If a bucket of sandy water is left to sit, the sand will settle to the bottom of the bucket. The sand falls to the bottom because the sand is more dense than the water. Then the mixture can be separated into two containers by decantation—pouring, suctioning, or scooping off the less dense upper layer. The lower layer, the sediment, is left behind. These processes—sedimentation and decantation—can separate solids that are mixed into liquids but do not dissolve into the liquid. They can be used for many mixtures. For example, solids such as leaves and debris can be separated from storm water at a wastewater treatment plant. These processes are also used by doctors. Blood is a mixture of tiny red blood cells in clear liquid. When a doctor or nurse takes a blood sample and lets it sit, the blood cells settle to the bottom. This is one way a doctor can check if there are enough blood cells in a person’s blood! Sedimentation and decantation can also be used to separate liquids that do not dissolve and have different densities, such as oil and water.

Filtration

Mixtures with large particles can also be separated by filtration. A mixture of sand and water can be separated by filtration. The sand does not dissolve, so the pieces of sand are larger than the individual water molecules. If the mixture is poured through a filter that has holes that are smaller than the sand grains, the sand will be held back by the filter. The water molecules will pass through the filter.

Like sedimentation, filtration can be used to separate solids that are mixed into liquids but do not dissolve. Because they do not dissolve, the solid particles remain large enough to be caught by a filter. Filtration can be used as an alternative to sedimentation. Filtration can also be used to separate substances with similar densities, which do not separate by sedimentation. This makes filtration useful for separating out solids such as bacteria or certain plastics, which neither sink nor float in water.

InteractiveDifferent filters have different-size holes or pores. A filter with quarter-inch (6-millimeter) holes could be used to separate gravel from sand. A filter with 0.000004-inch (0.1-micron) holes can remove bacteria and make water safe to drink. Filters can be made of metal, plastic, or even paper.

Evaporation and Distillation

Solutions and colloids are not always easy to separate because they are so well mixed. The tiny particles do not settle out, so they cannot be separated by sedimentation. All of the particles are very tiny, so they also cannot be separated by filtration. In order to separate a mixture such as a solution, it is necessary to use evaporation.

If a sugar-water solution is left sitting in an open container, the water will eventually evaporate, or change from a liquid to a gas, leaving the sugar behind. Evaporation allows the two parts of the solution to separate, because the water evaporates more readily than the sugar does.

For some solutions, it may be necessary to heat up the mixture to get one substance to evaporate away. One of the properties of a substance is its boiling point, the temperature at which the substance turns from a liquid to a gas. When two substances are in a solution, each substance still has its own boiling point. For example, the boiling point of water is 212 °F (100 °C), and the boiling point of table salt is 2,669 °F (1,465 °C). If a liquid solution is heated, the substance with the lower boiling point will evaporate first.

For example, if a saltwater solution is heated to 212 °F (100 °C), the water in the solution will evaporate, while the salt in the solution will not. This lets the two substances be separated. To obtain pure salt, without any water, the solution can be heated until all of the water evaporates away. To obtain pure water, the evaporated water molecules can be collected and condensed back into liquid water. This process is known as distillation. An apparatus known as a distiller is designed to heat a solution, capture the evaporating gas, and condense it back into a liquid. Distillation can be used to produce pure water. Many stores sell bottles of distilled water, which is free from salts and other chemicals.

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