Pure Substances and Mixtures
In this lesson we will learn how matter is classified based on its macroscopic and microscopic properties.
By the end of this lesson you will be able to:
Define matter and explain how it is composed of particles.
Distinguish between a pure substance and a mixture.
Distinguish between a homogeneous mixture and heterogeneous mixture.
Explain how matter can be observed at the macroscopic and microscopic level.
Distinguish between solutions, colloids and coarse mixtures.
- Matter is all around us. It’s what things are made of. You’re made of matter. So is the chair you’re sitting on. So is the air you breathe and the water you drink.
- Matter can be defined as anything that takes up space and has mass.
- But if everything is made of matter, why does everything look different
- It has to do with the different particles making up matter.
- Particles are the tiny building blocks of matter.
- You, your chair, air and water are all made up of different types of particles.
- Scientists classify matter based on the types of particles making it up.
Matter is anything that takes up space and has mass.
- A pure substance is something that is made up of only one type of particle.
- A pure substance cannot be physically separated into other substances, because all of the particles are the same.
- Pure substances have fixed physical properties, such as melting and boiling point.
- Examples of pure substances include oxygen, water and iron.
Oxygen, water and iron are examples of pure substances.
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- Two or more pure substances can combine to form a mixture.
- Therefore mixtures are made up of more than one type of particle.
- The formation of a mixture does not result in a new substance.
- A mixture is just a physical combination of pure substances.
- Mixtures can be physically separated into their components.
- Mixtures have variable physical properties, depending on their components.
- Examples of mixtures include air, sea water and soil.
- Most substances around us are mixtures.
Air, sea water and soil are examples of mixtures.
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Homogeneous and Heterogeneous Mixtures
- Mixtures can be divided into two types: homogeneous mixtures and heterogeneous mixtures.
- Homogeneous mixtures have a uniform composition.
- This means that regardless of where a sample is taken from, the sample would contain the same blend of components.
- Examples of homogeneous mixtures include mouthwash, paint and stainless steel.
Mouthwash, paint and stainless steel are homogeneous mixtures.
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- Heterogeneous mixtures have a non-uniform composition.
- This means that if samples were taken from different places within the mixture, the blend of components would not be the same.
- Examples of heterogeneous mixtures include mud and a bowl of cereal.
Breakfast cereals, mud and concrete are heterogeneous mixtures.
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- A phase is a part of a mixture that has a uniform composition.
- Since homogeneous mixtures have a uniform composition throughout, they have only one phase.
- Since heterogeneous mixtures do not have a uniform composition throughout, they have more than one phase.
- For example, an oil-water mixture has an oil phase and a water phase.
Oil and water form separate phases within a mixture.
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Classification of Mixtures as Solutions, Coarse Mixtures or Colloids
- Although mixtures are generally divided into two categories – homogeneous mixtures and heterogeneous mixtures – this is based on their appearance when viewed with the naked eye.
- However, certain substances, like milk, sunscreen and paint, might appear homogeneous, but when they are viewed under a microscope, separate phases are visible.
- In other words, they don’t have a homogeneous (uniform) composition.
Milk appears homogeneous with the naked eye, but under a microscope separate phases are visible.
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- Therefore, whether you are describing the composition of a mixture as homogeneous or heterogeneous depends on whether you are basing this on its appearance when viewed with a naked eye (its macroscopic appearance) or its appearance when viewed under a microscope (its microscopic appearance).
- When considering both the macroscopic and microscopic composition of mixtures, three categories of classifications become necessary: solutions, coarse mixtures and colloids.
- Solutions are mixtures that appear homogeneous with the naked eye as well as under a microscope.
- In other words, they are macroscopically and microscopically homogeneous.
- Examples of solutions include the mouthwash and stainless steel mentioned earlier, as well as petrol, methylated spirits and bleach.
Petrol, methylated spirits and bleach are solutions.
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- Coarse mixtures are mixtures that appear heterogeneous with the naked eye as well as under a microscope.
- In other words, they are macroscopically and microscopically heterogeneous.
- Examples of coarse mixtures include the mud and cereal mentioned earlier, as well as smoke, soup and concrete.
Smoke, soup and concrete are coarse mixtures.
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- Colloids are mixtures that appear homogeneous with the naked eye, but heterogeneous under a microscope.
- In other words, they are macroscopically homogeneous, but microscopically heterogeneous.
- Examples of colloids include the milk, sunscreen and paint mentioned earlier, as well as plain yoghurt, moisteriser cream and shaving foam.
Yoghurt, hand cream and shaving foam are colloids.
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Distinguishing Between Solutions and Colloids
- Coarse mixtures are quite easy to identify as their non-uniform appearance can be observed with the naked eye.
- Solutions and colloids can be harder to differentiate, as they both appear homogenous with the naked eye, and we don’t always have a microscope on hand to help us.
- One way to distinguish between solutions and colloids is to observe how light passes through them.
- Solutions are transparent, which means they are clear, because light passes easily through them.
- Note, transparent is different to colourless – solutions can be colourless, like methylated spirits, or they can be coloured, like petrol; in both cases, light passes through.
Solutions are transparent (clear).
(Image: LHcheM, Wikimedia Commons)
- Colloids, on the other hand, are opaque, which means they are cloudy, because light does not pass easily through them.
- For example, gels, ink and stained glass all appear homogeneous, but have an opaque appearance.
Colloids are opaque (cloudy).
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- Another way to distinguish between solutions and colloids is to test whether different phases can be separated.
- One way to test this is to pass the mixture through a fine filter. If nothing gets trapped by the filter, then the mixture is a solution; if some parts get trapped by the filter, then the mixture is a colloid.
- Another way to test this is to spin the mixture very fast in a machine called a centrifuge. If the mixture is a solution, it will look the same after spinning; if the mixture is a colloid, the different phases will separate into layers.
Colloids, such as honey and blood, contain different phases, which can be physically separated.
(Images: Luc Viatour, Wikimedia Commons; Wheeler Cowperthwaite, Wikimedia Commons)
- Matter is anything that takes up space and has mass.
- Matter can be divided into pure substances and mixtures.
- Pure substances are made up of one type of particle and cannot be physically separated into other substances.
- Mixtures are made up of more than one type of particle and can be physically separated into other substances.
- Mixtures can be classified as homogeneous or heterogeneous.
- Homogeneous mixtures consist of a single phase and have a uniform composition.
- Heterogeneous mixtures consist of two or more phases and do not have a uniform composition.
- Solutions are mixtures that are homogeneous at the macroscopic and microscopic level.
- Coarse mixtures are mixtures that are heterogeneous at the macroscopic and microscopic level.
- Colloids are mixtures that are homogeneous at the macroscopic level but heterogeneous at the microscopic level.
- Colloids can be distinguished from solutions as they are opaque whereas solutions are transparent; colloids can also be spun into different phases whereas solutions cannot.
The classification of matter.
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