Heterogeneous and Homogenous Mixtures

THE PARTICLE THEORY OF MATTER

Homogeneous Mixtures

   As we already learned, homogeneous mixtures contain two or more substances that combine to look like one substance.

Homogeneous mixtures are also called

solutions

.

Examples:

Homogeneous Mixtures

  Q: Why do some substances combine to form solutions?

A: To answer this question, consider a salt water solution.

 Salt is added to water.

  The salt particles are attracted to the water particles and completely intermingle (mix) with the water particles.

To an observer, the particles are so intermingled that is appears as one substance.

Heterogeneous Mixtures

   A heterogeneous mixture is a combination of two or more different types of matter that retain their own properties and that can be detected quite easily.

These mixtures may also be known as

mechanical mixtures

.

Examples:

Heterogeneous Mixtures

  Q: Why do some substances combine to form mechanical mixtures?

A: Consider the mixture of pepper and water.

    Pepper is added to the water The pepper particles are more attracted to themselves than they are to the water particles.

The pepper particles “clump” together forming parts for the observer to see.

Pepper does NOT dissolve in water.

Terminology

 Example: Oil , Gasoline & Water  What does this mean?

  Oil dissolves in gasoline because the oil particles are attracted to the gasoline particles. Oil does not dissolve in water (oil particles are not attracted to the water particles). The oil and water do not mix.

Solution or Mechanical Mixture?

  Q: How can you distinguish between a solution and a mechanical mixture?

A: The textbook discusses four methods: 1.

2.

 Look at the mixture: If you can see the different parts of the mixture, it is a mechanical mixture.

 Use a microscope: If you see only one type of mixture, it is a solution.

Solution or Mechanical Mixture?

3.

4.

  

Filtration:

  If the mixture is a liquid, pour it through a filter. If there is a

residue

, it is a mechanical mixture (see page 247, Activity 7-2B).

Residue

is the substance that is left behind in the filter.

The

filtrate

is the substance that passes through the filter.

Shine a light through the mixture (Tyndall Effect): Solutions (homogeneous mixtures) do NOT scatter light, so you should not see a beam of light as it passes through. However, you should see the light on the other end!

Heterogeneous mixtures scatter light, so you can see a beam of light through the mixture. However, you shouldn’t see the light on the other end!

Mixtures that are Mixtures

   Q: Is orange juice a solution or a mechanical mixture?

A: This answer depends on whether the orange juice has pulp.

 Without pulp, orange juice is a solution.

 Q: Does this mean that with pulp the orange juice is considered a mechanical mixture?

 NO! Scientists consider orange juice with pulp to be a

mixture of mixtures

. The juice is considered a solution while the juice and pulp is a mechanical mixture. It’s both!

Q: What are some more examples of mixtures that are mixtures? (See page 245)