Mixtures Lesson: Understanding How Substances Combine

Lesson Overview

• What Is a Mixture?
• Types of Mixtures: Homogeneous and Heterogeneous
• What Is a Solution?
• Mechanical Mixtures
• Solubility: What Can Dissolve?
• How Salt and Oil React
• Physical vs. Chemical Properties
• Mass, Volume, and Density
• Mass vs. Weight
• Testing for Density with Objects in Water
• Volume vs. Mass: Same Volume, Different Density
• Chemical Changes vs. Physical Changes
• Pure Substances and Density Consistency
• Liquids vs. Gases: Shape and Volume

We see and use mixtures every day. From the milk in your cereal to the air you breathe, mixtures are all around us. But what exactly is a mixture? How is it different from a pure substance? In this lesson, we'll explore different types of mixtures, learn how to tell them apart, and understand how mixtures relate to physical properties like mass, volume, and density.

Let's explore how substances come together without changing what they are-and how we can describe and separate them.

What Is a Mixture?

A mixture is made when two or more substances are combined but not chemically joined. This means:

• Each part of the mixture keeps its own identity
  
• You can often separate the parts by physical methods
  

Unlike a pure substance, which has only one type of material, a mixture contains different components.

Examples of mixtures:

• A bowl of salad
  
• A smoothie
  
• A bag of mixed candy
  

Types of Mixtures: Homogeneous and Heterogeneous

There are two main types of mixtures:

A homogeneous mixture is also called a solution, because everything looks smooth and well mixed. A heterogeneous mixture looks uneven or chunky, and its parts can often be picked out or seen.

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Quiz - Mixtures and Solutions

What Is a Solution?

A solution is a type of homogeneous mixture. It has:

• A solute: the substance being dissolved (like salt or sugar)
  
• A solvent: the substance doing the dissolving (usually water)
  

In a sugar-water solution, sugar is the solute and water is the solvent.

Mechanical Mixtures

Mechanical mixtures are a type of heterogeneous mixture where you can clearly see the parts. These parts can usually be separated by hand, straining, or sorting.

Examples:

• A chocolate bar with almonds
  
• Orange juice with pulp
  
• Pizza toppings
  

In these mixtures, each part keeps its own look and can be taken out or sorted easily.

Solubility: What Can Dissolve?

Some substances dissolve in water and others don't. This ability is called solubility.

Soluble substances disappear into the liquid to make a solution. Insoluble substances remain visible and can be filtered out. For example, sugar dissolves in water, but pepper does not.

How Salt and Oil React

When you try to mix salt and oil, they don't dissolve. This is because:

• Salt dissolves in water, not oil
  
• Oil is not a good solvent for salt
  

The result is a heterogeneous mixture where the salt remains in small, visible pieces inside the oil. This shows that not every combination of substances forms a true solution.

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mixtures and solutions

Physical vs. Chemical Properties

Mixtures can usually be separated by using their physical properties, not by changing their chemistry.

A chemical property, like reactivity, would involve changing what the substance is. But in mixtures, we only focus on physical differences.

Mass, Volume, and Density

Let's understand three important physical properties:

• Mass: How much matter something contains (measured in grams or kilograms)
  
• Volume: How much space something takes up (measured in mL or cm³)
  
• Density: How compact the matter is (measured by dividing mass by volume)
  

Formula for Density:

• Density = Mass \ Volume

If two objects have the same volume but different densities, one may float and the other may sink.

Mass vs. Weight

People often confuse mass and weight, but they are different:

Mass is used in mixture calculations because it doesn't change based on location.

Testing for Density with Objects in Water

If you place an object in water and it sinks, that means it is denser than water. If it floats, it is less dense.

Example:

• A rock sinks: rock is denser than water
  
• A piece of plastic floats: plastic is less dense than water
  

This helps us compare the density of unknown substances using water as a test.

Volume vs. Mass: Same Volume, Different Density

Two substances can have the same volume but different masses if their density is different.

Example:

• 40 mL of water weighs more than 40 mL of alcohol
  
• This means water is denser
  

Volume tells us the size, but density and mass tell us how much matter is packed inside.

Chemical Changes vs. Physical Changes

Mixtures are made using physical changes, not chemical ones. Here's how to tell the difference:

Changing states of matter (solid, liquid, gas) is a physical change, not chemical.

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Solutions and Mixtures

Pure Substances and Density Consistency

A pure substance always has the same density when measured under the same conditions.

That means:

• If you measure the density of pure water at the same temperature, it will always be about 1 g/cm³.
  
• This is not true for mixtures, since their composition can change.
  

Scientists use density as a tool to identify substances.

Liquids vs. Gases: Shape and Volume

All matter takes up space and has mass-but different states of matter behave differently.

Liquids are not easily compressed. Gases are easily compressed and can spread in all directions.