Conservation of Mass and Solutions Lesson

Learning Objectives

• I can describe the law of conservation of mass.
• I can explain how solutions form.
• I can explain dissolving in terms of particles of matter.
• I can explain diffusion in terms of particles of matter.

Introduction

If you build a campfire like this one, you start with a big pile of logs. As the fire burns, the pile of logs slowly shrinks. By the end of the evening, all that’s left is a small pile of ashes. What happened to the matter that you started with? Was it destroyed by the fire?

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Guided Learning

Where’s the Matter?

It may seem as though burning destroys matter, but the same amount, or mass, of matter still exists after a campfire as before. Look at the sketch in the figure below. It shows that when wood burns, it combines with oxygen and changes not only to ashes but also to carbon dioxide and water vapor. The gases float off into the air, leaving behind just the ashes. Suppose you had measured the mass of the wood before it burned and the mass of the ashes after it burned. Also suppose you had been able to measure the oxygen used by the fire and the gases produced by the fire. What would you find? The total mass of matter after the fire would be the same as the total mass of matter before the fire.

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Burning is a chemical process.

Q: What can you infer from this example?

A: You can infer that burning does not destroy matter. It just changes matter into different substances.

Law of Conservation of Mass

This burning campfire example illustrates a very important law in science: the law of conservation of mass. This law states that matter cannot be created or destroyed. Even when matter goes through a physical or chemical change, the total mass of matter always remains the same.

Q: How could you show that the mass of matter remains the same when matter changes state?

A: You could find the mass of a quantity of liquid water. Then you could freeze the water and find the mass of the ice. The mass before and after freezing would be the same, showing that mass is conserved when matter changes state.

Solutions

When rocks or other substances dissolve in water, they form a solution. A solution is mixture of two or more substance where the particles of a solution are mixed evenly throughout it. The particles are too small to be seen or to settle out. An example of a solution is salt water.

A solution forms when one substance dissolves in another. The substance that dissolves is called the solute. The substance it dissolves in is called the solvent. For example, ocean water is a solution in which the solute is salt and the solvent is water. In this example, a solid (salt) is dissolved in a liquid (water). However, matter in any state can be the solute or solvent in a solution. Solutions may be gases, liquids, or solids. In the table below and this four-minute video, you can learn about solutions involving other states of matter.

When a solute dissolves in a solvent, it changes to the same state as the solvent. For example, when solid salt dissolves in liquid water, it becomes part of the liquid solution, salt water. If the solute and solvent are already in the same state, the substance present in greater quantity is considered to be the solvent. For example, nitrogen is the solvent in Earth’s atmosphere because it makes up 78 percent of air.

How a Solute Dissolves

When a solute dissolves, it separates into individual particles that spread evenly throughout the solvent. Exactly how this happens depends on the type of bonds the solute contains. The individual particles (molecules) spread apart and are surrounded by particles (molecules) of the solvent. This is illustrated in the figure and videos below.

Dissolving Salt Video (0:47)

Solute and Solvent - Dissolving Video (1:38)

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These two images show how solutes (salt and sugar) dissolve in a solvent (water).

Diffusion

What happens if you put a few drops of food coloring in water?

Over time, the molecules of color spread out through the rest of the water. When the molecules are evenly spread throughout the space, the water will become an even color. This process of molecules moving from an area where there are lots of molecules to an area where there are fewer molecules is known as diffusion.

Diffusion is the movement of molecules from an area where there is a higher concentration (larger amount) of the substance to an area where there is a lower concentration (lower amount) of the substance. The amount of a substance in relation to the total volume is the concentration. During diffusion, molecules are said to flow from an area of high concentration to an area of low concentration. This is a natural process and does not require energy. Molecules will continue to flow in this manner until equilibrium is reached. At equilibrium, there is no longer an area of high concentration or low concentration, the entire solution is equally balanced. See it in action with this short video.                                                                                                                                 ^[4]

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The red solute in the images above moves around from container A until it is diffused in container C.

Rate of Dissolving

When you add sugar to a cold drink, you may stir it to help the sugar dissolve. If you don’t stir, the sugar may eventually dissolve, but it will take much longer. Stirring is one of several factors that affect how fast a solute dissolves in a solvent. Temperature is another factor. A solid solute dissolves faster at a higher temperature. For example, sugar dissolves faster in hot tea than in ice tea. Both of these factors are increasing the energy between the solute and solvent. Stirring increases the motion of the particles as they collide with one another. Increases in temperature also increase the motion of the particles as their energy increases.

A third factor that affects the rate of dissolving is the surface area of the solute. For example, if you put granulated sugar in a glass of ice tea, it will dissolve more quickly than the same amount of sugar in a cube. That’s because granulated sugar has much more surface area than a cube of sugar. The solvent has more opportunity to collide with the granulated sugar particles than the cube sugar particles because the surface area is larger.

You can see videos of all three factors at these links:

Rate of Dissolving Related to Stirring Video (1:04)

Rate of Dissolving Related to Surface Area Video (1:04)

Rate of Dissolving Related to Temperature Video (1:38)

The Almost-Universal Solvent

Solutes that can dissolve in a given solvent, such as water, are said to be soluble in that solvent. So many solutes are soluble in water that water is called the universal solvent.

However, there are substances that don’t dissolve in water. Did you ever try to clean a paintbrush after painting with an oil-based paint? It doesn’t work. It is insoluble in water. Instead another solvent such as paint thinner must be used to dissolve paint. ee this concept in action with this two-minute video.

Review

• Burning and other changes in matter do not destroy matter. The mass of matter is always the same before and after the changes occur.
• The law of conservation of mass states that matter cannot be created or destroyed.
• A solution forms when a solute dissolves in a solvent.
• The rate of dissolving is faster with stirring, a higher temperature, or greater surface area. Many solutes are soluble in water.
• Diffusion is the movement of molecules from an area of high concentration to an area of low concentration.

Concentration

        Concentration of a substance is the amount of that substance in relation to its volume.

Diffusion 

Movement of molecules from an area where there is a higher concentration (larger amount) of the substance to an area where there is a lower concentration (lower amount) of the substance.

Dissolving

Dissolving is the process of breaking down of a solute into a solvent.

Insoluble

        A substance is insoluble if it is not capable of being dissolved in a solvent.

Law of Conservation of Mass

Law stating that matter cannot be created or destroyed in chemical reactions.

Soluble

        A substance is soluble if it is capable of being dissolved in a solvent.

Solute

The substance that dissolves in a solution is called the solute.

Solution

        A solution is an evenly distributed mixture of two or more substances.

Solvent

The substance a solute dissolves in is called a solvent.