Investigation Four continues students’ exploration of the interactions of matter by demonstrating how chemical changes can occur when different types of matter interact.

Begin this part of the investigation with a brief review of what was learned about how matter interacts in Investigation Three.

• Remind students that in Investigation Three they learned that different types of matter interact to form mixtures. Ask students: What do we call a mixture that is the same throughout? Students should indicate that a mixture that is the same throughout is called a solution.

• Ask students: Do all mixtures form solutions? Students should indicate that all mixtures do not form solutions.

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Divide students into groups.

• Direct students to discuss among their groups whether each item is a mixture and a solution or only a mixture. 
• After allowing time for discussion, ask students to indicate which items qualify as solutions. 
• If groups disagree with one another on an answer for a particular item, encourage each group to explain their reasoning to the rest of the class.

• Cinnamon sugar is a mixture of solids. Cinnamon may coat the sugar crystals but both solids are still clearly visible.
• Iced tea is a solution because the tea (and sugar if sweetened) has been dissolved into the water, leaving a uniform mixture.
• Chicken noodle soup is a mixture because the noodles and chicken are clearly visible in the broth.
• Vanilla ice cream is a solution because the sugar has been dissolved into the liquid ingredients, which are all miscible with one another.
• The oats and other solids are clearly definable in a granola bar, and therefore it is a mixture.
• All the ingredients of a soda have dissolved into a uniform solution.
• Vinegar and oil salad dressing is a mixture, as vinegar and oil are not miscible, although they may briefly appear blended after vigorous shaking.

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Explain to students that they will continue their study of matter, solutions, and interactions of matter by exploring the differences between physical and chemical changes in matter.

Direct students’ attention to the terms physical change and chemical change in their Scientist’s Glossary. 

Explain that when a physical change occurs the substance or substances involved do not change chemically, only their state of matter or form changes. For example, when sugar is dissolved in water it simply changes from a solid to a liquid state and can be recovered if the water is evaporated away. When paper is cut, its form changes but its chemical composition remains the same. In contrast, when a chemical change occurs new substances are created.

Ask students: How can you tell if a new substance has been created? Student answers will vary. Students may suggest a change in the substance occurs, or a change in color. 

Suggest the following examples to help students understand chemical change in terms of previous knowledge: Milk turning sour or cake ingredients becoming a cake after baking.

Explain to students that each example mentioned was the result of a chemical change and that a characteristic of a chemical change occurring is that the original substances cannot be recovered.

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Tell students that a chemical change can be detected when the combination of two or more substances creates a new substance by focusing on the properties of the substance. Explain to students that chemists look for at least one of ten common signs when deciding if a chemical change has occurred:

• A change in volume.
• A change in temperature.
• A change in color.
• A change in electrical conductivity.
• A precipitate forms (a chemical falls out of solution as a solid or a liquid).
• A gas is produced.
• A change in pH.
• A change in chemical or physical properties occurs.
• A change in odor.
• A change in melting or boiling point.

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Explain to students that sometimes more than one of these common signs occurs when a chemical change takes place. For example, a change in volume might occur when a gas is produced or a precipitate forms.

Ask students: Which do you think you observed in Investigation Three, physical or chemical changes? Student answers will vary. 

Explain that they observed a physical change in salt when salt dissolved in water because only its form changed. Salt was a solid and when dissolved became a liquid. As in the sugar example earlier, if the water was evaporated, the salt would remain behind as a solid.

Tell students that in Investigation Three they did not create any new substances when they mixed their solutions. Remind students that two solutions were created during Investigation Three: saltwater and vinegar water. The salt dissolved into the water but did not create a new substance in doing so. Vinegar was miscible in water, creating a weaker solution of vinegar. However, no new substance was created even though the vinegar was no longer visible.

Ask students: Is it possible to create new substances when you mix solutes and solvents? Student answers may vary. Tell students this is one of the questions they will investigate in the lab.

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Explain to students that one way they can help answer the question is to focus on three signs of a chemical change: an unexpected change in color, a change in temperature, and the production of a gas. 

Tell students that in order to better understand these changes they will conduct several trials that show these changes. These include:

• • Browning of an apple slice
  • Production of gas by combining baking soda and vinegar
  • A change in temperature caused by adding bleach to water

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Tell students that as they prepare for their laboratory experiments you would like them to use the Cognitive Tool, Look For. 

Look For should be used before students begin activity that will demonstrate something. Students should ask themselves, “What is important?” and “What should I be looking at?” to help direct their attention toward important experimental outcomes.

In their lab experiments, students should remember to look for the three signs that a chemical change may have occurred:

• A change in color
• Production of a gas
• A temperature change

Ask students: Why do you think it will be important to look for the signs of a chemical change? Students should indicate that knowing what to look for will help them be more aware of their observations and will help them to remember the results of their experiments.