Chemical Reactions

Students begin this unit by focusing on how butterflies undergo metamorphosis as they move through their life cycle. Students observe the different stages of painted lady butterflies as they develop and grow.

• Molecules are made up of different combinations of individual atoms, which combine in various ways in chemical reactions to form molecules.
• Matter can only change when energy is present, so all chemical reactions require energy.

• How do water molecules form?
• What evidence is there that water molecules are formed in a chemical reaction?
• What evidence would support the claim that cooking often involves chemical reactions?
• Why do all chemical reactions need an input of energy to get started?
• How does the concept of energy transfer relate to chemical reactions?
• How is a system defined in a chemical reaction?

Misconception: Conservation of mass does not apply to atoms.

Fact: Mass is the measure of the amount of atoms in a substance, so conservation of mass refers to atoms, which cannot be created or destroyed.

Misconception: The atoms of the reactants in a chemical reaction are transformed into other atoms.

Fact: The atoms aren’t changed into other atoms. Instead, the atoms stay the same but rearrange to form new molecules. 

Chemical reaction : a change that rearranges the atoms of the original substances into a new substance that has different properties from the original substances

Endothermic : a process in which energy is absorbed from the environment

Exothermic : a process in which energy is released into the environment

Baking Bread

Have you ever watched someone make bread? The baker takes a variety of different ingredients. They follow a recipe to combine the ingredients in a specific way. Then they add heat to the mixed ingredients. After some time, a loaf of bread is created.

When you bake a loaf of bread, chemical reactions occur. In a chemical reaction, the atoms of the original substances are rearranged into one or more new substances that have different properties from the original substances.

Chemical vs. Physical Changes

The changes that occur as a result of a chemical reaction are not the same as physical changes. Physical changes are changes that do not affect the chemical structure of a substance. For example, if you cut up a piece of paper, it is smaller but it is still paper. When you add enough thermal energy to an ice cube, it will turn into liquid water, but the molecules are still water molecules. The ingredients of salad dressing can be separated, and they will still have the same properties.

In contrast, a chemical change is a change that rearranges the chemical structure of the substances through a chemical reaction. When you bake bread, the ingredients are combined in such a way that the bread that is produced has different properties from the ingredients it came from. When you cook pasta, the noodles are chemically changed to become sticky and springy.

When you cook, it is good to know about the properties of the different ingredients. These properties are determined by the number and kind of atoms that make up the different ingredients.

But scientists often don’t work with individual atoms because they are so small. Instead they work with elements (substances made up entirely of one kind of atom). For example, salt is a staple of kitchens. It is made up of the elements sodium (Na) and a form of chlorine (Cl), so it is called sodium chloride.

Periodic Table of Elements

Scientists use a chart called the Periodic Table of Elements to organize all of the known elements according to their properties. There are currently 118 known elements. The last four were just added in 2016. Scientists are continuing to search for new elements.

Each element on the periodic table is assigned a symbol and a number. The symbol comes from the name of the element’s atom (in English or Latin), and the atomic number comes from the number of protons found in the atom’s nucleus.

For example, sodium has 11 protons, while chlorine has 17. Neutrons and electrons don’t define an element because the number of neutrons and electrons in an atom can fluctuate. Salt is made up of 2 elements: sodium and a form of chlorine.

The periodic table is organized from top to bottom in groups by increasing atomic number. The periodic table indicates some patterns among elements. For example, an element’s place on the periodic table tells us how reactive its atoms are and whether it is a metal, nonmetal, or metalloid.

• Metals are shiny, malleable, and good conductors of electricity and heat. Almost 75 percent of all elements are metals, including mercury, zinc, gold, copper, iron, and other elements in columns 1-12 of the periodic table.
• Nonmetals are brittle, dull, and poor conductors of electricity and heat. There are only 18 nonmetals on the periodic table. Gasses and elements on the far right of the periodic table are nonmetals. Coal is an example of a nonmetal.
• Metalloids, also called semiconductors, have properties of both metals and nonmetals, such as being shiny and hard, but brittle. Boron, silicon, and arsenic are metalloids. Metalloids are found between metals and nonmetals on the periodic table. They are called semiconductors because depending on what other molecules are around, they can sometimes conduct electricity.

Whenever two or more atoms bond together to form molecules or when bonded atoms are broken apart, a chemical reaction occurs. A result of this process is that the substance formed has different properties than the original substances. For example, at normal room temperature, both oxygen and hydrogen elements are gasses. When hydrogen and oxygen bond in a molecule of water, they change into a liquid instead of a gas at room temperature.

In this lesson, students explore matter and its interactions by investigating conservation of mass, endothermic and exothermic chemical reactions, and how concentration affects the temperature change of a reaction. Students collect and analyze data on the mass of a bottle, cap, water, and effervescent tablet before and after they are combined. Then they analyze data on the temperature change of different chemical reactions. Finally, they collect and analyze data on the temperature change of a reaction when the concentration of one of the reactants increases. Students use the data they gather to explain how matter is changed in a chemical reaction and how the amount and kind of matter affects how much energy is needed to change it.