Chemical Change

Unit Review Sheet

Facts and Definitions

Lesson 1: Protons, Neutrons, and Electrons

• An atom is the smallest particle of matter that cannot be broken down by chemical means. It is made up of a nucleus of protons and neutrons, surrounded by a cloud of electrons.
• The nucleus is the central part of an atom.
• Protons are particles with a positive electrical charge located in the nucleus of an atom.
• Neutrons are particles with no electrical charge located in the nucleus of an atom.
• Electrons are particles with a negative electrical charge that move around the nucleus of an atom.
• An element is a substance made up entirely of one kind of atom. Sodium and carbon are examples of elements.
• A molecule is a group of atoms that are bonded together.
• The atomic number refers to the number of protons found in the nucleus of an atom.
• Atoms have the same number of protons and electrons, but an atom becomes an ion if it gains an extra electron (negative ion) or loses an electron (positive ion).
• Electron shells are groupings of electrons around a nucleus. Each shell can hold a specific number of electrons.
• A chemical reaction takes place when atoms in the substances you start with (called the reactants) are rearranged, and become different substances (called the products).
• The outermost shell, called the valence shell, contains electrons that can move to or bond with other atoms.

Lesson 2: Elements, Compounds, and Mixtures

• A pure substance contains only one type of molecule. Examples include elements like silver or copper or compounds like sugar and salt.
• Compounds are made of two or more elements held together by chemical bonds. For example, water is a compound made from the elements hydrogen and oxygen.
• A mixture is made of two or more elements or compounds that are mixed together but not bonded together. Mixtures can be separated relatively easily. Salad dressing is a good example.
• A solution is a mixture in which one substance has been dissolved into another. Solutions are harder to separate than most mixtures. An example is sugar water.
• A solute is the dissolved substance in a solution, like sugar dissolved in water.
• An alloy is a mixture of two or more metals. Brass is an alloy of the elements copper and zinc.

Lesson 3: Physical Changes

• Viscosity is a fluid's resistance to flowing.
• As molecules, solids have less energy than liquids, and liquids have less energy than gases.
• Physical changes include changes in state, which do not change what a substance is chemically. For example, water in liquid form and water in solid form (ice) both have 2 hydrogen atoms and 1 oxygen atom even though they look different and have some different properties.
• In a chemical change, the original substances change into something else. For example, when sodium (Na), a silvery metal, combines with chlorine (Cl), a greenish-yellow gas, the two substances react with each other and form table salt.
• Freezing is the process of changing from a liquid to a solid when a liquid reaches its freezing point.
• Melting occurs when a solid changes to a liquid.
• Boiling is the process of changing from a liquid to a gas by heating a liquid to its boiling point.
• Evaporation occurs when a liquid changes to a gas at a temperature below the liquid's boiling point.
• Condensation is the process of a gas changing into a liquid as it cools down.
• A gas can change into a solid without becoming a liquid (for example, frost on a cold day). This process is called deposition.
• Sublimation is the process of a solid changing into a gas without becoming a liquid.

Lesson 4: Chemical Changes

• There are 4 changes that indicate that a chemical change as opposed to a physical change has occurred: a change in temperature, a change in color, the formation of a precipitate, and the production of a gas from solids or liquids.
• Precipitation is when a chemical reaction creates a solid in a solution. The solid is called a precipitate.
• Temperature changes can involve exothermic reactions, which give off heat, or endothermic reactions, which absorb heat.

Lesson 5: Acids and Bases

• pH is a measure of how acidic or basic a substance is. It measures the number of hydrogen ions. pH ranges from 0 to 14.
• An acid is a substance that donates a hydrogen ion to another substance. Acids have a pH of 0 to 7. The closer to 0, the more acidic the substance.
• A base is a substance that accepts a hydrogen atom from another substance. Bases have a pH of 7 to 14. The closer to 14, the more basic (also called alkaline) the substance.
• Litmus paper is a special paper made from lichens that turns blue in a base and pink or red in an acid.

Final Project: Demonstrating the Concepts

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