Changes in Matter Section 3.2

Water What is water? What are some of the physical properties of water? Does it have taste? Color? Smell? Temperature?

Physical Changes Matter can exist as solids, liquids, or gases. Energy causes phase changes. When a substance changes from one phase to another, it undergoes a physical change. Physical change- some of the physical properties of a substance change, but the identity of the substance remains the same.

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Mixtures and Solutions Sometimes a physical change occurs when substances combine. Mixture-two or more substances that do not combine chemically. Atmosphere- mixture of gases but they keep their separate chemical properties. Salt water is a solution. Solution- type of mixture in which one substance is evenly mixed with another substance. The taste changes but the physical water and physical salt are not changed in any way.

Chemical Changes Chemical change- when the substance is turned into many different substances. (chemical identity is changed) A chemical reaction takes place during a chemical change. New substances are produced when a chemical reaction occurs. Rust Wood Burning

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Chemical Changes vs. Physical Changes physical-changes-source.jpg

Chemical Reactions and Equations Chemical Equations- symbols represent elements and formulas represent compounds. Chemical equations contain reactants (raw materials in a chemical reaction) and products ( substances produced by the reaction). A plus sign in a chemical equation shows that two or more reactants will combine. Reactants are usually on the left side of the equation. Products are usually on the right side of the equation. The arrow means produces or yields the products. The arrow also shows the direction of the reaction.

What have we learned? Look at Figure 3.11 on page 60 in your book. Name the reactants in this chemical equation. Name the products in this chemical reaction. How many molecules of carbon dioxide are indicated on the right of the chemical equation? You decide that instead of water today you want to add a flavor packet to it to make fruit punch. Is this a physical change or a chemical change? When you light a match, is this a physical change or a chemical change? Given the equation 2HCl  H 2 + Cl 2 : What are the Reactants? What are the Products?

Balanced Chemical Equations H 2 + O 2  H 2 O What’s wrong here? Atoms do not disappear during during chemical reactions. Therefore, we need to balance this equation. We balance equations by adding coefficients in front. Coefficient shows how many molecules or atoms of a substance are involved in the reaction. Number of atoms must be the same on both sides. 1 st we need to place a coefficient before the product. H 2 + O 2  2H 2 O Now oxygen are balanced but the hydrogen are not. 2H 2 + O 2  2H 2 O Balancing Chemical Equations

What have we learned? 1.Explain how water changes from a solid to a liquid. 1.What are the reactants and products in a chemical equation? 1.A lump of sugar burns. Is this an example of a physical change or a chemical change? Explain why. 1.In your own words, describe what the following chemical reaction for hydrogen peroxide means: H 2 + O 2  H 2 O 2.