Section 3.2: Changes in Matter Chapter 3 Section 3.2: Changes in Matter

Objectives Define physical change and list several common physical changes. Define chemical change and list several indications that a chemical change has taken place. Apply the law of conservation of mass to chemical reactions.

Physical Changes A physical change is a change that alters a substance without changing its composition. Cutting your hair or crumpling a sheet of paper are examples of physical changes. Verbs describing a physical change include: TEAR, BREAK, BEND, SPLIT, CRUSH, BOIL, FREEZE, MELT. Note: Changes in state and ability to dissolve are physical changes.

Chemical Changes A chemical change is the change of one substance into a new, different substance. A chemical change is more commonly referred to as a chemical reaction. Examples of chemical changes are the fermentation of grapes, the rusting of iron, and the changing color of leaves in the fall. Verbs describing a chemical reaction would include…?

A Closer Look Iron reacts with oxygen in the air to form a new, different substance – rust. Rust is really a compound called iron oxide. Iron & rust have different compositions and different physical & chemical properties. In the reaction, iron and oxygen, the starting substances, are called the reactants. The new substances produced are called the products.

Evidence of a Chemical Reaction A color change – The formation of rust is an excellent example of this. Production of a gas – Bubbles or a change in odor indicate a gas was produced. Formation of a precipitate – An example of the production of a solid in a liquid is seen in the photo to the right. An energy change – A change in temperature indicates that energy was absorbed or released.

Law of Conservation of Mass Following the invention of the analytical balance, scientists, like Antoine Lavoisier, observed many chemical reactions and always found that the mass before and after a reaction did not change.

Law of Conservation of Mass They summarized this phenomenon in a law: mass is neither created or destroyed during a chemical reaction – it is conserved. Massreactants = Massproducts

Practice Problems 10.00 g of mercury (II) oxide is heated until it is converted to liquid mercury and oxygen gas. If the mercury has a mass of 9.26 g, what is the mass of the oxygen formed? 10.3 g of Al are allowed to react with 100.0 g of Br. After the reaction, no Al remains but 8.5 g of Br remains unreacted. How many grams of the compound aluminum bromide were formed?