5.1 and 5.2 notes

Properties of matter: Two kinds of properties of matter: physical properties and chemical properties. A physical property is a characteristic of a substance that can be observed without changing the substance into another substance. Temperature of melting point, boiling point, color, texture, density, and conductivity are physical properties of matter. A chemical property is a characteristic of a substance that describes its ability to change into other substances. To observe the chemical properties, you have to try to change it into another substance.

When a shiny penny turns green, the color change demonstrates a chemical property of the penny’s copper coating. When copper is exposed to air, it reacts over time to form a dull, crusty solid. Another chemical property is a material’s ability to burn in the presence of oxygen (flammability).

Changes in matter: Two types of changes in matter Changes in matter can be described in terms of physical changes and chemical changes.

A physical change is any change that alters the form or appearance of a substance but does not change it into another substance: bending, crushing, and cutting. Changes in the state of matter: freezing, boiling, and melting are physical changes.

A change in matter that produces one or more new substances is a chemical change, or chemical reaction. In a chemical change, atoms rearrange to form new substances, which results in different physical properties, as well. Burning, rusting, and even when an apple slice left out in the air turns brown - the cut reacts with oxygen in the air to form new compounds. Substances that undergo the chemical changes are called reactants. The new substances that form are the products.

Bonding and Chemical Change Chemical changes occur when existing bonds break and new bonds form. New substances are produced. Atoms form bonds when they share or transfer electrons. Example: Oxygen gas (O2) in the air consists of molecules made up of two oxygen atoms bonded together.

These bonds break when oxygen reacts with magnesium (Mg) and a new ionic bond forms. The compound magnesium oxide (MgO) is produced. Magnesium oxide, a white powder, has properties that differ from those of either shiny magnesium or invisible oxygen gas. And magnesium melts at 650*C, but magnesium oxide melts at 2,800*C.

One way to detect chemical reactions is to observe changes in the physical properties of the materials. Formation of a precipitate, gas production, and a color change are all possible evidence that a chemical reaction has taken place. Many times, physical properties such as texture and hardness may also change.

Example: sodium (Na) and chlorine (Cl) react to form an ionic compound, sodium chloride (NaCl). Both reactants are very reactive elements, but the product, sodium chloride, is very stable.

+ = Precipitate - a solid that forms from liquids during a chemical reaction.

Gas production can be seen as bubbles. Bread dough rises from gas bubbles produced when yeast reacts with sugar. What evidence in a slice of bread shows the presence of gas? Cakes rise when baked... A color change can signal that a new substance has formed. Leaves changing color in the fall… Bananas turning brown in the air…

Changes in Energy - Exothermic reaction - energy is released when bonds are breaking and when the products are forming - usually released as heat, but can also be released as light

Endothermic reaction - energy is absorbed while breaking the bonds. The energy can be absorbed from nearby matter. When the energy is absorbed, it causes the surroundings to become cooler.

Baking soda undergoes an endothermic reaction when mixed with vinegar. The reaction absorbs heat from its surroundings, so the reaction feels cold. Not all endothermic reactions result in a temperature decrease. Many endothermic reactions only occur when heat is constantly added, like when frying an egg. Heat must be applied through the whole process in order for the reactions that cook the egg to continue. 5.2 Describing Chemical Reactions A chemical equation is a way to show a chemical reaction, using symbols instead of words. Remember, a chemical formula is a combination of symbols that represents the elements in a compound. For example, CO₂ is the formula for carbon dioxide. The formula tells you that the ratio of carbon atoms to oxygen atoms in this compound is 1 to 2. Carbon dioxide is a molecular compound. A chemical equation tells you the substances you start with in a reaction and the substances that are formed at the end. The substances you have at the beginning are the reactants, and the new substances are called the products.

Reactant + Reactant → Product + Product Reactant → Product + Product Reactant + Reactant → Product The number of reactants and products can vary. Some reactions have only one reactant or product. Others have two, three, or more reactants or products. Law of conservation of mass: In a chemical reaction, all of the atoms present at the start of the reaction are present at the end of the reaction. Atoms are not created or destroyed; however, they may be rearranged to form new substances. The amount of matter does not change. Total mass stays the same before and after the reaction. Open and Closed Systems: It is not always easy to measure all the matter involved in a reaction. Products can escape into the air, but how much? A fish bowl is an example of an open system. In an open system, matter can enter from or escape to the surroundings. If you want to measure all the matter before and after a reaction, you have to be able to contain it. In a closed system, matter does not enter or leave. A chemical reaction that occurs inside a sealed, airtight container is a closed system. What are Three Types of Chemical Reactions? The three types of chemical reactions are synthesis, decomposition, and replacement. Synthesis – when two or more elements or compounds combine to make a more complex substance Example: The reaction of phosphorus with oxygen P₄ + 3 O₂ → P₄O6 Decomposition – when compounds break down into simpler products Example: If you keep a bottle of hydrogen peroxide for a very long time, (several years) you’ll have water instead. Hydrogen peroxide decomposes into water and oxygen gas. 2 H₂O₂ → 2 H₂O + O₂ Replacement – when one element replaces another element in a compound, or if two elements in different compounds trade places, the reaction is called a replacement. Single replacement - 2 Cu₂O + C →4 Cu + CO₂ Double Replacement – FeS + 2 HCl → FeCl₂ + H₂S