Compounds, Mixtures, and Reactions 8th Grade Science-2015

What is a Pure Substance? A pure substance is a classification of matter that includes both elements and compounds Pure substances cannot be separated by physical means such as distillation, filtration, or chromatography

Elements An element is made of one kind of atom Found on the periodic table Watch the Element and Compound video!

What Is A Compound? A compound is a pure substance that is created by two or more elements chemically reacting and joining together Ex: NaCl, H2O, CO2, NH3, NaHCO3, and C6H12O6 Notice that elements combine in many ways to make compounds Ex: H2O, H2O2, CO, and CO2

Why Do Compounds Form? Compounds form to allow elements to become more stable Na is flammable when it comes in contact with H2O, and Cl2 is a toxic gas NaCl is a very stable compound that is neither flammable nor toxic (in normal quantities) Watch Video Animation Compounds that are extremely unstable will break down to form the more stable elements

How Do Compounds Form? Compounds form by the interaction of the nuclei and valence electrons of 2 or more elements THE OCTET RULE: an element is most stable with 8 valence electrons Elements will join chemically to get 8 valence electrons Ex: CO2 : oxygen has 6 valence electrons and carbon has 4 If the carbon shares 2 with each oxygen, everyone will have 8 valence electrons! Watch how it happens!

What Do Compounds Have To Do With My Life? Compounds are substances that make up ALL living and non-living things Examples: Where would you be without: H2O —water NaCl—table salt C3H8O —rubbing alcohol C55H98O6 —an example of an unsaturated fat

What Is A Mixture? A mixture is the physical combination of 2 or more substances It is important to understand that a mixture is not chemically combined Mixtures can be separated by physical means such as distillation, filtration, and chromatography Mixtures can be divided into 2 groups Homogenous mixtures Heterogeneous mixtures

How Do Mixtures Form? See it explained... Mixtures form by physically “junking” 2 or more substances together Remember no chemical change is occurring The formation of a mixture is not a result of lowering energy

What Is a Homogenous Mixture? A homogeneous mixture is a mixture that is evenly distributed Homogeneous mixtures are commonly called solutions. Like this!

What Is a Homogenous Mixture? Solution = Solute + Solvent Solute: “stuff” being dissolved Solvent: “stuff” doing the dissolving The solvent is present in greater quantity The solute is present in the lesser quantity Ex: Salt water: Salt= solute, Water= solvent

What Is a Heterogeneous Mixture? A heterogeneous mixture is a mixture that is unevenly distributed. Like this!

What Is a Heterogeneous Mixture? Examples: Iced tea: The ice is floating at the top and therefore is not evenly distributed throughout the tea Chex Mix: You may find a different number of pretzels or Chex cereal in each handful; therefore, the mixture is unevenly distributed

How Are Mixtures Important To My Life? We encounter mixtures everywhere in our lives Where would you be without: Ice cream Soup Shampoo Orange Juice Kool-aid Milk

How Can We Change Matter Into New Substances? Chemical reaction (also known as a chemical change) is a change in a substance or substances that results in a totally new substance Chemical Reaction Basics Ex: 2H2(g) + O2(g)  2H2O(l) Notice that the reactants (the substances you start with) combine to form a new substance (the product)

How Do I Know If A Chemical Reaction Has Occurred? There are 5 indicators of a chemical reaction Evolution of a gas Evolution of light Evolution of heat Color change Evolution of a precipitate (Precipitate: an insoluble substance that is produced as result of a chemical reaction) Indications of Chemical Changes Examples

Why Do Chemical Reactions Occur? Chemical reactions occur to produce a more stable product than the existing reactants Ex: 2Na(s) + Cl2(g)  2NaCl(s) *The sodium is highly unstable and the chlorine gas is somewhat unstable. The resulting Sodium Chloride is VERY stable. **It is important to understand that the products have totally different properties than the reactants

Where Does The Matter Go? It is important to understand that when matter undergoes a chemical reaction (ie a chemical change) it does not disappear or appear The atoms are rearranged and form new bonds, but no matter is lost nor gained This is called the Law of Conservation of Matter Watch this experiment to prove it!

What Kind of Chemical Reactions Do I Experience? The acidic milk and basic baking soda that produce CO2 gas when a cake bakes Paper burning to produce ashes, CO2, and H2O vapor Hydrogen peroxide decomposing to produce water and oxygen gas And this one!

Classifying Matter Matter Pure Substances Mixtures milk, tea Fe, O Homogeneous Heterogeneous Elements Compounds Rocky Road ice cream, Trail Mix, Fruit Salad milk, tea Fe, O H2O, CO2