Ch. 2.2-2.4 Mixtures, Elements, Compounds. Symbols Chemists use chemical symbols to represent...
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Transcript of Ch. 2.2-2.4 Mixtures, Elements, Compounds. Symbols Chemists use chemical symbols to represent...
Ch. 2.2-2.4
Mixtures, Elements, Compounds
Symbols
• Chemists use chemical symbols to represent elements
• Chemical Symbol: 1st 1-2 letters of the name of the element
– 1st letter is always Capitalized
– 2nd letter is always lower case Examples: Co = Cobalt; O = Oxygen;
Be = Berylium
Exception: Some use the letters from the Latin name Examples: Fe = Iron (Ferrum)
K = Potassium (Kalium)
Cu = Copper (Cuprium)
Formulas
• Chemists use chemical formulas to represent compounds– Compounds: 2 or more elementsExamples: NaCl = Sodium Chloride
H2O = Water
Breaking up
• Compounds can be broken down into simpler substances by chemical means, but elements cannot
• Chemical changes can be used to break down the substances– Heating sugar can be heated until it
breaks down into solid carbon and water vapor
• “Heating sugar can be heated until it breaks down into solid carbon and water vapor”
• Do you think either of those substances that are formed can be broken down any?• Yes, the water can be broken down into
hydrogen and oxygen
Properties
• Properties of compounds differ greatly from the properties of the individual elements that make them up– Solid sodium is extremely reactive– Chlorine is a gas– Combined they make up sodium
chloride, aka table salt
Na + Cl2 NaClSodium + Chlorine Table salt
H2 + O2 H2O
Classifying mixtures
• A mixture is a physical blend of two or more components
• Based on the distribution of the components, mixtures can be classified as heterogeneous or homogeneous mixtures
• Heterogeneous mixture: a mixture in which the composition is not uniform throughout– Ex: oil and vinegar, sand in water
• Homogeneous mixture: a mixture in which the composition is uniform throughout– Also known as a solution
Homogeneous mixtures
• Many solutions are liquid, but can also be gas (air) or solids (stainless steel)
• The term “phase” can be used to describe any part of a sample with uniform composition and properties– Homogeneous mixtures have 1 phase
Heterogeneous V. Homogeneous
Distinguishing Substance and Mixtures
Matter
Substance:Definite Composition
(Homogeneous)
Mixture of Substance:
Variable Composition
ElementEx: Sodium Na
CompoundEx: Sodium Chloride
Homogeneous
Mixture:Uniform; also called
a SolutionEx: Salt in Water
Heterogeneous Mixture:
Non-uniform;Distinct phases
Ex: Milk
Separating mixtures
• Different physical properties can be used to separate mixtures– Different BPs or MPs can be used
• Filtration: separates a solid from a liquid– Coffee filters separate grounds from water
• Distillation: a liquid is boiled to produce a vapor then condensed into a liquid
Paper chromatographySeparating a mixture
Chemical reactions
• Chemical changes are signs of chemical reactions happening– Iron reacting with oxygen to form rust
– Fe + O2 Fe2O3
• During a chemical reaction, the composition of matter always changes
• Fe + O2 Fe2O3
• A substance present at the start of a chemical reaction is called a “reactant”
• A substance produced in the reaction is the “product”
• Reactants react to produce products
Recognizing chemical reactions
• Signs a reaction may have occurred:– Production of gas or bubbles– Change in color– Change in temperature– Precipitate forms
• A solid that forms and settles out of a liquid mixture
Law of Conservation of Mass
• During any chemical reaction or physical change, the mass of the products is always equal to the mass of the reactants
• Bottom line: you cannot create or destroy matter. Matter is conserved.
Discuss
• What about burning a candle? Where does the mass go??
Review questions
• How are mixtures classified?
• Come up with a way to separate a mixture of sand and salt
• How is a compound different from an element?
• How much water would form if 4.8 grams of hydrogen reacted with 38.4 g oxygen?