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?