8.1 Chemical Properties and Changes. chemical property chemical change dissolving

Transcription

1 8.1 Chemical Properties and Changes chemical property chemical change dissolving

2 Ability to Change 8.1 Chemical Properties and Changes In a chemical change, the properties that give a substance its identity change. Properties of matter help identify objects. Properties are either physical or chemical. What properties do elements have?

3 Ability to Change (cont.) 8.1 Chemical Properties and Changes A chemical property is the ability or inability of a substance to combine with or change into one or more new substances.

4 Chemical Properties Examples of chemical properties include burning or rusting. Some substances do not react. 8.1 Chemical Properties and Changes

5 Physical Properties 8.1 Chemical Properties and Changes Physical properties can be observed without changing the substance.

6 8.1 Chemical Properties and Changes Chemical and Physical Changes Chemical changes change one substance into another substance. Usually chemical changes cannot be easily reversed. Property Changes

7 Forming New Substances 8.1 Chemical Properties and Changes All chemical changes produce substances that are different from the starting substances. A compound is decomposed into its elements. Two elements join to form a compound.

8 Physical Changes 8.1 Chemical Properties and Changes A physical change is a change in which the properties of a substance change but the identity of the substance remains the same. Dissolving is a process in which substances mix evenly with one another.

9 law of conservation of mass reactants products diatomic molecule coefficient

10 Is matter conserved in chemical reactions? Chemical equations show that in chemical reactions, atoms rearrange, but no atoms are gained or lost.

11 Conservation of Mass The law of conservation of mass states that the total mass before a chemical reaction is equal to the total mass after the reaction.

12 Writing a Chemical Equation Reactants are the starting materials in a chemical reaction and are placed on the left side. Products are the ending materials in a chemical reaction and are placed on the right side.

13 Word Equations Tin + oxygen tin oxide reads as tin plus oxygen produces tin oxide. Word equations can be long and do not show that mass is conserved.

14 Elements, Compounds, and Molecules Instead of writing long word equations, scientists use symbols and formulas. Symbols represent atoms. Formulas represent molecules. Molecules may be elements or compounds.

15 Elements, Compounds, and Molecules (cont.) Some elements are diatomic, that is, they contain two atoms. Molecules may also contain two different elements, such as hydrogen and oxygen in a water molecule.

16 Elements, Compounds, and Molecules (cont.)

17 Chemical Equations Use the symbols and formulas instead of words.

18 Balancing a Chemical Equation Atoms are neither gained nor lost in a reaction, so both sides of the equation must have the same number of atoms.

19 Counting Atoms A subscript tells how many atoms of an element are in one molecule. A coefficient tells how many atoms, molecules, or formula units are in a reaction.

20 Counting Atoms (cont.)

21 Writing Balanced Equations Step 1 Determine the correct symbols and formulas for reactants and products. Step 2 Write reactant symbols and formulas to the left of an arrow and product symbols and formulas to the right. Step 3 Count the number of each kind of atom on both sides. Step 4 Use coefficients to make the number of each kind of atom the same on both sides of the arrow. Step 5 Check to see that each kind of atom balances.

22 Equations for Common Chemical Reactions Reaction of methane: CH 4 + O 2 CO 2 + H 2 O balance hydrogen CH 4 + O 2 CO 2 + 2H 2 O balance oxygen CH 4 + 2O 2 CO 2 + 2H 2 O

23 Equations for Common Chemical Reactions (cont.) Baking soda and vinegar: NaHCO 3 + HC 2 H 3 O 2 CO 2 + H 2 O + NaC 2 H 3 O 2

24 Using Parentheses with Formulas When counting atoms inside parentheses, the subscript multiplies all atoms inside. Ca + H 2 O Ca(OH) 2 + H 2 Ca + 2H 2 O Ca(OH) 2 + H 2