Evidence of a Chemical Reaction

Chemical Reactions

http://www.youtube.com/watch?v=MTFOxR2cg0g&feature=related&safety_mode=true&persist_safety_mode=1&safe=active

Chemical Change

-vs- Physical Change

Physical Change Physical change: a change that

occurs that does not change the identity of the substance– Melting ice– Freezing Kool-Aid– Tearing paper– Boiling water

Chemical Changes

Chemical change: a change that occurs causing the identity of the substance to change– Burning– Digesting food – Reacting with other substances

A chemical change is called a chemical reaction

Chemical reactions A chemical reaction involves changing the

ways the atoms are grouped.

H2 + Cl2   -->   2 HCl 2 H2 + O2   -->   2H2O

A process in which one or more substances, the reactants, are converted to one or more different substances, the products. A chemical reaction rearranges the

atoms of the reactants to create different substances as products

Chemical Reaction

How do we know a

chemical reaction has occurred?

http://www.youtube.com/watch?v=cZMkqagL8Ps&safety_mode=true&persist_safety_mode=1&safe=active

PrecipitationA solid formed from a solution can result from

a chemical change.

Evidence for a Chemical Reaction

Gas ProductionFizzing, bubbling

Color Changes

Odor Changes

Evidence for a Chemical Reaction

Energy ChangesExothermic

Heat is released (feels hot)Endothermic

Heat is absorbed (feels cold) Light can be absorbed or released also without a

temperature change.

Evidence for a Chemical Reaction

Step 1 – Write the reactants as they actually exist before any reaction occurs.

Step 2 – Consider the various solids that could form.

Step 3 – Use the solubility rules to decide whether a solid forms, and, if so, to predict the identity of the solid.

Steps for predicting precipitates

Soluble SolidSolid that readily dissolves in water.

Insoluble solid and slightly soluble solidVery small amount dissolves in water, the majority stays solid.

Chemical Equation

A representation of a chemical reaction showing the relative numbers of reactants and products.

Reactants Products“Yields”

Parts of a chemical Equation

SUBSCRIPTS VS. COEFFICIENTS

The subscripts tell you how many atoms of a particular element are in a compound.

The coefficient tells you about the quantity, or number, of molecules of the compound.