Physical and Chemical Changes, Chemical Equations, and Reactions in General Chemistry

Physical vs. Chemical Changes

Definitions and Key Concepts

Understanding the distinction between physical and chemical changes is fundamental in chemistry. These changes affect substances in different ways and are identified by specific indicators.

• Physical Change: A change that affects the form or state of a substance without altering its chemical composition. Examples include melting, freezing, and dissolving.

• Chemical Change: A process in which one or more substances are transformed into new substances with different properties. Examples include burning, rusting, and digestion.

Examples

• Physical Change: Melting ice, tearing paper, dissolving sugar in water.

• Chemical Change: Burning wood, rusting iron, baking a cake.

Evidence of a Chemical Change

Chemical changes are often identified by observable indicators:

• Color change

• Formation of a gas (bubbling or fizzing)

• Formation of a precipitate (a solid formed in a solution)

• Temperature change (hotter or colder)

• Light production

• Other changes (such as odor)

Parts of a Chemical Equation

Key Terms

• Reactants: Substances that start a reaction.

• Products: Substances formed as a result of a reaction.

• Arrow (→): Indicates the direction of the reaction, from reactants to products.

• Coefficients: Numbers placed in front of chemical formulas to balance equations and indicate the number of molecules or moles.

• Subscripts: Small numbers within a chemical formula showing the number of atoms of each element in a molecule.

Example Equation

• $2H_2O \rightarrow 2H_2 + O_2$

• Here, 2 is the coefficient for $H_2O$ and $H_2$.

• Subscript 2 in $H_2O$ indicates two hydrogen atoms per molecule of water.

Balancing Chemical Equations

Key Ideas

Balancing chemical equations ensures the Law of Conservation of Mass is obeyed: the number of atoms of each element is the same on both sides of the equation.

• Only coefficients are changed to balance equations, never subscripts.

Steps to Balance an Equation

1. Write the unbalanced equation.

2. Count the number of atoms of each element on both sides.

3. Add coefficients to balance the atoms.

4. Check your work to ensure all elements are balanced.

Example

• Unbalanced: $H_2 + O_2 \rightarrow H_2O$

• Balanced: $2H_2 + O_2 \rightarrow 2H_2O$

Classifying Chemical Reactions

Common Types

Chemical reactions can be classified into several types based on the rearrangement of atoms and molecules.

• Synthesis (Combination): $A + B \rightarrow AB$

• Decomposition: $AB \rightarrow A + B$

• Single Displacement (Replacement): $A + BC \rightarrow AC + B$

• Double Displacement (Replacement): $AB + CD \rightarrow AD + CB$

Reactions Between Metals and Acids

Key Ideas

When metals react with acids, they typically produce a salt and hydrogen gas ($H_2$).

• General Reaction: Metal + Acid $\rightarrow$ Metal Salt + Hydrogen Gas

• Example: $Mg + 2HCl \rightarrow MgCl_2 + H_2$

• Metal Carbonate or Hydrogencarbonate + Acid: Produces a salt, water, and carbon dioxide gas.

• Example: $Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2$

Testing for Gases

• Hydrogen Gas: A burning splint produces a 'pop' sound in the presence of hydrogen.

• Carbon Dioxide: Turns limewater milky due to the formation of calcium carbonate.

Summary Table: Types of Chemical Reactions

Type	General Equation	Example
Synthesis	$A + B \rightarrow AB$	$2H_2 + O_2 \rightarrow 2H_2O$
Decomposition	$AB \rightarrow A + B$	$2H_2O \rightarrow 2H_2 + O_2$
Single Displacement	$A + BC \rightarrow AC + B$	$Zn + 2HCl \rightarrow ZnCl_2 + H_2$
Double Displacement	$AB + CD \rightarrow AD + CB$	$Na_2SO_4 + BaCl_2 \rightarrow 2NaCl + BaSO_4$

Additional info:

• Physical and chemical changes are foundational concepts for understanding chemical reactions and laboratory observations.

• Balancing equations is a critical skill for predicting the outcomes of reactions and for quantitative chemical calculations.

• Classifying reactions helps in predicting products and understanding reaction mechanisms.