Comprehensive Study Guide: Chemical Changes and Reactions

Chemical Change (Chemical Reaction): A process that occurs when one or more substances, known as reactants, are transformed into entirely different substances, called products. This change results in the formation of new chemical identities.
Reactant: The starting substances in a chemical reaction, typically located to the left of the arrow in a chemical equation.
Product: The substances formed as a result of a chemical reaction, typically located to the right of the arrow in a chemical equation.
Physical Change: A change that affects the form of a chemical substance but does not change its chemical composition. Unlike chemical changes, physical changes do not create new substances.
Dissolving: The process where a solute (like ammonium nitrate) incorporates into a solvent (like water). In the Gizmo, the example of ammonium nitrate ( $NH_4NO_3$ ) dissolving in water to produce ammonium ( $NH_4^+$ ) and nitrate ( $NO_3^-$ ) ions is noted as a point of debate among chemists regarding whether it constitutes a physical or chemical change.
Ion: An atom or molecule with a net electric charge due to the loss or gain of one or more electrons, such as the hydroxide ion ( $OH^-$ ).

Temperature Changes:
- Exothermic Reaction: A reaction that releases heat into its surroundings, causing the temperature of the system to rise.
- Endothermic Reaction: A reaction that absorbs heat from its surroundings, causing the temperature of the system to decrease.
Indicator Tests (Phenol Red): Chemists use indicators to detect the presence of acids and bases. Phenol red is a specific indicator that changes color based on the pH of the solution:
- Yellow: Indicates an acidic solution.
- Orange: Indicates a neutral solution.
- Pink: Indicates a basic (alkaline) solution.
Acids and Bases:
- Bases: These chemicals often contain the hydroxide ion ( $OH^-$ ). For example, sodium hydroxide ( $NaOH$ ) is a strong base.
- Acids: Detected by a yellow color in phenol red.
Gas Production: Evidence of a reaction often includes the appearance of bubbles. In a laboratory setting, a gas collection apparatus can trap these gases. As gas travels through a tube into a cylinder of water, it displaces the water until the cylinder is filled with the gas.

Glowing Splint: A wooden stick that has been lit and then blown out, leaving a red-hot glowing tip.
Burning Splint: A splint with an active flame at the tip.

The following table outlines how different gases respond to splint tests:

Gas	Formula	Glowing Splint	Burning Splint
Carbon dioxide	$CO_2$	Goes out	Goes out
Ammonia	$NH_3$	Goes out	Goes out
Oxygen	$O_2$	Ignites (burns)	Continues to burn
Hydrogen	$H_2$	Does not ignite	Small explosion and "pop" sound

Chemical Equation: A shorthand method to describe a reaction using elemental symbols (e.g., $H$ for hydrogen, $O$ for oxygen, $Na$ for sodium).
Subscript: Small numbers within a chemical formula that indicate the number of atoms of a specific element in a molecule.
- Example: In $H_2O$ , there are two hydrogen atoms and one oxygen atom.
Coefficient: Large numbers placed in front of chemical formulas in a balanced equation. They indicate the number of molecules of that substance involved in the reaction.
- Example: $2H_2O$ signifies two water molecules, totaling four hydrogen atoms and two oxygen atoms.
Balanced Equation: An equation that accurately reflects the Law of Conservation of Matter by showing the same number of atoms for each element on both the reactant and product sides.

The Law: In a chemical reaction, matter can neither be created nor destroyed.
Mass Consistency: The total mass measured at the start of a reaction will equal the total mass at the end, provided the system is closed and nothing escapes.
Closed vs. Open Systems:
- In a Normal Setup (open system), mass may appear to be lost if a gas is produced and escapes into the atmosphere.
- In a Gas Collection Setup or Closed System, the mass remains constant because gases are trapped and accounted for on the balance.
Catalyst: A substance that helps a chemical reaction proceed more efficiently but is not consumed or changed by the reaction itself.
- Example: In the reaction of hydrogen peroxide ( $H_2O_2$ ), potassium iodide ( $KI$ ) acts as a catalyst.

Synthesis (Combination): Two or more reactants combine to form a single product.
- General Form: $A + B \rightarrow AB$
- Example: $2H_2 + O_2 \rightarrow 2H_2O$
Decomposition: A single reactant breaks down into two or more simpler products.
- General Form: $AB \rightarrow A + B$
- Example: $2KCl \rightarrow 2K + Cl_2$
Single Replacement: An uncombined element reacts with a compound and replaces one of the elements in that compound.
- General Form: $A + BC \rightarrow AC + B$
- Example: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$
Double Replacement: Two compounds react to form two new compounds through an exchange of ions.
- General Form: $AB + CD \rightarrow AD + CB$
- Example: $FeS + 2HCl \rightarrow FeCl_2 + H_2S$

Sodium and Water: Sodium is a soft metal that can be cut with a knife. When added to water, it reacts to form sodium hydroxide (a base, indicated by pink phenol red) and hydrogen gas (identified by a "pop" sound with a burning splint). This reaction is exothermic.
Hydrogen Peroxide and Potassium Iodide: In this reaction, $KI$ acts as a catalyst. Even though a gas is produced, if the reaction is performed in a gas collection setup, the mass remains identical to the starting mass.
Sodium and Chlorine: Solid sodium reacts with yellowish chlorine gas to form sodium chloride ( $NaCl$ ), commonly known as table salt. In a normal (open) setup, the mass might increase as the gaseous chlorine from the environment reacts with the solid sodium to form a solid product.
Ammonium Nitrate and Water: When ammonium nitrate dissolves in water, the temperature drops, signifying an endothermic process. This specific process results in the formation of $NH_4^+$ and $NO_3^-$ ions.

Question: If baking soda and vinegar are mixed in a glass, are new substances created?
Observation: Yes, evidence such as bubbling (gas production) indicates a chemical change is occurring.
Question: Would the mass change if the reaction was done on a balance?
Response: In an open glass, the mass would decrease as the gas produced escapes into the air.
Question: What happens to the mass if the reaction is in a sealed plastic bag?
Response: The mass would remain the same because the bag is a closed system that prevents the gas from escaping.
Question: How do the products of the Sodium and Water reaction relate to the phenol red and splint tests?
Response: The products are sodium hydroxide ( $NaOH$ ) and hydrogen gas ( $H_2$ ). The sodium hydroxide is a base, causing the phenol red to turn pink. The hydrogen gas causes the characteristic "pop" in the burning splint test.