Acids, Bases, Physical & Chemical Changes

• Chemical changes result in new substances that possess different properties.
• Physical changes modify a substance’s form or appearance without changing its chemical composition.

Physical Changes

• Changes of state, dissolving, and altering shape or size are physical changes.
• Changes of state include melting, boiling, freezing, evaporation, condensation, and sublimation.
• Physical changes are typically reversible.
• During a physical change, the substance retains its original chemical properties and no new substances are formed.
• Dissolving a substance in water can be a physical change. Examples include dissolving salt in water.
• Examples include melting ice, boiling water, and cutting paper.

Chemical Changes

• Burning, rusting, cooking, and acid-base reactions are chemical changes.
• Chemical changes are usually irreversible.
• Atoms are rearranged in chemical reactions, but are never created or destroyed.
• Chemical reactions involve the breaking and forming of chemical bonds.
• Examples include burning wood, rusting iron, and cooking an egg.
• Signs of a chemical change include color change, gas production, precipitate formation, temperature change, and odor change.
• The rate of a chemical reaction can be influenced by factors such as temperature, concentration, and the presence of catalysts.

Acids

• Acids produce hydrogen ions (H+) when dissolved in water.
• Acids have a sour taste.
• Acids can corrode metals.
• Blue litmus paper turns red in contact with acids.
• Acids have a pH less than 7.
• Common acids: hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3).
• HCl is the chemical formula for hydrochloric acid.
• H2SO4 is the chemical formula for sulfuric acid.
• HNO3 is the chemical formula for nitric acid.
• CH3COOH is the chemical formula for acetic acid.
• C6H8O7 is the chemical formula for citric acid.
• Acids donate protons (H+) or accept electrons.
• Acids react with bases in a neutralization reaction to form salt and water.
• Strong acids completely dissociate into ions in water, while weak acids only partially dissociate.

Bases

• Bases produce hydroxide ions (OH-) when dissolved in water.
• Bases taste bitter
• Bases feel slippery
• Red litmus paper turns blue in contact with bases.
• Bases have a pH greater than 7.
• Common bases include sodium hydroxide (NaOH), potassium hydroxide (KOH), and ammonia (NH3).
• NaOH is the chemical formula for sodium hydroxide.
• KOH is the chemical formula for potassium hydroxide.
• Ca(OH)2 is the chemical formula for calcium hydroxide.
• NH3 is the chemical formula for ammonia.
• Bases accept protons (H+) or donate electrons.
• Bases react with acids in a neutralization reaction to form salt and water.
• Strong bases completely dissociate into ions in water, while weak bases only partially dissociate.

Salts

• Salts are ionic compounds from an acid and a base reacting.
• Neutralization is the reaction when forming salts
• Salts consist of a positive ion (cation) from the base and a negative ion (anion) from the acid.
• Common salts: sodium chloride (NaCl), potassium nitrate (KNO3), and calcium sulfate (CaSO4).
• NaCl is the chemical formula for sodium chloride.
• KNO3 is the chemical formula for potassium nitrate.
• CaSO4 is the chemical formula for calcium sulfate.
• MgCl2 is the chemical formula for magnesium chloride.
• (NH4)2SO4 is the chemical formula for ammonium sulfate.
• Common salt is calcium carbonate (CaCO3).
• Salts can be soluble or insoluble in water.
• The properties of salts depend on the ions they are composed of.
• Salts are essential for various biological and industrial processes.

Neutralization

• Neutralization is the reaction between an acid and a base.
• The products of neutralization are salt and water.
• Neutralization reactions are exothermic, meaning they release heat.
• The pH of the resulting solution depends on the strengths of the acid and base.
• If a strong acid reacts with a strong base, the resulting solution has a pH of 7, which is neutral.

Common Acids and Their Chemical Formulas

• Hydrochloric acid: HCl
• Sulfuric acid: H2SO4
• Nitric acid: HNO3
• Acetic acid: CH3COOH
• Carbonic acid: H2CO3

Common Alkalis and Their Chemical Formulas

• Sodium hydroxide: NaOH
• Potassium hydroxide: KOH
• Calcium hydroxide: Ca(OH)2
• Ammonia: NH3
• Magnesium hydroxide: Mg(OH)2

Testing for Hydrogen Gas

• Hydrogen gas (H2) is colorless and odorless.
• Steps for hydrogen gas testing:
  • Collect the gas in a test tube.
  • Put a lit splint at the test tube’s mouth.
  • Hydrogen ignites with a "pop" sound if present.
• The "pop" sound is due to the rapid combustion of hydrogen gas with oxygen in the air.

Testing for Carbon Dioxide

• Carbon dioxide (CO2) is colorless and odorless.
• Steps for carbon dioxide testing:
  • Bubble the gas through limewater (calcium hydroxide solution).
  • The limewater turns milky or cloudy due to calcium carbonate formation if carbon dioxide is present.
• The cloudiness is due to the formation of calcium carbonate (CaCO3), which is insoluble in water.
• The reaction: CO2(g) + Ca(OH)2(aq) -> CaCO3(s) + H2O(l)

pH Indicators

• pH indicators change color depending on the solution's pH.
• pH indicators are weak acids or bases.

Litmus Paper

• Red in acidic solutions when pH is less than 7.
• Blue in basic solutions when pH is greater than 7.
• Purple in neutral solutions when pH equals 7.

Phenolphthalein

• Colorless in acidic solutions when pH is less than 8.3.
• Pink to magenta in basic solutions when pH is greater than 8.3.

Methyl Orange

• Red in acidic solutions when pH is less than 3.1.
• Yellow in basic solutions when pH is greater than 4.4.

Universal Indicator

• It is a mixture of several indicators that show a range of colors across the pH scale.
• Red indicates strongly acidic solutions.
• Orange/yellow indicates mildly acidic solutions.
• Green indicates neutral solutions.
• Blue indicates mildly basic solutions.
• Purple indicates strongly basic solutions.
• pH indicators are used to estimate the pH of a solution.
• The choice of indicator depends on the pH range of interest.