Chemical and Physical Changes

Chemical vs. Physical Changes

• Physical changes don't alter the substance's chemical properties or composition, and no new substance forms.
• Physical changes are usually reversible, like dissolving or changing states.
• Chemical changes involve the combination of substances to create new ones or the decomposition of a substance into different ones.
• In chemical changes, atoms rearrange to form new substances, which are usually irreversible.
• Chemical reactions can be exothermic (producing heat) or endothermic (absorbing heat).

Evidence of Chemical Change

• Odor change
• Color change
• Temperature change
• Form change
• Emission of heat, light, or sound
• Effervescence (bubbles in liquid)
• Precipitate formation (insoluble substance)
• Decomposition of organic matter

Initiating Chemical Reactions

• Mixing
• Heating
• Exposure to light
• Passing electricity

Representing Chemical Reactions

• Reactant 1 + Reactant 2 → Product 1 + Product 2

Types of Chemical Changes: Combustion

• Combustion is a reaction where a substance is heated with oxygen, producing new substances and usually heat and light.
• Example: carbon + oxygen → carbon dioxide
• Other examples:
  • methane + oxygen → carbon dioxide + water vapor
  • hydrogen + oxygen → water vapor
  • magnesium + oxygen → magnesium oxide

Types of Chemical Changes: Thermal Decomposition

• Thermal decomposition is when a substance breaks down into simpler substances upon heating.
• Example: calcium carbonate → calcium oxide + carbon dioxide
• Another example: sugar → carbon + water vapor

Types of Chemical Changes: Oxidation

• Oxidation is when a substance gains oxygen.
• Cellular respiration: glucose + oxygen → carbon dioxide + water
• Corrosion of copper: copper + oxygen → copper oxide
• Rusting (with moisture): iron + oxygen → iron oxide

Rusting

• Rusting occurs when iron reacts with oxygen and water to form rust (reddish-brown solid).

Respiration

• Respiration breaks down food molecules (like glucose) with oxygen to provide energy, producing carbon dioxide and water.
• Glucose + oxygen → carbon dioxide + water

Acids

• Acids have a sour taste and can be corrosive.
• Common lab acids: hydrochloric acid, sulfuric acid, nitric acid
• Common everyday acids: vinegar, citrus fruits, car batteries
• Hydrochloric acid forms chloride salts.
• Nitric acid forms nitrate salts.
• Sulfuric acid forms sulfate salts.

Alkalis

• Alkalis have a bitter taste and a soapy feel; some are corrosive.
• Common lab alkalis: sodium hydroxide, calcium hydroxide, potassium hydroxide, aqueous ammonia.
• Common everyday alkalis: soaps, baking soda, cleaning liquids.
• Common alkalis include potassium hydroxide, sodium hydroxide, calcium hydroxide (limewater), and ammonia solution.

Chemical Reactions Between Acids and Alkalis

• Neutralization reaction: acid + alkali → salt + water
• Salt is a general term for a compound containing a metal.
• Water is always formed in neutralization reactions.
• Examples:
  • hydrochloric acid + sodium hydroxide → sodium chloride + water
  • nitric acid + sodium hydroxide → sodium nitrate + water
  • sulfuric acid + sodium hydroxide → sodium sulfate + water
  • hydrochloric acid + potassium hydroxide → potassium chloride + water
  • nitric acid + calcium hydroxide → calcium nitrate + water

Acid and Reactive Metal Reactions

• acid + metal → salt + hydrogen
• Example: hydrochloric acid + magnesium → magnesium chloride + hydrogen
• Test for hydrogen gas: a lighted splint extinguishes with a "pop" sound.
• Some metals (e.g., copper, silver, gold) do not react with acids.
• Examples:
  • hydrochloric acid + magnesium → magnesium chloride + hydrogen
  • nitric acid + magnesium → magnesium nitrate + hydrogen
  • sulfuric acid + magnesium → magnesium sulfate + hydrogen
  • hydrochloric acid + zinc → zinc chloride + hydrogen
  • nitric acid + aluminum → aluminum nitrate + hydrogen

Acid and Metal Carbonate Reactions

• acid + carbonate → salt + carbon dioxide + water
• Example: hydrochloric acid + magnesium carbonate → magnesium chloride + carbon dioxide + water
• Test for carbon dioxide: limewater turns chalky/cloudy (white precipitate forms).
• Examples:
  • hydrochloric acid + copper carbonate → copper chloride + water + carbon dioxide
  • hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide
  • nitric acid + copper carbonate → copper nitrate + water + carbon dioxide
  • sulfuric acid + calcium carbonate → calcium sulfate + water + carbon dioxide

Indicators

• Indicators change color in acids or alkalis.
• Acidic solutions turn blue litmus paper red; no effect on red litmus paper.
• Alkaline solutions turn red litmus paper blue; no effect on blue litmus paper.
• Neutral solutions have no effect on either red or blue litmus paper.

How Chemical Changes Occur: Exposure to Light

• Photosynthesis: carbon dioxide + water → glucose + oxygen (with chlorophyll and light)
• Photodecomposition: substances break down into simpler ones when exposed to light (used in photography).
• Silver bromide → silver + bromide gas
• Silver chloride → silver + chlorine gas

How Chemical Changes Occur: Using an Electric Current

• Electrolysis: decomposition of compounds into simpler substances using electricity.
• Water → oxygen gas + hydrogen gas
• Electroplating: coating a metal with a different metal using electricity (for corrosion prevention or improved appearance).

Additional Notes: Atomic Structure

• Protons have a +1 charge, and electrons have a -1 charge.
• Atoms are neutral, so the number of protons equals the number of electrons.
• The valence shell is the outermost shell of an atom; valence electrons participate in chemical reactions.

Stable Electron Configuration of Noble Gases

• Noble gases (Group 18) are unreactive because they have fully filled valence shells.
• Other elements react to achieve a stable noble gas configuration by losing, gaining, or sharing valence electrons.

Covalent Bond

• Non-metallic atoms share valence electrons to achieve a noble gas configuration, forming covalent bonds.
• A covalent bond is formed by the sharing of electrons between two atoms
• Covalent bonding:
  • Occurs between non-metallic atoms
  • The number of electrons to be shared is determined by the number of valence electrons present
  • Number of electrons shared between the two atoms must be the same
  • After bonding, there must be a total of 2 valence electrons (in the first shell) or 8 valence electrons
• Each hydrogen atom needs one more electron to attain the electronic configuration of a noble gas
• Each chlorine atom needs one more electron to attain the electron configuration of a noble gas (eight valence electrons in the third shell)
• Double bond: oxygen and carbon dioxide
• Triple bond: nitrogen

Ionic Bond

• Metals transfer valence electrons to non-metals, forming ionic bonds.
• Ionic bonds are strong electrostatic forces between positive and negative ions.
• Ionic bonding:
  • Occurs between metallic and non-metallic atoms.
  • Metallic atoms lose valence electrons to form positive ions.
  • Non-metallic atoms gain electrons to form negative ions.
  • The total number of electrons lost by positive ions equals the total number gained by negative ions.
• Example: Sodium atom gives its valence electron to a chlorine atom to form sodium chloride.
• After the transfer of valence electron, both sodium ion and chloride ion have the stable electronic configuration of a noble gas
• The positive sodium ion is attracted to the negative chloride ion, forming sodium chloride.