Science Exam Review

Salty Rainbows (Density is the Driver)

1. Compounds in the Solution

• Sodium chloride (NaCl) + Water (H₂O)

2. Why Did the Solutions Stack?

• Density differences caused solutions with more salt (higher density) to sink below less salty (lower density) solutions.

  • Reference: Density is the relationship between mass and volume.

  • The more salt added, the more mass the solution had, increasing its density and causing it to sink.

3. Definition of Density

• Density is the mass per unit volume of a substance.

  • Formula: Density = Mass ÷ Volume

  • If a substance has more mass in the same volume, it has a higher density.

4. How Increasing Salt Affected Density

• More NaCl (salt) in the solution = higher density because more mass was dissolved in the same volume of water.

  • Solutions with less salt (lower density) floated above solutions with more salt (higher density).

5. Why Did the Solutions Stay in the Straw?

• Capillary action (water moving through narrow spaces) kept the solutions in place.

  • Cohesion: Water molecules stick to each other.

  • Adhesion: Water sticks to other surfaces (straw walls).

  • Surface tension helped prevent mixing.

  • Density layering kept the solutions from mixing immediately.

Water Properties & the Hydrologic Cycle

Water’s Unique Properties

• Universal solvent: Dissolves many substances (like salt).

• Cohesion: Water molecules stick together.

• Adhesion: Water sticks to other substances.

• Surface tension: Forms a "skin" on water’s surface.

• Density is an intensive property: It does not change with sample size.

• Temperature and density relationship:

• As temperature rises, volume increases, density decreases (water expands when heated).

Water Cycle (Hydrologic Cycle)

1. Evaporation – Liquid → Gas (water rises from oceans/lakes).

2. Condensation – Gas → Liquid (cloud formation).

3. Precipitation – Rain, snow, sleet, hail falls to Earth.

4. Transpiration – Plants release water vapor through their stomata.

5. Evapotranspiration – Combined evaporation + transpiration.

6. Infiltration & Percolation – Water moves through soil into groundwater.

7. Human Impact:

• Deforestation: Increases runoff, erosion, and lowers water tables.

  • Irrigation: Depletes lakes/rivers and increases evaporation.

  • Dams: Increase evaporation and infiltration.

  • Water shortages cause global conflicts.

  • Only 1.2% of Earth's water is drinkable.

Photosynthesis & Cellular Respiration

Photosynthesis Equation

• Reactants: 6CO₂ + 6H₂O + Energy (Sunlight)

• Products: C₆H₁₂O₆ (Glucose) + 6O₂

• Occurs in: Chloroplasts

Cellular Respiration Equation

• Reactants: C₆H₁₂O₆ + 6O₂

• Products: 6CO₂ + 6H₂O + Energy (ATP)

• Occurs in: Mitochondria

Key Concepts

• Photosynthesis stores energy in glucose.

• Respiration releases energy (ATP) from glucose.

• Carbon (C), Hydrogen (H), and Oxygen (O) are recycled in both processes.

• Without sunlight → No photosynthesis → Plants die.

• Without water → No photosynthesis → Plants die.

• Without oxygen → Animals can’t make ATP → Cells die.

• Energy used to walk your dog comes from the sun (via food chains).

Properties of Water & States of Matter

Changes in Water States (Kinetic Particle Theory)

1. Solid (Ice)

• Particles vibrate in fixed positions (low energy).

  • Water freezes at 0°C.

  • Ice is less dense than liquid water (why ice floats).

2. Liquid (Water)

• Particles move freely but remain close together.

  • Water is most dense in liquid form.

3. Gas (Water Vapor)

• Particles move fast and spread out.

  • Water boils at 100°C.

  • Gases are compressible; liquids and solids are not.

Atomic Structure & Bonding

1. Structure of an Atom

• Nucleus: Contains protons (+) and neutrons (0).

  • Electrons (-): Orbit nucleus in shells.

  • First shell: Holds 2 electrons.

  • Second shell: Holds 8 electrons.

  • Third shell: Holds 18 electrons.

  • Number of protons = Element identity.

  • Most of an atom’s mass is in the nucleus.

2. Covalent Bonding in Water

• Oxygen (O) is more electronegative → pulls electrons closer.

  • Hydrogen (H) is partially positive due to uneven electron sharing.

  • Covalent bonds: Strong, sharing electrons.

  • Water is a polar molecule (unequal charge distribution).

3. Hydrogen Bonds in Water

• Attraction between water molecules (not within the molecule).

  • Each water molecule forms up to 4 hydrogen bonds.

  • Hydrogen bonds = weaker than covalent bonds.

  • Effects of Hydrogen Bonds:

  • Cohesion: Water sticks together.

    • High heat capacity: Resists temperature changes.

    • Ice floats: Because solid water is less dense than liquid water.

Environmental Issues & Solutions

1. Halocene Era & Climate Effects

• 9% of rainforests remain due to deforestation (palm oil industry).

  • Deforestation removes carbon sequestration (storage).

  • Aerosols damage the ozone layer → more radiation reaches Earth.

  • Carbon particulates in atmosphere trap heat.

2. Solutions & Key Terms

• Solutions: Homogeneous mixtures of 2+ substances.

  • Solvent: Dissolving agent (e.g., water).

  • Solute: The substance that dissolves (e.g., salt).

  • Solubility: The ability of a solute to dissolve in a solvent.

  • Water = Universal solvent.

  • Hydrophilic substances mix well with water (polar/charged).

  • Hydrophobic substances do not mix with water (non-polar).

Understanding Density

• Density is defined as the relationship between the mass of a substance and the volume it occupies, expressed mathematically as density = mass/volume.

• A substance with greater mass and volume than another will have a higher density, which is crucial in understanding how solutions stack based on their densities.

• Increasing the concentration of salt (sodium chloride) in a solution raises its density, as the mass increases while the volume remains relatively constant.

• The stacking of solutions occurs because less dense solutions float atop denser ones, demonstrating the principle of buoyancy in liquids.

• Water's unique properties, such as cohesion and adhesion, play a significant role in how solutions behave in a straw, allowing for capillary action.

The Water Cycle and Its Importance

• The water cycle consists of interconnected pathways involving evaporation, condensation, precipitation, and transpiration, which are essential for maintaining ecological balance.

• Water exists in various states: solid (ice), liquid (water), and gas (water vapor), with changes in state influenced by temperature and energy.

• The majority of Earth's water (97%) is found in oceans, with only 1.2% being readily available for drinking, highlighting the importance of water conservation.

• Deforestation and agricultural practices can disrupt the water cycle, leading to increased runoff, erosion, and reduced transpiration, which affects water availability.

• Understanding the water cycle is crucial for addressing global water shortages and conflicts over water resources.

Photosynthesis and Cellular Respiration

The Process of Photosynthesis

• Photosynthesis occurs in chloroplasts, where plants convert carbon dioxide and water into glucose and oxygen using solar energy.

• The chemical equation for photosynthesis is: 6CO2 + 6H2O + energy → C6H12O6 + 6O2, illustrating the transformation of reactants into products.

• Key components required for photosynthesis include sunlight, carbon dioxide, and water, emphasizing the interdependence of these elements.

• Without sunlight, plants cannot perform photosynthesis, leading to their eventual death due to lack of energy.

• The products of photosynthesis (glucose and oxygen) are essential for cellular respiration in animals, creating a cyclical relationship between the two processes.

Cellular Respiration and Its Relationship to Photosynthesis

• Cellular respiration occurs in mitochondria, where glucose and oxygen are converted into carbon dioxide, water, and energy (ATP).

• The chemical equation for cellular respiration is: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy, showing the reverse process of photosynthesis.

• Both processes recycle carbon, hydrogen, and oxygen, which are vital for sustaining life on Earth.

• Deprivation of oxygen in animals leads to severe consequences, including organ failure and death, highlighting the importance of respiration.

• The energy derived from food, which originates from sunlight through photosynthesis, powers all biological activities, including movement and growth.

Atomic Structure and Bonding

Structure of the Atom

• An atom consists of a nucleus containing protons (positive charge) and neutrons (neutral), surrounded by electrons (negative charge) arranged in shells.

• The first shell can hold 2 electrons, the second 8, and the third 18, with the number of protons determining the element's identity.

• Water (H2O) is a polar molecule due to the uneven distribution of charges, with oxygen being more electronegative than hydrogen.

• The unique structure of water contributes to its properties, such as high surface tension and solvent capabilities.

Covalent Bonding and Hydrogen Bonds in Water

• Covalent bonds in water involve the sharing of electron pairs between oxygen and hydrogen atoms, resulting in a stable molecule.

• Each water molecule can form up to four hydrogen bonds with neighboring molecules, leading to cohesive behavior and resistance to temperature changes.

• Hydrogen bonds are weaker than covalent bonds, making them easier to break, which is essential for processes like evaporation and condensation.

• The octet rule explains how atoms interact to achieve stable electron configurations, influencing the behavior of water and its interactions with other substances.