A1.1

Why is water considered the medium for life?

Water provides the environment for most biochemical reactions; early life likely evolved in water near hydrothermal vents where dissolved molecules could react.

Why do scientists look for water when searching for extraterrestrial life?

Because water is essential for metabolic reactions, its presence suggests potential for life.

What type of bonding occurs within a water molecule?

Covalent bonding between one oxygen atom and two hydrogen atoms.

Why is water a polar molecule?

The oxygen atom attracts shared electrons more strongly than hydrogen, creating a partial negative charge (δ-) on oxygen and partial positive charges (δ+) on hydrogens.

What is a dipole?

A molecule with regions of slight positive and negative charge due to uneven electron distribution.

What is hydrogen bonding?

A weak attraction between the slightly positive hydrogen of one polar molecule and the slightly negative atom of another.

How do hydrogen bonds affect water's properties?

They give water high cohesion, adhesion, specific heat capacity, and surface tension, all essential for life.

Why is hydrogen bonding biologically important?

It helps stabilize the structures of DNA, proteins, and cellulose and allows water to act as a solvent.

How much of a cell's mass is water?

Between 70-95% of a cell's mass is water.

What is cohesion in water?

The attraction between water molecules due to hydrogen bonding.

How does cohesion help plants?

It allows continuous columns of water to move under tension through the xylem in transpiration.

What is surface tension?

The cohesive forces between surface water molecules create a film-like surface, allowing insects like pond skaters to move across water.

What is adhesion in water?

The attraction between water molecules and other polar or charged surfaces, such as cellulose in plant cell walls.

How does adhesion help in plants?

It assists water movement up the xylem and through plant tissues via capillary action.

What is the difference between cohesion and adhesion?

Cohesion is water molecules sticking to each other; adhesion is water molecules sticking to other materials.

Why is water a universal solvent?

Because its polarity allows it to dissolve polar and ionic substances by forming hydrogen bonds.

What types of substances dissolve easily in water?

Polar and ionic compounds such as salts, glucose, and amino acids.

What types of substances are hydrophobic?

Non-polar molecules like lipids and fats that do not form hydrogen bonds with water.

What are hydrophilic substances?

"Water-loving" substances that dissolve easily due to polarity or charge, such as glucose or ions.

What are hydrophobic interactions?

Non-polar molecules cluster together in water because they cannot form hydrogen bonds with it.

How does water's solvent property support life?

It allows transport of nutrients and waste in blood, cytoplasm, and xylem/phloem.

Why are fats and lipids insoluble in water?

They are non-polar and hydrophobic, preventing them from forming hydrogen bonds with water molecules.

How do organisms transport insoluble molecules?

By combining them with carrier proteins, lipoproteins, or hemoglobin (for oxygen).

Why is oxygen less soluble in water at 37°C than at 20°C?

Higher temperature reduces gas solubility, making oxygen less soluble at body temperature.

How do organisms overcome oxygen's low solubility?

By using hemoglobin to bind and transport oxygen efficiently in blood.

Why do enzymes require water?

Water helps maintain enzyme structure and allows enzyme-substrate interactions in aqueous solutions.

How does water participate in enzyme activity?

Hydrogen bonds facilitate substrate binding to the enzyme's active site, forming enzyme-substrate complexes.

What is specific heat capacity?

The amount of energy required to raise the temperature of 1 kg of a substance by 1°C.

Why does water have a high specific heat capacity?

Because hydrogen bonds absorb a large amount of energy before breaking, resisting temperature changes.

How does water's high specific heat capacity benefit organisms?

It stabilizes aquatic habitats and internal body temperatures, maintaining optimal enzyme activity.

What are examples of animals benefitting from stable water temperatures?

Arctic species such as the ringed seal survive year-round due to minimal sea temperature fluctuation.

Why does ice float on water?

Ice is less dense than liquid water because hydrogen bonds hold molecules further apart in a lattice structure.

Why is ice floating important for life?

It insulates the water below, allowing aquatic organisms to survive in cold environments.

What is thermal conductivity?

The ability of a substance to transfer heat energy.

Why is water's thermal conductivity important?

It helps distribute heat evenly in aquatic environments, stabilizing temperatures.

What is the thermal conductivity of water compared to air?

Water's thermal conductivity is nearly 30 times higher than air's, making air a better insulator.

How do aquatic animals regulate temperature?

Seals use blubber as insulation, while diving birds trap insulating air in feathers.

What is buoyancy?

The ability of an object to float in water due to density differences.

How do loons overcome buoyancy when diving?

They have solid bones to reduce buoyancy and compress air from feathers and lungs.

How does blubber help seals?

It improves buoyancy and insulates against cold.

What is viscosity?

The resistance of a fluid to flow.

How does viscosity affect aquatic organisms?

Water's higher viscosity compared to air provides resistance, shaping aquatic animal adaptations.

What adaptations help movement in water?

Seals use flippers; loons use webbed feet and streamlined bodies to reduce drag.

Why can both common and scientific names be used in exams?

IB accepts either as long as species identification is clear.

What is the origin of water on Earth according to the HL syllabus?

Water likely came from asteroid impacts containing ice and organic materials around 4.5 billion years ago.

What are carbonaceous chondrites?

A group of ancient meteorites rich in hydrogen isotopes similar to seawater, supporting the asteroid hypothesis.

What are eucrite achondrites?

Meteorites with hydrogen isotope ratios resembling Earth's, further supporting the same hypothesis.

How did these meteorites contribute to water on Earth?

Upon impact, they released water vapour that condensed and was retained by Earth's gravity.

Why is the asteroid hypothesis favored?

It explains the isotopic similarity between meteoritic water and Earth's oceans.

What is the Goldilocks zone?

The region around a star where temperatures allow liquid water to exist—"not too hot, not too cold."

Why is the Goldilocks zone essential for life?

Only planets in this zone can maintain liquid water, a prerequisite for life as we know it.

What are exoplanets?

Planets outside our solar system, some located within their stars' Goldilocks zones.

How do scientists detect water on exoplanets?

Using transit spectroscopy to analyze starlight passing through a planet's atmosphere for water signatures.

What are signs that an exoplanet could support life?

Presence of a water signature, being in the Goldilocks zone, and being large enough to retain an atmosphere.