IB BIOLOGY — A1.1, WATER

SOURCE: IB BIOLOGY TEXTBOOK

How much of the Earth is covered in water?

71%.

Water uses hydrogen bonds, what properties does this type of bond give it?

Cohesion/Adhesion.

What type of/Why are bonds formed between water molecules?

Hydrogen bonds, because the hydrogen atom is positively charged, and the oxygen atom is negatively charges.

What is the term for a molecule that has an uneven distribution of charge?

A polar molecule.

Cohesion: Definition and example(s).

Definition: Water molecules sticking together.

Example(s): The column of water travelling up the xylem of a plant, the Jesus bug (the bug that can wallk on water), surface tension.

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Adhesion: Definition and example(s).

Definition: When water molecules cling to other molecules.

Example(s): When water travels up the xylem, capillary action (plants).

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What are the properties of water (and their definitions)?

Specific Heat Capacity: The amount of energy needed to raise the temperature of a substance by 1°C.

Thermal Conductivity: How easily the substance can conduct heat.

Buoyancy: Allows for swimming/bobbing/floating.

Viscosity: How easy it is to move through a substance.

What is the most popular theory of the origin of water?

Due to the fact that the Earth was too hot for water when it formed, most scientists believe that water has extra-planetary origins, for example, an asteroid.

Such asteroids also contain amino acids (used for building proteins).

These asteroids are called carbonaceous chondrites, and are some of the oldest asteroids in the solar system. They contain up to 28% water, with the water having a very similar composition to ocean water.

Before the water vapour (it would have to be a gas because the Earth’s average temperature was well above the boiling point of water [100ºC] at 700ºC) condensed into liquid water after the Earth cooled down, gravity would have kept the water inside the atmosphere, rather than letting it disperse into space.

What else supports the hypothesis that Earth’s water has an extra-planetary origin?

Two 4.5 billion year old meteorites (containing liquid water) were found on Earth. The Earth’s current deuterium to protium ratio also matches that of ancient achondrites (another type of meterorite, originating from asteroid Vesta) in the asteroid belt.

What are the (three) isotopes of hydrogen?

Protium (Hydrogen-1)

Deuterium (Hydrogen-2)

Tritium (Hydrogen-3)

How can the composition of water (and by extent its possible origin) be assessed?

Using isotopes of hydrogen:

• Deuterium (Hydrogen-2) has a nucleus with one proton and one neutron, while…

• …Protium (Hydrogen-1) has only got one proton in its nucleus.

• With hydrogen isotopes that closely match Earth’s seawater, the water in these meteorites was likely to be the source of water on Earth.

What is the ‘Goldilocks Zone’?

The zone between the star and the planets that isn’t too hot, nor too cold.

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How is the water content on a distant planet measured?

We measure water content on other planets by using a technique called ‘transit spectroscopy’. As a planet passes in front of its nearest star (‘transit’), light passes through the planet’s atmosphere. This light is measured to see which wavelengths are being absorbed/deflected. This analysis shows which elements/molecules are present in the atmosphere.

In this way, planets outside our solar system (exoplanets) may be said to have a ‘water signature’.

Scientists have already discovered planets such as Kepler-186f (an Earth sized exoplanet located in the Goldilocks zone of another star).