Water

A molecule with an **uneven spread of charge** — a slightly negative end and a slightly positive end.

A bond in which two atoms **share a pair of electrons**.

How strongly an atom **pulls shared electrons** toward itself.

**Oxygen** — it is more electronegative, so it pulls the shared electrons closer.

The **two hydrogen** atoms.

A **partial** (slight) charge — much smaller than a full ionic charge.

About **104.5°** — the molecule is **bent**.

Its **bent shape** means the partial charges **do not cancel** — they act on the same side.

A **polar covalent** bond (electrons shared, but unequally).

It lets water molecules attract each other and other charged particles — the basis of water's life-supporting properties.

A **weak attraction** between a **δ+ hydrogen** of one water molecule and a **δ− oxygen** of another molecule.

**Between** separate water molecules (a covalent bond acts **within** one molecule).

A **δ+ hydrogen** of one molecule and a **δ− oxygen** of a neighbouring molecule.

Because each molecule is **polar** — it has a δ− oxygen and δ+ hydrogens.

**Weak** — much weaker than a covalent bond.

Because there are a **huge number** of them, so a **lot of energy** is needed to separate the water molecules.

A **strong** bond **within** a molecule, where atoms **share a pair of electrons** (the O–H bonds in water).

As a **dashed line** from a δ+ hydrogen of one molecule to the δ− oxygen of another, labelled **'hydrogen bond'**.

Water forms **many hydrogen bonds** that hold its molecules together; methane is **non-polar** and forms none.

High **boiling point**, **cohesion** (and a high **heat capacity**) — any two.

Yes — each molecule can hydrogen-bond to **several** neighbours at once.

A **partial** (slight) charge — much smaller than a full ionic charge.

The attraction of water molecules to **other water molecules** (water sticking to itself).

The attraction of water molecules to a **different surface** (water sticking to something else).

A 'skin-like' effect at the water surface, caused by **cohesion** pulling the surface molecules together.

**Hydrogen bonding** between water molecules, which happens because water is **polar**.

**Surface tension** (produced by **cohesion**).

**Adhesion** — water sticking to a different surface.

It holds water in an **unbroken column** in the xylem, so water can be pulled up from roots to leaves.

Water sticks to the **walls** of the narrow xylem vessels, helping support and lift the water column.

Cohesion + adhesion together let an unbroken water column be **pulled up the xylem** from roots to leaves.

**Adhesion** — the water sticks to the leaf surface.

**Cohesion** — water molecules pulling on each other at the surface.

Each bond is weak, but there are **so many** of them that together they hold the molecules tightly.

The amount of **heat needed to change the temperature** of a substance. Water's is **high**, so its temperature changes slowly.

Heating it must **break/stretch many hydrogen bonds** between the polar molecules, which takes a lot of energy.

The **heat needed to turn a liquid into vapour**. Water's is high, so evaporation removes a lot of heat.

Cooling caused because **evaporating water carries heat away** from the surface left behind (e.g. sweating).

Evaporating the sweat **takes heat from the skin** (high heat of vaporisation), lowering body temperature.

How well a material **lets heat pass through it**. Water conducts heat well, so it draws heat from a warm body.

Water's **high thermal conductivity** makes it lose body heat fast; **blubber insulates** against this.

Water's **high specific heat capacity** means its temperature changes only slowly, day and night.

The **hydrogen bonds** between polar water molecules — breaking them costs energy.

**Water changes far less** (high specific heat capacity); **air swings more widely**.

Water's **high specific heat capacity** — the sea releases a lot of heat for only a small temperature drop.

High **specific heat capacity**, high **heat of vaporisation**, and high **thermal conductivity**.

A liquid that **dissolves** another substance to form a solution. In cells the solvent is **water**.

A substance that **dissolves** in a solvent.

It is **polar** — its δ+ and δ− ends are attracted to charged/polar particles and surround them, pulling them into solution.

**Polar** molecules (e.g. glucose, amino acids) and **ionic** substances (salts, mineral ions).

**Non-polar** substances such as **fats and oils** (no charged parts for water to grip).

“Water-loving” — a **polar/charged** substance that **dissolves** in water.

“Water-fearing” — a **non-polar** substance that does **not** dissolve in water.

**Metabolism** (reactions between dissolved solutes) and **transport** (carrying dissolved substances).

In **xylem** (water + dissolved minerals) and **phloem** (dissolved sugars).

It is taken up by plants as **ions dissolved in soil water**, then passed to **animals that eat the plants**.

Low soil copper → grass takes up **little copper** → the animal eats that grass and takes in **too little copper**.

Roots absorb minerals as **ions in solution**, so the mineral must first **dissolve** in the soil water.