Grade 9 Biology: Properties of Water

Flashcards covering the molecular structure, polarity, hydrogen bonding, and essential properties of water (cohesion, adhesion, universal solvent, high specific heat, and density of ice) based on Grade 9 Biology Module 6.

Why is a water molecule classified as a polar molecule?

Oxygen pulls shared electrons more strongly than hydrogen, resulting in a slightly negative charge ($\delta-$) at the oxygen end and slightly positive charges ($\delta+$) at the hydrogen ends.

How does a hydrogen bond form between water molecules?

The slightly positive ($\delta+$) hydrogen end of one water molecule is attracted to and pulls on the slightly negative ($\delta-$) oxygen end of another water molecule.

Which property of water is demonstrated by the steel paper clip resting on the surface?

Surface tension, which occurs because cohesion causes water molecules to stick together via hydrogen bonds, forming a strong "skin" across the surface.

What is cohesion, and what is a real-life UAE example of it?

Cohesion is the tendency of water molecules to stick to each other through hydrogen bonds; a real-life example is water forming round beads on a car windscreen after rain in Dubai.

What is adhesion, and how does it enable capillary action?

Adhesion is the sticking of water molecules to other surfaces; along with cohesion pulling subsequent molecules along, it allows water to climb upward through narrow tubes.

How do adhesion and capillary action support plants such as the Ghaf tree?

They enable water to travel upward against gravity through narrow plant vessels from deep underground roots to the top leaves.

Why is water referred to as the universal solvent?

Water dissolves more substances than any other liquid because its charged ($\delta+$ and $\delta-$) ends surround solute particles (like salt and sugar) and pull them apart.

Why does water have a high specific heat capacity?

A large amount of thermal energy must first be absorbed to break hydrogen bonds before the water molecules can move faster and increase in temperature.

Why does the sand at Jumeirah Beach feel burning hot at noon while the sea water feels cool?

Water has a much higher specific heat capacity than sand, meaning it absorbs substantial thermal energy before heating up, keeping the sea cool.

What would happen to marine life in the Arabian Gulf if water had the same low specific heat as sand?

Water temperatures would swing rapidly between daytime highs and nighttime lows, causing extreme stress or death to fish and triggering coral bleaching.

Why is solid ice less dense than liquid water?

When water freezes, hydrogen bonds lock the molecules apart into a fixed, open lattice pattern, making ice take up more space and lowering its density.

Why does a sealed bottle completely full of water crack when placed in a freezer?

Water expands upon freezing because hydrogen bonds hold the molecules apart in an open crystal structure.

How does the floating of ice benefit aquatic ecosystems during cold weather?

Ice freezes from the top surface down and floats, creating an insulating barrier that preserves liquid water underneath where aquatic life can survive.

How does water's high heat of vaporisation assist in temperature regulation in humans?

As sweat evaporates from the skin, it absorbs and removes significant amounts of heat energy from the body, producing a cooling effect.