Hydrogen Bonding In Water
● H2O is bonded with covalent bonds (two non metals)
● Hydrogen atoms are at one end of the molecule
● Oxygen is electronegative - “hogs electrons” which causes a partial negative charge
on the non-hydrogen side
● Hydrogen bond between partial negative and partial positive ends of water molecules
This structure makes water able to have the properties that it does, such as: being solvent,
cohesion and adhesion, specific heat and density.
Water is classed as a polar molecule because of its polar covalent bonds and its bent shape.
● A polar molecule that interacts with water is hydrophilic
● Non polar molecules do not interact with water (Such as fats, oils etc) and they are
hydrophobic
Capillary Action
Water is more attracted to a glass beaker than itself because glass is usually made from a
silicon oxide lattice - which is a polar molecule.
The water meeting the glass is called adhesion.
Water sticking to itself is called cohesion.
Surface Tension
Molecules at the surface don’t have anything pulling them from above, but do from every
other direction, because of this, they are able to get more densely packed - resulting in a
stronger intermolecular force.
Cohesion and Adhesion
Cohesion refers to the attraction of molecules for other molecules of the same kind, and
water has strong cohesive bonds because of their ability to form hydrogen bonds with one
another. Cohesive forces are responsible for surface tension.
Adhesion is the attraction of molecules of one kind of molecules for a different kind of
molecules. A good example of adhesion is water inside a capillary glass, because glass has
stronger polar bonds than water, the water is more attracted to glass than to itself, meaning
a concave meniscus is formed.
Water as a Solvent
Solvent - other molecules can be dissolved in water.
The cytoplasm - which is mostly made of water - allows many reactions to occur between
different types of molecules.
Water’s polarity makes it a good solvent.
If something is an ion, it is very easy to dissolve in water.
, Solute - the substance that’s being dissolved.
Water: Solid, Liquid and Gas
In liquid water, bonds are constantly being formed and broken as water molecules slide past
each other. The breaking of these bonds is caused by kinetic energy due to the heat in the
system.
When heat is raised, kinetic energy increases - which causes the bonds to be broken
completely, allowing the water molecules to escape into the air as gas.
However, when temperature decreases, water molecules form a crystal structure,
maintained by hydrogen bonding. This structure makes ice less dense than liquid water.
Key Terms
Term Meaning
Polar Molecule A neutral molecule that has asymmetric internal distribution of
charge, leading to partially positive and partially negative regions.
Cohesion The attraction of molecules of the same kind
Adhesion The attraction of molecules of a different kind
Density The mass per unit volume of a substance
Specific Heat The amount of heat needed to raise 1g of a material by 1 degree
Capacity celsius
Heat of The amount of energy needed to change one gram of a liquid
vaporisation substance to a gas at constant temperature.
Properties of water:
1. Water is polar
2. Water is an excellent solvent
3. Water has a high specific heat capacity
4. Water has high heat of vaporisation
5. Water has cohesive and adhesive properties
6. Water is less dense than a solid than as a liquid
Elements and Atoms
Protons - Defines the element (atomic number), + charge.
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