Transcript Properties of Water

Properties of Water
FS Unit 5
HUM-FS-6 Explain why water and acidity are important factors in
food preparation and preservation.
Properties of Water
 Chemical Description of Water
- One atom of oxygen bound to two atoms of
hydrogen make up water.
 Polarity – uneven distribution of charge
 Because the hydrogen atoms are attached to one side
(positive charge) and the oxygen atom is on the other
side (negative charge)
Properties of Water
 Adhesion – the attraction of the water
molecules to other materials. (Water’s
attraction to materials other than itself )
LET’S TRY IT
Properties of Water
 Cohesion – attraction between water molecules.
(Water’s attraction to itself )
 This is why water is kind of “sticky”
 With the molecules attracting to each other they tend to
clump together. That’s why they are drops.
 If it wasn’t for earth’s forces like gravity a drop of water would be
a perfect SPHERE.
LET’S TRY IT
Properties of Water
 Capillary Action – water molecules being
attracted to molecules of other materials
as well as themselves.
 Adhesion and Cohesion are working together
WATCH IT IN ACTION
Properties of Water
 Surface Tension – a tension is created at the
surface as the water molecules at the surface
are pulled into the body of water.
 Acts as the water’s skins
 Is strong and can support a paper clip or needle
LET’S TRY IT
Properties of Water
 In the cleaning process you have to reduce surface
tension so that water can spread and wet the surface.
 Chemicals called surface active agents or Surfactants make a surface “wetter”.
 Surfactants functions can also loosen, disperse water
and hold soil in suspension until it can be rinsed.
 Surfactants can provide alkalinity which can remove
acidic soils.
Properties of Water
 Water is the “Universal Solvent”.
 It dissolves more substance than any other liquid
 Wherever water goes (through the ground
or our body) it takes along valuable
chemicals, minerals and nutrients.
Properties of Water
 Water is the only natural substance that is
found in all states
 Liquid
 Solid (ice)
 Gas (steam)
 Water Freezes at 32 degrees Fahrenheit
 Water Boils at 212 degrees Fahrenheit
Atmospheric Pressure
 Force of the weight of gases in the air pressing down
on a surface.
 Measured by the average pressure on an object at sea
level.
 Similar to the pressure that water puts on your body
when you dive into the pool
 Gases in the air are pressing against your body all the
time.
Boiling Point
 The boiling point of a substance is the temperature at
which its vapor pressure equals the air pressure above the
liquid.
 When water reaches its boiling point, water molecules in the
liquid are escaping into the air as steam.
 As the atmospheric pressure changes, the boiling point of
water changes.
 Less AP on the water, molecules need less energy to escape.
 More AP on the water, molecules need more energy to
escape.
 Temperature is a measure of energy.
 Heat of Vaporization is the amount of heat needed to change
1.0g of a substance from a liquid to a gas.
Function of water in
Food Preparation
 Two main functions in the preparation
of foods...
1. Water is an important medium for
transferring heat.
2. Water is a necessary ingredient in forming
many food mixtures.
Heat Medium
 Water in both liquid and gaseous stages is used to
transfer heat into foods.
 When water is heated, energy is transferred from the
heat source by both conduction and convection.
 The slower/lower temp you cook the food on the more
even the heat distribution will be.
 Boiling certain food too high can affect the texture.
 Ex: pasta needs a higher temp to boil and potatoes
need a lower temp (they can get mushy from banging
around in the pan)
Melting Point
 The melting point is the temperature at which a
substance changes from a solid to a liquid.
 Melting occurs as heat energy enables molecules to
breakout of their solid structure.
 For pure substances melting and freezing occur at the
same temperature.
Water as a Solvent
 Solvent – the substance that dissolves another
substance.
 Solute – a substance that can be dissolved into
another substance.
 As a polar covalent compound water can dissolve
many substances. Remember it’s the universal
solvent.
Water as a Solvent
Examples…
 When sodium chloride (table salt) is mixed with water
the negative oxygen ends of water molecules attract
the positive sodium ions, while the positive hydrogen
ends attract negative chloride ions. (Food Science, chapter 9, figure 9-8)
 Sugar dissolves in the free water of strawberries to
form sweet syrup.
 Water dissolves the flavor molecules in tea leaves and
coffee beans to create our beverages.
Water as a Disperser
 Colloidal Dispersion (kuh-LOYD-ul dis-PUR-zhun)Homogeneous mixture that is not a true solution.
 Contain relatively large solute particles.
 Particles are called Colliods (KAHL-oyds) (Food Science, Ch 9, pg 139)
 Not large enough to settle out of the colloidal
Dispersion.
 Scattered or dispersed in a solvent.
Water as a Disperser
 Emulsion– a mixture of two liquids containing
droplets that don’t normally blend with each other.
 Ex: Oil and water
 Immiscible (ih-MISS-suh-fy-ur) – Liquids that don’t
blend or mix.
 Emulsifier – a substance composed of large
molecules that are polar at one end of nonpolar at
the other. (Simply “glues” the two types of molecules together)
Water in Food
 Water Content – the amount of water that is in a
food.
 Ex: Many fruits and vegetables are made up of over 70%
water
 Water Activity – the measure of a the particular water
pressure over a food.
 Takes into account the water available to support
activity of enzymes and growth of microorganisms.
Water Content
 Free Water – readily separates from foods that are
sliced, diced and dried.
 Ex: Bread to toast heat evaporates the free water.
Grapefruit squirts when you cut it open.
 Bound Water – water that cannot be easily separated
in food.
 Ex: Proteins and starches
 It can’t react with other substances as free water can.
 Freezes only at VERY low temperatures & doesn’t
evaporate.
 Ex: adding dry gelatin to water. Within a few minutes the
water is unable to flow and is bound.
Water & Food Preservation
 Water content alone will not predict a food
perishability.
 Both carrots and milk are 88% water…
 A carrot is safe in a lunch bag all day because it has
starch molecules that attach to the water.
 Milk doesn’t have the same insulation and will go bad if
let in a lunch bag.