Lecture 3: Water Activity Flashcards
What is the liquid angle of the two O-H polar covalent bonds in water?
104.5°
What is polar covalent bond? Where is it in water.
Covalent bond that has a positive and negative end
•O has larger nucleus and more protons than H, so shared electron pairs orbit more around oxygen
- Oxygen end has slightly negative charge
- Hydrogen has slightly positive charge
What is H-bond? Describe how it affects water.
Negative oxygen from next molecule somewhat attracts the positive hydrogen end of first water molecule due to polarity.
Explains a lot of characteristics of water
• Surface tension - water molecules forming H-bonds, pulling molecules on outside edge towards them
• High solubility - proteins, starches, and other sugars have a lot of OH groups, making it easy to form H-bonds with water. This helps the substance to stay distributed inside water
• Why water has a relatively high BP - Have to break all the H-bonds before water can escape and turn to gas
This is why water can dissolve so many substances in it
Much weaker than covalent bond - eg: two people holding hands vs. an arm attached to body; less energy needed to break this bond
What kind of compounds forms H-bond with water?
Proteins, starches, and other sugars - Anything with a lot of OH groups will readily form hydrogen bonds with water
Why does ice has a lower density compared to water?
B/c of H-bonding - when try to pack H2O into solid form, H wants to repel H and O wants to repel O (due to slight polarity); O and H try to form H-bonds.
• When reaches equilibrium forms unique pyramid shape
distance between oxygen molecules is smaller compared to liquid form
• However, distance between water molecules is larger than in liquid; D=M/V (mass stays the same, but volume is larger, so is less dense)
Closer O distance but is less dense
What is the O-H bond angle for ice vs liquid?
Ice: 109°
Liquid: 104.5°
What two factors influence boiling point of water? Describe their effect.
- Atmospheric pressure - Higher you go up, less pressure
• Effect on BP - water molecules need to escape from liquid surface; if have less air pressing on it, easier for water to escape → Lower boiling point
• Principle behind the pressure cooker - once water reaches boiling point, it cannot go any higher, just turns to gas; if want to cook >100°C you have to add pressure to increases bp of water
- Can also add a lid - water vapor can’t escape, so builds up pressure, increasing BP to 108C - Impurities - (Salt, sugar considered impurities) when added to water begins forming H-bonds; so now you need more energy to boil the water to break the H-bonds first → BP increases with impurities in it
What are the advantages of using steam instead of water for cooking?
A lot of flavor, color and texture compounds dissolve more readily in water than steam
Describe Free Water
- Readily separates from foods that are sliced
- Held inside cells or tiny tunnels
- Most susceptible to bacterial growth
Describe Bound Water
•Cannot be easily separated in foods
•Is part of molecular structure
- water slides between other molecules, forms H-bonds with water limiting water movement. Locked unless apply heat, acid, etc to break bond
•Cannot react with other substances as free water
•Freezes only at very low temp
•Does not evaporate
Describe a Hydrate
(eg: coffee)
Forms a stronger chemical bond (sometimes covalent), tightly locking water; Almost impossible to separate water from molecule
Describe three water pollution sources
- Biological pollutants
• Human & animal feces
• Daily waste of organic compounds - Chemical contaminants
• Acids – carbonic acids, sulfuric acid…
• Toxic compounds
• Metal ions - calcium & magnesium - Physical contaminants - garbage, diapers, cans, plastics
Describe water activity and its relation to chemical potential
The driving force that determines how matter flows (high energy → low energy until all domains reach equilibrium/same amount of energy)
- Chemical potential is movement from high energy to low energy; so if one domain has higher energy is has higher chemical potential
- Water activity is the same as chemical potential, but just specific to water
How is water activity a thermodynamic concept?
- Is the same as chemical potential, just specific to water
* It is what determines how water moves from one domain to another
What is the equation for determining water activity?
Aw = Pwater above food/Ppure water
- This is for in a closed space
- Range of Aw is always from 0-1 (1 is pure water)
