Food Science Chapter 9: Water
What temperature does water boil at?
100°C at normal atmospheric pressure.
What is the measure of the angle between the nuclei of the hydrogen atoms and the nucleus of the oxygen atoms?
105°
What is the measure of the angles in a perfect tetrahedron?
109.5°
The boiling point of water drops how many degrees as altitude rises?
1°C for every 293m rise in altitude.
Where does water achieve its minimum volume and maximum density?
3.98°C
Oceans, lakes, and rivers cover how much of the earth's surface?
3/4
What is the heat of vaporization of water?
540 calories per gram (540 cal/g).
What percentage water are fruits, vegetables, and meats?
70-90%
What is the heat of fusion for water?
80 calories per gram (80 cal/g).
What is sublimation?
A change from the solid phase directly into the gas phase.
What is colloidal dispersion?
A homogeneous mixture that is not a true solution.
What is electronegativity?
A measure of how strongly an atom attracts electrons in a chemical bond.
What is a polar molecule?
A molecule with a clear division of opposite electrical charges.
What does soluble mean?
A substance that can be dissolved in another substance.
What is a medium?
A substance through which something is transmitted or carried for nearly every chemical reaction that sustains life.
What is density?
A substance's mass per unit of volume.
Heat of vaporization doesn't increase what?
A substance's temperature.
What is a hydrogen bond?
An attractive force between any molecules in which hydrogen is covalently bound to a highly electronegative element.
What is surface tension?
An inward force or pull that tends to minimize the surface area of a liquid.
Underground beds that store huge amounts of water, which on occasion percolate to the surface as vapor in geysers and volcanoes are called...?
Aquifers
When does melting occur?
As heat energy enables molecules to break out of their solid structure.
When does water have a more open structure?
At temperatures below 4°C.
Four pairs of shared electrons around a central atomic nucleus arrange themselves where?
At the angles of a tetrahedron.
For pure substances, when does melting and freezing occur?
At the same temperature.
Why is water called "the universal solvent"?
Because as a polar covalent compound, it can dissolve many substances.
Why does compound CO2 sublime?
Because at normal atmospheric pressure, the vapor pressure of its solid state is greater than that of its liquid state.
Why can both phases of water (solid and liquid) be present when the temperature is 0°C?
Because freezing and melting occur at the same temperature.
Why is compound CO2 popular for refrigeration?
Because it doesn't melt, so it's never wet, and it also absorbs heat efficiently.
Why are foods put on ice to keep cold?
Because of heat of fusion.
Why will ice at 0°C keep food cold longer?
Because of latent heat.
Why is water's melting point so high?
Because the strong hydrogen bonds between molecules take considerable energy to break.
Water that is temporarily hard contains what?
Bicarbonate ions.
Boiling water over a higher heat causes what to happen?
Causes it to boil more rapidly and evaporate more quickly.
What is one of the few substances that sublimes?
Compound carbon dioxide.
The water molecule is an example of what kind of bonding?
Covalent bonding.
Bonding in water diagram.
Covalent bonds are between the hydrogen molecules.
Hydrogen bonds between molecules are much weaker than what?
Covalent bonds that hold a single molecule together.
Solid CO2 is also know as what?
Dry ice.
What happens when water dissolves ionic compounds?
Each charged end of the water molecule attracts ions of the opposite charge, pulling them apart and surrounding them with other water molecules.
What commercial method of food preservation is sublimation induced in?
Freeze-drying.
What happens when water freezes?
Heat energy releases from the ice (80 cal of heat energy per 1.0g of liquid water that freezes).
Air pressure over liquid molecules controls what?
How rapidly water molecules can escape into the air.
Oxygen is more electronegative than what element?
Hydrogen
As water freezes, what happens to the molecules?
Hydrogen bonding causes the molecules of water to separate slightly from one another.
What is one property of a solid?
It can be compressed only slightly.
What does boiling do to bicarbonates?
It changes them to insoluble carbonates.
What happens when steam contacts your skin?
It condenses because your body temperature is well below the boiling point of water.
As altitude increases, what happens to the atmospheric pressure?
It drops.
Why does ice float in cold drinks, and icebergs drift in oceans?
It is 10% less dense than water
How can permanently hard water remove the calcium or magnesium ions?
It must pass through an ion exchange filter or undergo chemical treatment.
As the steam that contacts your skin condenses, what happens?
It releases heat of vaporization.
What does it mean when the heat of vaporization of water is 540 cal/g?
It takes 540 calories to change 1.0g of water from liquid to steam.
Why can water as a solvent be a problem?
It's also effective with some vitamins and minerals in foods. The nutrients may dissolve in cooking water and be lost during food preparation.
What do the properties that make water valuable to everything that lives come from?
Its molecular makeup.
What is one way that detergents clean clothes?
Lowering surface tension.
The oxygen end of the molecule is what charge?
Negative.
How can water put out fires if hydrogen and oxygen together are flammable?
Once hydrogen and oxygen have combined to form water, they no longer have a need to react.
Bound water freezes at...?
Only very low temperatures and it doesn't evaporate.
Hydrogen is highly flammable, reacting with what element in the air in order to burn?
Oxygen.
The hydrogen end of the molecule is what charge?
Positive
What can free water do that bound water can't?
React with other substances.
What is free water?
Readily separates from foods that are sliced, diced, or dried.
What does latent heat do during a phase change?
Rearranges molecules.
Water that doesn't contain calcium or magnesium ions is considered what?
Soft.
What does it mean when water's heat of fusion is 80 cal/g?
That it takes 80 calories of heat energy to change 1.0 gram of water from solid to liquid.
What is the heat of vaporization?
The amount of heat needed to change 1.0g of a substance from the liquid phase to the gas phase.
As a liquid cools, what happens?
The atoms or molecules become more compact
What happens to the boiling point of water when atmospheric pressure lowers?
The boiling point lowers.
As oxygen exerts a stronger pull on shared electrons, what happens?
The electrons are drawn closer to the oxygen atom.
What is heat of fusion?
The energy needed to change 1.0g of a substance from solid to liquid phase.
What is latent heat?
The energy required to cause a phase change without a change in temperature.
Why is steam at 100°C more dangerous than boiling water at 100°C?
The heat absorbed when a liquid turns to gas is released when the gas condenses to a liquid.
Which will keep food cold longer in an ice chest? Water at 0°C or ice at 0°C?
The ice.
What is solubility?
The maximum amount of solute that can dissolve in a given quantity of solvent at a specific temperature.
The arrangement of the electrons affects what?
The molecule's shape
What happens when water turns to ice at 0°C?
The molecules arrange into a solid that is 10% less dense than its liquid.
The properties of oxygen and hydrogen affect what?
The nature of the bond.
What happens when sodium chloride (NaCl=table salt) is mixed with water?
The negative oxygen ends of the water molecules attract the positive sodium ions, while the positive hydrogen ends attract negative chloride ions.
When the metal ions bond to bicarbonate ions, what is the result?
The resulting compound dissolves in the water.
The surface tension of water directly results from what?
The strength of the hydrogen bonds between water molecules.
What is a solvent?
The substance that dissolves another substance in a solution.
What is a solute?
The substance that is dissolved in a solution.
What is the melting point?
The temperature at which a substance changes from a solid to a liquid.
What is a liquids boiling point?
The temperature at which its vapor pressure equals the air pressure above the liquid.
What do the electrons do in covalent compounds where fewer than four atoms are bonded to a central atom?
The unshared pairs of electrons take up more space than the shared electrons pairs, forcing the shared electron pairs closer together.
How is bound water held so tightly?
The various chemical groups in a food's molecules.
What happens if the metal ions are present as compounds called sulfates?
The water is considered permanently hard.
What happens to the molecules on the surface of a body of water?
They experience a pull from below and to sides, but not from above.
How do detergents lower the surface tension?
They interfere with the formation of hydrogen bonds.
What do electrons do to one another?
They repel each other because of their negative charge.
What do the insoluble carbonates do?
They settle to the bottom of the container.
What do the electron pairs do in a water molecule?
They're driven closer together
What is the water molecule made of?
Two hydrogen atoms that share electrons with one oxygen atom.
What is bound water?
Water that cannot be easily separated in food.
What is hard water?
Water that contains calcium or magnesium ions.
Hard water interferes with what?
Water's tenderizing effect.
When can vaporization only occur?
When molecules gain enough energy to exert a force equal to the force of the air pressure over the liquid.
What is freezer burn?
When sublimation occurs in the ice in food that has been kept too long in the freezer and it causes a grayish, off tasting state.
As the 100°C water cools on your skin, what happens?
You absorb more heat, which intensifies the burn.
