Chapter 12: Properties of Water Flashcards
What is the “latent heat” of a substance?
Latent heat is the energy needed to change the phase of a substance at it’s melting/boiling point. Symbol used is l, and common units are kJ mol^-1 and J kg^-1.
What is the latent heat of fusion and latent heat of vaporisation of water?
Latent heat of fusion is the amount of energy needed to change a substance from solid to liquid phase. 6.02 kJ mol^-1 of energy at 0 degrees Celsius for water.
Latent heat of vaporisation is the amount of energy needed to change a substance from liquid to gas phase. 40.7 kJ mol^-1 of energy at 100 degrees Celsius for water.
Why does water have relatively high latent heat values? Why is this property useful?
Water has strong intermolecular forces such as hydrogen bonding and dipole-dipole interactions, requiring more energy to break the bonds between the molecules.
The relatively high latent heat of vaporisation of water is useful in keeping organisms cool through perspiration. The drops of liquid water on the skin evaporate by absorbing unwanted heat energy from the body.
What is the “specific heat capacity” of a substance?
Specific heat capacity is the amount of energy needed to raise the temperature of one gram of a substance by 1 degree Celsius. Q=mc(delta T).
What is the specific heat capacity of water, and why is it useful?
The specific heat capacity of water is 4.15 J g^-1 C^-1. This is relatively high, and makes water an excellent insulator of heat.
Water is also important for climate moderation and life in the oceans, effectively acting as a buffer against large fluctuations in temperature. Thus, marine temperatures are made more stable.
What is the “density” of a substance?
Density is a measure of the amount of mass that is contained in a certain volume of that substance. It is calculated by using the formula d=m/v. Typical units are g cm^-3 and kg m^-3.
Why does ice float on water?
Water has it’s maximum density at 3.98 degrees Celsius. Water that is approaching 0 degrees, close to freezing, floats to the top as it is less dense than the surrounding water. Ice also has a significantly lower density than water.
Why is the density of ice and water useful for marine life?
Due to it’s lower density, ice stays above water, acting as an insulator, preventing the water below from freezing. Thus, aquatic life can survive even in sub-zero conditions.
Can pure water conduct electricity? Explain.
The covalent bonding in water results in neutral, uncharged molecules, but the ability of water to self-ionise results in the formation of a small number of ions. Pure water is still classified as a non-conductor, as this reaction occurs to a very slight extent.
Why does water expand when freezing?
As the water molecules slow down, they join together to form open, hexagonal crystalline lattice structures. This places them further apart than in liquid state, making it less dense.
Water is often called “the universal solvent”. What effect does hydrogen bonding have on water’s ability to dissolve other substances?
Water can form hydrogen bonds, giving it a very strong attraction to other polar molecules and ionic substances.
What is “solubility”?
Solubility is the extent to which a solute can dissolve in a solvent.
What does the “like dissolves like” rule predict?
Generally, a polar solvent dissolves a polar solute, and a non-polar solvent dissolves a non-polar solute.
Ionic substances may dissociate in water. How does this happen?
Water molecules pull ions from the ionic lattice structure, freeing them from their rigid bonds, and spread throughout to form a solution.
How is the solubility of water useful in everyday life?
The solubility of water can be useful for industrial applications (diluting chemicals, forming solutions), biological applications (water containing nutrients, minerals and hormones in cells) and domestic applications (beverages, soaps, medicine).
