SC.912.L.18.12: Discuss the special properties of water that contribute to Earth’s suitability as an environment for life: cohesive behavior, ability to moderate temperature, expansion upon freezing, and versatility as a solvent.
Water consists of just a single oxygen atom attached to two hydrogen atoms. This chemical covers about 71% of the Earth’s surface, but only 3% of that water is actually drinkable for human consumption (potable). Water is necessary for every form of life currently known to scientist and enables most of the biochemical reactions needed to sustain life as we know it. In this unit you will learn about the about the two key characteristics of water that produce it’s unique properties that allow life to exist on Earth.
2 key characteristics of water
1. Polar structure of water
Polar means that a molecule has a separation of charges, one side of the molecule tends to be slightly negative while the other side of the molecule may be slightly positively charged. Polarity occurs when the electrons of an atom gather or concentrate themselves towards one end of the molecule. The electrons in a water molecule have a tendency to get pulled towards the Oxygen atom, making that end more negatively charged and therefore polar.
Hydrogen bonds form between water molecules
In chemistry the old “adege” (an often stated observation regarding something from common experience. “the early bird gets the worm”) “opposites attract” holds very true. Positively charged atoms are attracted to negatively charged atoms this is called “Electromagnetism”. Due to the polar structure of water molecules the negative oxygen atom on one water molecule will become attracted to the positively charged hydrogen atoms of nearby water molecules. This process results in the formation of hydrogen bonds that link together many water molecules in an area. Hydrogen bonds (H – bonds) are weak enough to allow water to flow, but strong enough to create “Surface Tension”. H-bonds are present in the double helix structure of DNA and allow the nucleotides of DNA to bind together.
4 unique properties of water
1. The solid state of water is less dense than the liquid state
Density is a measurement of how much matter (atoms/particles) occupies a certain amount of space (volume). Density can be calculated by dividing the mass of a sample (m) by the volume (v) of a sample.
Most chemicals have a lower density in their liquid state than their solid state. If you compare the two figures below you will notice that the atoms are more tightly packed in the solid state, and more spaced out in the liquid state
If you compare the two figures you will notice that the atoms are more tightly packed in the solid state, and more spaced out in the liquid state. This is what happens in most chemicals, one of the few exceptions to this pattern is water.
[Insert Picture Here]
As water molecules decrease in kinetic energy/temperature they will arrange themselves to form a lattice structure called ice. The solid state of water is significantly less dense than the liquid state. The least dense form of water is the gaseous state “steam”, the molecules are not closely associated and are arranged in no particular pattern. Because the solid state of water is less dense than the liquid state, Ice can float on top of water. This property is also causes water to expand when it freezes, which is why a soda can could burst when left in the freezer. The ability of ice to float on water is pivotal for life to exist on Earth because this causes water to remain liquid in frozen lakes and portions of the ocean. Water freezing from the top and floating lets fish and other wildlife survive the cold winter months. When bodies of water are frozen at the top this can create a layer of insulation and trap some heat, allowing fish to remain warmer.
2. Water is the universal solvent
During a previous unit we discussed what a solvent, solute, and solution is. In most scenarios water can act as a solvent and break apart the molecules of other substances (solutes). This combination of solvent and solute is referred to as a “solution”. Water can act as a solvent due to its polarity, the positive end of a water molecule is attracted to the negative end of the solute while the negative end of the water molecule is attracted to the positive end of the solute. The polar structure of water allows the molecules of water to break apart a variety of substances by surrounding individual molecules of solvent.
3. Water is cohesive
Water unlike many chemicals has cohesion. In science cohesion can be defined as “molecules of the same kind being attracted to each other”, so basically water molecules are attracted to other water molecules. This is due to the formation of hydrogen bonds mentioned earlier. Cohesion is partially responsible for plants being able to pull water in through their roots and up their bodies, a process called “transpiration” that you will learn about in future sections. Cohesion is the reason water can form droplets, the molecules of water are so strongly attracted to one another that they form rounded droplets.
When molecules are attracted to a surface or an object this is called “adhesion“. An everyday example of adhesion is painting a wall or surface. When the molecules of paint stick to the molecules making up the surface this is adhesion. In a lab setting the cohesive nature of water is easily observed, when water is placed into a graduated cylinder it will form a meniscus at the surface of the water instead of laying flat.
4. Water has a high specific heat
Specific heat is defined as “the amount of energy required to raise a substance’s temperature by 1 degree”. Heat is an example of a kind of energy that can raise a substances energy. Unlike many chemicals water has a very high specific heat. This means that water has the capacity to store large amounts of heat. Water can do this because of its H-bonds. “Kinetic energy” or heat will get trapped in the spaces between the hydrogen bonds of water molecules.
Because heat can be stored between the hydrogen bonds of water molecules water can carry heat energy away from an area as it flows over, through, or nearby. This property is how the radiator of a car keeps the engine from over-heating. This is also one of the reasons why temperatures on Earth do not tend to fluctuate/change wildly in short periods of time. Places with large bodies of water nearby (like Florida) tend to have pretty consistent temperatures year round due to water’s high specific heat.
Summary
2 key characteristics
- Polar Structure (water molecules have a positive and negative end)
- Formation of Hydrogen Bonds (positive ends of water molecules are attracted to negative ends of other water molecules)
Unique properties
- 1. Less Dense when solid (ice floats and insulates)
- 2. Universal Solvent (can dissolve lots of things)
- 3. Cohesion (water molecules can stick together)
- 4. High Specific Heat (cooling properties)