AP Biology - Chemistry of Life - Unit I

Define 'functional groups'

'A functional group is defined as an atom or group of atoms within a molecule that has similar chemical properties whenever it appears in various compounds.' (Chemistry - LibreTexts)

Define 'macromolecule'

'A molecule containing a very large number of atoms, such as a protein, nucleic acid, or synthetic polymer.' Most macromolecules (carbon backbone molecules) are not classified as hydrocarbons because they contain other atoms in addition to carbon + hydrogen, such as nitrogen, oxygen and phosphorus. Carbon chains with attached hydrogens are a key structural component of most macromolecules.

What does water's lighter density as a solid mean for nature?

- Ice floats on the surface of liquid water - Layer of ice forms on top of the liquid water in lakes/ponds, which creates an insulating barrier that protects the animals + plant life in the pond below from freezing

Give three examples of the surface tension of water.

1) A water droplet's shape; it is a rough circle, because the water molecules on the surface are more attracted to each other than to the surrounding air. 2) Some insects are able to walk on the surface of water. 3) A cup of water filled all the way to the brim; the water does not overflow, but it does form a type of bulge, because the water molecules on the surface are more attracted to each other than the air.

Give 2 examples of water's abilities because of its high heat capacity:

1) Land cools faster than the sea once the sun goes down; slow-cooling water can release heat to nearby land during the night 2) Water is used by warm-blooded animals to distribute heat through their bodies.

What are the two regions of an atom?

1) Tiny atomic nucleus: The center of the atom; contains positively charged particles called protons + neutral, uncharged particles called neutrons 2) 'Cloud' of electrons: Much larger region, negatively charged particles that orbit around the nucleus. The attraction between the two regions - positively charged protons + negatively charged electrons - is what holds the atom together.

Name all of the unique properties of water.

1) Water is polar - partial positive charge on the hydrogen, partial negative charge on the oxygen, bent overall structure. 2) Water is an excellent solvent - can dissolve many polar and ionic substances, which is important to all living things; as water travels through the water cycle, it takes many nutrients with it. 3) Water has a high heat capacity - takes a lot of energy to raise the temp of a certain amount of water by a degree; helps with regulating temp in the environment. 4) Water has a high heat of vaporization - takes a lot of energy needed to change one gram of water to gas at a constant temp; this property is used by animals + humans to cool off, because sweat is made of mostly water, the evaporating water absorbs heat + releases into into the atmosphere, this is known as evaporative cooling. 5) Water has cohesive + adhesive properties - this is because of water's ability to form hydrogen bonds with one another. This forces are responsible for surface tension + capillary action, which allows nutrients to be transported to the top of a tree against gravity. 6) Water is less dense as a solid than as a liquid - water freezes, molecules form a crystalline structure that spaces the molecules further apart than in liquid water, so ice is less dense than water. This keeps ponds + oceans from freezing solid + allows life to continue under the icy surface.

What is the percentage of oxygen and carbon in the average human being?

65% oxygen by mass (this is because of all of the water in the human body + the fact that oxygen forms the bulk of the water molecule) 18% carbon

Define a hydrocarbon, and give four examples.

A hydrocarbon is an organic chemical compound composed of only hydrogen and carbon. Two examples are methane, butane, propane, and hexane. Hydrocarbons make good fuels because their covalent bonds store a lot of energy, which is released when molecules are burned.

What is the cause for a concave meniscus?

A meniscus occurs within a glass of water because the water is, in fact, more attracted to the glass than it is to itself. The molecules in the glass are polar, typically made of silicon oxide lattice, and for every 1 silicon atom, there are 2 oxygen atoms. Because of this, the electronegativity difference between oxygen and silicon is higher than that of oxygen and hydrogen. With this, silicon is less electronegative than hydrogen, meaning it's not as 'hoggy' of electrons. Oxygen, on the other hand, can still 'hog' electrons. The hydrogen atom's partially positive end would be attracted to the partially negative ends of the oxygens in the glass - this is a stronger partial charge than water sticking to itself, as there's a larger electronegativity difference between silicon + hydrogen in a glass than oxygen + hydrogen in water. Water is sticking to the glass, called adhesion, causing a meniscus.

Define 'non-polar'

A molecule in which the electrons are shared equally between the nuclei = the distribution of charge is even + the force of attracted between different molecules is small; non-polar molecules show little reacting.

Define organic molecules

A molecule that contains carbon

What is a nonpolar covalent bond?

A nonpolar covalent bond is a type of chemical bond where two atoms share a pair of electrons with each other; electrons are halfway between the two atoms, and they're shared equally.

What is a polar covalent bond?

A polar covalent bond is a pair of electrons that are shared UNEQUALLY between two atoms. This is shown in a water molecule, where the electrons are not shared equally because oxygen is more electronegative than hydrogen, so partial charges exist on the O and H atoms.

How much to protons, neutrons and electrons way in atomic mass units (amu)

A single neutron or proton has a weight close to 1 amu, while electrons are smaller in mass at 1/1800 of an amu. Because of this, electrons do not contribute much to an atoms mass, but they do contribute to an atom's charge.

Why is the water molecule a polar molecule?

A water molecule consists of two hydrogen atoms bonded to one oxygen atom. This structure is known as a BENT structure because the oxygen atom carries two pairs of unshared electrons, and all pairs of electrons - paired or unpaired - repel each other, forming a bent shape. The water molecule reaches a STABLE arrangement when these pairs are far away from each other, forming a tetrahedron. Specifically, because this molecule contains lone pairs, and lone pairs are more repulsive than bonded electrons, the angle between the O-H bonds is less than 109 degrees, which is a perfect tetrahedron, at 104.5 degrees. Because of oxygen's electronegativity and therefore partial positive and negative charges, its bent shape and polar covalent bond, it is a polar molecule. This polarity keeps water in its liquid state and gives water cohesiveness.

How is a water molecule formed?

A water molecule is comprised of one oxygen atom and two hydrogen atoms. The oxygen atom has two electrons in its first shell, meaning that shell is full. In the oxygen's second shell, there are two PAIRS of electrons and two UNPAIRED electrons, meaning this shell only holds 6 electrons, when it can hold a maximum of 8. Hydrogen atoms, on the other hand, only have one electron in their first and only shell. Therefore, the lone electron from one hydrogen atom bonds with one of the lone electrons from the oxygen. The remaining lone electron from the oxygen bonds with another lone electron from a hydrogen. This forms two covalent bonds, as the electron pairs formed by the two hydrogen atoms + one oxygen atom are shared to fill oxygen's valence shell with 8 electrons and the hydrogens' valence shell with two, individually. A water molecule is formed.

What does adhesion enable?

Adhesion enables water to 'climb' upwards through thin glass tubes (capillary tubes) placed in a beaker of water. This is known as capillary action, or the upward motion against gravity.

How does adhesion relate to the importance of water for life?

Adhesion is the property of water that allows it to adhere to other 'things'. This property allows water to transfer from the roots of trees to the top of a tree. It is also present within our capillary vessels, which involves capillary action to transfer blood throughout our body due to the cohesion and adhesion of water.

What is the main universal rule of matter?

All matter and elements, along with the chemical reactions between them, obey the same chemical and physical laws, regardless of whether they are apart of the living or nonliving world.

What is in the OH functional group?

An OH functional group is composed of a carbon backbone, a hydroxyl group (this is polar and can be dissolved in water)

Define 'ion'

An ion is an atom or group of atoms (molecule) that carries a positive or negative charge as a result of having lost or gained one or more electrons. It is also chemically stable, meaning it has two or eight electrons in its outer shell.

What does evaporative cooling mean?

As water molecules evaporate and the surface they evaporate from gets cooler, this process occurs. This is because the molecules with the highest kinetic energy are lost to evaporation. In humans and other organisms, the evaporation of sweat (99% water) cools the body to maintain a steady temperature.

What is the octet rule?

Atoms tend to be more stable when they at least pretend that they're sharing or have 8 electrons in their outermost shell.

Why is water a good solvent?

Because of its polarity. This positive/negative bond is also shown in sodium chloride (NaCl). Sodium and chloride are attracted by ionic bonds, as sodium has a positive charge (it is stripped of an electron), and chloride has a negative charge (it gains an extra electron). This positive/negative bond is called an ionic bond, and it makes molecules good solvents.

Why is water polar?

Because of the distribution of electrons between the covalently bonded atoms is not even - this is because the oxygen atom is more electronegative, so it 'hogs' more electrons than the hydrogen.

Describe the hydrogen bond that forms between water molecule:

Because of the partial negative charge at the non-hydrogen end of the water molecule, and the partial positive charge on the other end of the molecule - at the hydrogen end - one water molecule's partially negative side is attracted to another water molecule's partially positive side (POLARITY). This attraction between opposites is a hydrogen bond. These hydrogen bonds are key to the behavior of water.

Describe capillary action, an example of it and why it occurs.

Capillary action is the ability of a liquid to flow in an upward motion without the assistance of, or even in opposition to, external forces such as gravity. An example of capillary action is the ability of water to 'climb' upwards through thin glass tubes that are placed in a beaker of water. This occurs and is dependent on the attraction between water molecules and the glass walls of a tube (adhesion) and the interactions between water molecules (cohesion). The water molecules are more strongly attracted to the glass than the other water molecules because glass molecules are more polar than water molecules.

How does carbon and the octet rule relate?

Carbon can maintain the octet rule by forming 4 covalent bonds. 4 covalent bonds can be formed by a carbon atom and 4 hydrogen atoms; this carbon has 4 electrons in its outermost shell, and pairing with 4 hydrogen atoms (each of which have one electron in their valence shell), the carbon now has 8 electrons in its valence shell, and each hydrogen has 2 electrons in their valence shells, making them stable. This molecule is known as methane, and because it has carbon, it is known as an organic molecule (because this only has carbon + hydrogen in it, it is also considered to be a hydrocarbon).

Why is carbon so popular for making molecular backbones?

Carbon-carbon bonds are unusually strong, so carbon can form a stable + sturdy backbone for a large molecule. A carbon atom can form covalent bonds to up to 4 other atoms, it's well suited to form the basic skeleton/backbone of a macromolecule. This quality stems from its number + configuration of electrons (c atom has an atomic number of 6, which is 6 protons and 6 electrons. Carbon also wants to achieve stability; it must find four more electrons to fill its outer shell, thus, they may form bonds to as many as 4 other atoms.)

How does cohesion relate to the importance of water for life?

Cohesion - the property allowing for water molecules to be attracted to other water molecules. When looking at a macroscale, you'll see water droplets form, though they couldn't form without cohesion (one drop can be an environment where thousands of microorganisms can live).

Why are cohesive + adhesive forces important for life?

Cohesive and adhesive forces are important for life as they play a role in many water-based processes in biology, such as the movement of water to the tops of trees + their leaves, and the drainage of tears from tear ducts in the corners of eyes.

Compare and contrast protons and neutrons

Compare: Both subatomic particles have the same mass at ~ 1.67 x 10^-24 (alternative unit of measure is the dalton or atomic mass unit (amu))

What is electronegativity and how does it relate to the water molecule?

Electronegativity is a measure of an atom's ability to attract the shared electrons of a covalent bond to itself. Electrons participating in a covalent bond are not always shared equally between the atoms involved, as some atoms hold electrons in covalent bonds more tightly. These atoms - that hold electrons in covalent bonds more tightly - are electronegative. More simply, an electronegative atom essentially 'hogs' electrons. In regards to the water molecule, the oxygen atom likes to keep electrons more around itself than the partners it is bonding with. This means that in a water molecule, the electrons from the covalent bond from the hydrogen atoms spend more time on the side without the hydrogen. This, therefore, creates a partial negative charge at the non-hydrogen end, symbolized by: δ-

How do electrons move in an atom?

Electrons are constantly moving/jumping/buzzing around the nucleus of the atom. The electrons are 'bound' to this nucleus because the protons have a positive charge and the electrons have a negative charge, so they attract each other. Electrons have a VERY high velocity.

Define cohesion.

First definition: When molecules are more attracted to each other than their surroundings, so they stick to each other. Second definition: The attraction of molecules for other molecules of the same kind. Water molecules have strong cohesive forces thanks to their ability to form hydrogen bonds with one another.

How many electrons can the first, second and third shell of an atom hold?

First shell: 2 electrons Second shell: 8 electrons Third shell: 18 electrons

How can carbon atoms form a tetrahedral shape?

Generally, when a carbon atom is bonded to four other atoms, the molecule, or part of one, will take on a tetrahedral shape that's similar to methane. Electron pairs that make up the bonds repel each other, and the shape that maximizes their distance from each other is a tetrahedron.

What makes an atom neutral?

Having an equal number of protons and electrons

What does the structure of a liquid look like in regards to hydrogen bonds?

Hydrogen bonds are constantly being formed + broken as the water molecules slide past each other. This is caused by the energy of motion (kinetic energy) of the water molecules due to the heat in the system.

Describe hydrogen bonds in general, not necessarily in relation to the water molecule.

Hydrogen bonds are weak interactions that form between a HYDROGEN with a partial POSITIVE charge and a more electronegative atom (meaning it must have a more negative charge), such as oxygen. Hydrogen atoms involved in bonding must be attached to electronegative atoms, such as oxygen, nitrogen, or fluorine.

Describe hydrogen bonds and what they mean for water's properties.

Hydrogen bonds are weaker than covalent bonds - they are formed by the weak attraction between the negative end of one water molecule and the positive end of another. Though they are weaker than covalent bonds, they are still strong enough to give water its 'fluid nature'. Hydrogen bonds allow the two opposite ends of water molecules to be attracted to each other but then break apart and reform. They are key for water's ability to flow past itself. They are also key for its properties that are: the ability to take in heat, the ability to regulate temperature, and water's properties regarding evaporation cooling, surface tension and cohesion. Most importantly, it seems, hydrogen bonds result in water's POLARITY, which makes water a solvent + allows it to dissolve polar molecules in itself.

Other than a glass tube flipped upside-down in a beaker of water, give an example of capillary action.

Imagine spilling water, and using a paper towel to clean it up. If you dip one tip of the paper towel into the puddle, you'll notice the water climbing up the towel. This is an example of capillary action, as the water is attracted to the molecules that make up the paper towel.

Give an example that displays water's attraction to glass.

In a glass of water, a meniscus forms. Known as, "the curved upper surface of a liquid in a tube,", the meniscus forms as a result of water's attraction to glass molecules MORE THAN water's attraction to water itself, because glass molecules are MORE POLAR than water molecules.

What are the polar covalent bonds in a water molecule?

In a water molecule, because of oxygen's electronegativity, the electrons are not shared equally, as the oxygen atom 'hogs' the electrons. This means that partial charges exist on the hydrogen and oxygen atoms. As oxygen 'hogs' the electrons, the oxygen side of the water molecule has a partial negative charge. Because the hydrogen atoms do not 'hog' the electrons, the hydrogen side has a partial positive charge. A polar covalent bond exists when atoms with different electronegatives share electrons in a covalent bond, as shown here, by the bond between the oxygen and hydrogens.

Why is water's lowest density in its solid form?

In ice, the water molecules are pushed farther apart than in liquid water, which means water expands when it freezes. Due to the way hydrogen bonds are oriented as it freezes, water's lowest density is in its solid form. With MOST other liquids, solidification (occurs when the temperature drops + kinetic energy of molecules is reduced) allows molecules to pack more tightly than in liquid form, giving the solid a greater density than the liquid. Water is the OPPOSITE, so it's an anomaly (weird standout) as it has a lower density as a solid.

Define hydrophilic

Interacting readily with water, typically polar compounds containing partially or fully charged atoms.

What does 'water has a specific heat capacity' mean?

It is defined as the amount of heat needed to raise the temperature of one gram of a substance by one degree celsius. The amount of heat needed to raise the temperature of one gram of water by 1 degree celsius has its own name, the calorie.

What does water being a solvent mean?

It means that it is 'easy for certain things' to be dissolved in water.

What does 'water has a high heat capacity' mean?

It takes a lot of heat to increase the temperature of liquid water because some of the heat must be used to break hydrogen bonds between the molecules.

What does 'water has a high heat of vaporization' mean?

It takes an unusual amount of heat to VAPORIZE a given amount of water, because hydrogen bonds must be BROKEN in order for the molecules to fly off as a gas. Water's heat of vaporization is around 540 calories/gram at 100 degrees celsius, water's boiling point. Some molecules of water that happen to have a high kinetic energy will escape from the surface of the water even at lower temperatures.

What is carbon?

Life is carbon-based - it is extremely valuable for life, and it forms the backbone of the molecules that make life possible. It is formed from 6 protons (the most common isotope is carbon-12, with 6 neutrons). Fun fact: it can form in bonds in 3-dimensional shapes, specifically the tetrahedron.

Why does water's high heat capacity matter for life?

Many life forms can only operate within a certain range of temperatures; if it was easy to raise/lower the temperatures of water, it would be harder for life to operate in water.

What is the simplest hydrocarbon molecule?

Methane; the carbon + four hydrogen atoms form the vertices of a tetrahedron, so it's said to have a tetrahedral geometry.

Define 'atom'

Most basic or 'broken down' unit of any of these elements. - The most broken down an element can be before it's no longer that element - There are 1 million carbon atoms across the width of a human hair.

If most of an atom is empty space, why don't solid objects pass through each other?

Negatively charged electron clouds of atoms will repel each other if they get too close together, resulting in our perception of solidity.

Define a polar molecule.

Neutral/uncharged molecule that has an asymmetric (not symmetric) internal distribution of charge, leading to partially positive and partially negative regions.

Are protons, electrons and neutrons very large?

No, they are very small; most of the volume in an atom (99%+) is empty space.

Define hydrophobic

Not readily interacting with water, typically nonpolar compounds that lack partially or fully charged atoms.

What are the four elements common to all living organisms?

Oxygen (O), Carbon (C), Hydrogen (H), and Nitrogen (N). These make up 96% of the human body.

What are a few important molecules in the human body that carbon makes the backbones of?

Proteins, DNA, RNA, sugars and fats. These molecules are called macromolecules, classified as organic molecules, meaning they contain carbon atoms. (There are a few exceptions: carbon dioxide + carbon monoxide contain carbon but usually won't be considered organic)

Most atoms contain what three types of subatomic particles?

Protons, neutrons + electrons. Hydrogen is an exception because it typically has 1 proton and one electron but no neutrons.

What do protons + electrons define in an atom?

Protons: # of protons in a nucleus determines which element an atom is Electrons: # of electrons surrounding the nucleus determines which kind of reactions the atom will undergo.

How much energy does it take for water to go from its liquid form to its gas form?

Proven valuable in many life forms for a form of cooling - evaporative cooling - where heat can be taken away from an organism through evaporation so it doesn't overheat.

Define 'matter'

Refers to anything that occupies space and has mass. All matter is made of substances called elements, which have specific chemical/physical properties and cannot be broken down into other substances through ordinary chemical reactions.

Define 'atom'

Smallest unit of matter that retains all of the chemical properties of an element. Example: Gold coin = large # of gold atoms molded into a coin, cannot be broken down into anything smaller WHILE STILL RETAINING TH PROPERTIES OF GOLD (gets its properties from the tiny subatomic particles it's made of)

What are examples of things that would not incorporate themselves well in water?

Substances that don't have charge or aren't polar. An example of this would be hydrocarbons, such as hexane, which is a major constituent of car gasoline. Hexane has no polarity, meaning that it DOESN'T FORM HYDROGEN BONDS. This means that it won't dissolve well in water, meaning substances like hydrocarbons are hydrophobic, meaning water-fearing.

Define 'element'

Substances that have specific properties at certain temperatures and react in certain ways. Examples: carbon, lead, gold, oxygen, nitrogen, silicon. Elements can be broken down into even more fundamental particles, they are defined by the arrangement of these particles, and if you were to change the number of fundamental particles, you could change the properties of the element + how it would react OR you could change the element itself.

Define 'heat of vaporization'

The amount of energy needed to change one gram of liquid substance to gas at a constant temperature.

Define 'specific heat capacity'

The amount of heat needed to change one gram of a substance by 1 degree celsius.

Define adhesion of water.

The attraction of molecules of one kind for molecules of a different kind. This can be quite strong for water, especially with positively/negatively charged molecules.

Define density

The mass per unit volume of a substance.

What are the characteristics a molecule must have in order to dissolve in water?

The molecule must be an ion (meaning it has charge) or have polarity. These molecules are known as hydrophilic, meaning water-loving.

Why does sodium chloride dissolve in water?

The negative ends of water molecules (the oxygens) are attracted to the positive sodium ion. The positive ends of water molecules (the hydrogens), will be repelled from the positive sodium ion, though they are attracted to the negative chloride ion (called an anion). Because water is polar, so it has negative and positive charges, it will be attracted to both the positive hydrogens and the negative chloride ions. Essentially, the water molecules form a 'shell' around the sodium or chloride atoms (many water atoms surround the chloride or sodium atom, therefore forming a 'shell'). This allows the surrounded sodium/chloride to easily interact with other water molecules + flow easily, thereby dissolving in water.

What 'defines' an element?

The number of protons in the nucleus of an atom (atomic number)

Why does the surface of water have surface tension?

The number of water molecules in a pool of water is extreme. All of the hydrogen bonds between these molecules of water push and pull the molecules in every direction. The top molecules, in contrast, do not have anything pulling them up from above, allowing them to get more densely packed and closer to 'neighbors', which allows them to have a stronger intermolecular force at the surface than you have within the body, causing the phenomenon known as surface tension.

Define 'electrically neutral'

The overall charge of the atom is zero, protons = electrons

What is a covalent bond?

The sharing of electron pairs between atoms.

Solute definition

The substance being dissolved (in a sodium chloride-water situation, this would be sodium chloride)

Solvent definition

The substance that dissolves the solute - the substance that's doing the dissolving (in a sodium chloride-water situation, this would be water)

Define 'chemically stable'

The tendency of a material to resist charge of decomposition in its natural environment or when exposed to air, heat, light, pressure / other natural conditions or due to internal reaction, 2 or 8 electrons in outer shell.

What is the structure of the water molecule?

The water molecule is comprised of two hydrogen atoms and one oxygen atom. The atoms are bonded together with covalent bonds, which is a type of molecular bond that involves the sharing of electron pairs between atoms, specifically, pairs from valence electrons.

Describe what would happen if a thin glass tube was placed upside down in a beaker full of water.

The water will begin to climb up the tube through capillary action, also revealing that the thin glass tube must be a hydrophilic container. In this scenario, capillary action is adhesion occurring more intensely because more of the water molecules are able to come in touch with the polar glass lattice. Because this is a POLAR container, it is HYDROPHILIC, which is why it fills with water.

Draw a water molecule, including where the partial positive and partial negative charges are.

There's a partial negative charge at the non-hydrogen end, as a result of the electronegative oxygen 'hogging' electrons. The deficiency of electrons on the side with the hydrogen atoms forms a partial positive charge:

Why is the phrase "water dissolves everything because it is a 'universal solvent'" misleading?

Though often called a 'universal solvent', water can actually only dissolve polar molecules and ions (molecules with charge). It CANNOT dissolve nonpolar molecules (like oils), which are molecules that lack partial positive/negative charges. Because of this, this phrase is misleading.

How did water become important to life because of its structure?

Water forms covalent bonds between the two hydrogens and the oxygen. This means that the oxygen shares electrons with each of the hydrogen atoms, but oxygen is more electronegative meaning they will spend more time around the oxygen than the hydrogens. Because of of this, oxygen is a bent molecule, and the side with the most electrons will be partially negative & the side with the hydrogens will be partially positive. Because both charges are partial, water molecule are polar molecules, allowing for hydrogen bonding. Hydrogen bonding allows water to have many properties and makes it special for harboring life and allowing life to be possible. Life needs a fluid environment; these hydrogen bonds - when temp + conditions are appropriate - allow water to be in that liquid form. They're strong enough so molecules stay together, weak enough so they allow the water molecules to flow past each other, which makes them a good solvent. Because of its polar structure, it dissolves polar molecules/things that have a charge.

Why does surface tension occur?

Water molecules at the surface of the water will form hydrogen bonds with their neighbors, called water-air interface. Because they are exposed to air on one side, they will have fewer neighboring water molecules to bond with. This forms stronger bonds with the neighbors they do have - making the surface more resistant of stress/pressure.

Why is it harmful for living things to freeze?

When a cell freezes, its watery contents expand + its membrane is broken into pieces.

What does the structure of water vapor look like in regards to hydrogen bonds?

When heat is raised (boiled), hydrogen bonds break completely which allows water molecules to escape the air as gas. These bonds break because of the higher kinetic energy of the water molecules. This is observed as water vapor or steam.

What does the structure of solid water (ice) look like in regards to hydrogen bonds?

When the temperature drops and water freezes, molecules form a crystal structure maintained by hydrogen, as there's too little heat energy to break the bonds. This structure makes ice less dense than water, meaning hydrogen bonding explains the low density of ice.

Give an example of the cohesion of water.

When water is filled to the top of the glass but does not overflow, and so a dome-like shape forms. This is because of the molecules' cohesive properties, which is their tendency to stick to one another, forming tension at the surface of the water.

What is an example of capillary action within a simple class of water?

When you fill a glass/beaker with water, the surface is NOT flat, it is higher when it's near the glass vs. in the center. This is a meniscus, and with water, it would be called a concave meniscus. It is caused by capillary action.

How does the fact that ice is less dense than water relate to the importance of water for life?

Where life first arose - in a pond - the floating of ice protects life in the pond underneath, and it prevents the lake/pond from completely freezing and killing all life. Life can continue to thrive under the ice, even when the air is much colder than what is suitable for life (water's high specific heat capacity allows for the temperature variation in the water to be much less than that of outside the water)

Are cohesive forces responsible for surface tension?

Yes. Surface tension is the tendency of a liquids surface to resist rupture when placed under tension or stress. The reason why the surface of the water is able to resist rupture is because of cohesion - the water's ability to be attracted to itself. The water can be more tightly packed at the surface of the water because there are no water molecules above the surface pulling the water upward, as the surface is exposed to air, not water, called water-air interface. This allows the water molecules at the surface to have less molecules to bond with, allowing it to form stronger bonds with its neighboring molecules & allowing it to pack very tightly at the top, causing surface tension. Water's cohesion is because of it's polarity, or it's partial negative and positive charges.