Water

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Water as the Medium for Life

The first cells are believed to have evolved in a watery environment, likely in deep oceans near hydrothermal vents.
Water in its liquid state allows dissolved molecules to move and collide, enabling chemical reactions.
Most life processes occur in water; it is the medium in which all metabolic reactions take place in cells.
Between 70% and 95% of the mass of a cell is water.
Water covers 71% of Earth's surface and is a major habitat for organisms.
Scientists searching for extraterrestrial life look for evidence of water as an indicator of possible life.

Hydrogen Bonds and the Polarity of Water

A water molecule consists of one oxygen atom covalently bonded to two hydrogen atoms.
Electrons are shared unevenly: oxygen attracts electrons more strongly, creating a weak negative region (δ-) on oxygen and weak positive regions (δ+) on hydrogen.
This separation of charge makes water a dipole and a polar molecule.
Hydrogen bonds form between the δ- oxygen of one water molecule and the δ+ hydrogen of another.
Hydrogen bonds are weak individually and constantly break and reform, but collectively they form a strong structure.
Hydrogen bonding explains many of water's biologically important properties.

Roles of Hydrogen Bonding in Biological Molecules

Hydrogen bonds are involved in dissolving solutes in water.
They provide cohesion and adhesion of water molecules, enabling water to move up tall trees.
They are responsible for base-pairing between the two strands of DNA.
They help form secondary and tertiary structure in proteins.
Hydrogen bonds between strands of cellulose and collagen give these molecules tensile strength.
They facilitate interactions between mRNA and tRNA during protein synthesis.
They contribute to surface effects on membranes between polar phosphate groups and water.

Cohesion and Adhesion

Cohesion is the attraction between water molecules due to hydrogen bonds.
Cohesion allows columns of water to move under tension (mass transport) through the xylem of plants.
Cohesion creates surface tension at the air-water interface, forming a film that supports insects such as pond skaters.
Adhesion is the bonding of water to other polar or charged molecules, such as cellulose.
Adhesion enables water to move up the xylem during transpiration and to be drawn up narrow soil channels by capillary action.
Spaces between cellulose fibres in plant cell walls draw water from xylem vessels by capillary action, allowing flow through plant tissue.

Water as a Solvent

Hydrophilic means 'water-loving'; hydrophobic means 'water-hating'.
Polar molecules and those with positive or negative charges can form hydrogen bonds with water and are generally hydrophilic.
Non-polar molecules cannot form hydrogen bonds with water and are generally hydrophobic; they tend to group together due to hydrophobic interactions.
Because most biological molecules are hydrophilic and can dissolve, water is regarded as the universal solvent.
Different metabolites have different solubilities: sodium chloride and urea are highly soluble; fats are insoluble.
Highly soluble molecules such as salts, glucose and amino acids are easily transported in solution.
Even amino acids with hydrophobic R groups are soluble enough to be freely transported in water.
Less soluble molecules require assistance: oxygen combines with haemoglobin for transport, and phospholipids have hydrophobic tails that form the core of cell membranes.

Enzyme Action in Water

Most enzymes require water to hold their shape and maintain stability.
Water enables enzymes to catalyse reactions in aqueous solutions.
Hydrogen bonds often facilitate binding between the enzyme active site and its substrate.
This binding forms an enzyme-substrate complex.

The lock and key model

Physical Properties of Water: Specific Heat Capacity

Specific heat capacity is the energy required to raise the temperature of 1 kg of a substance by 1°C.
Water has a high specific heat capacity (4200 J/kg/°C) compared to air (1000 J/kg/°C).
The high value is due to the many hydrogen bonds: a lot of thermal energy is needed to break and build them.
This means water temperature does not fluctuate greatly.
It provides stable aquatic habitats, as water temperatures change more slowly than air temperatures.
It helps organisms maintain a constant internal temperature and is vital for optimal enzyme activity.
Arctic and sub-Arctic species such as the ringed seal survive year-round due to stable sea temperatures.

Physical Properties of Water: Density, Thermal Conductivity, Buoyancy and Viscosity

Ice is less dense than liquid water, so it floats, forming a habitat for organisms such as the ringed seal.
Thermal conductivity is the ability of a substance to conduct heat; water's thermal conductivity is almost 30 times higher than air, making air a good insulator.
The black-throated loon traps an insulating layer of air in its feathers; the ringed seal relies on blubber for insulation.
Ice forms an insulating layer above water because its thermal conductivity is much lower than liquid water, trapping thermal energy and increasing sea temperature below.
Buoyancy is the ability to float; the loon has solid bones to overcome buoyancy, while the seal's blubber improves buoyancy and insulation.
Viscosity is resistance to flow; water's viscosity is much higher than air, and body shapes of loons and seals reduce drag in water.
The seal uses flippers to propel itself; the loon uses webbed feet and lateral foot placement to reduce drag.

Origin of Water on Earth

When Earth formed about 4.5 billion years ago, conditions were too hot for water vapour to condense into liquid water.
The asteroid hypothesis proposes that asteroids and their meteorites brought water to Earth, as many contain ice and organic materials.
Carbonaceous chondrites contain hydrogen isotopes similar to those in seawater.
Eucrite achondrites contain hydrogen isotope ratios similar to Earth's, supporting the hypothesis.
Impacts may have released water vapour that was trapped by Earth's gravity and later condensed to liquid water.

Water and the Search for Extraterrestrial Life

Liquid water is a requirement for life, so its presence is key in the search for extraterrestrial life.
The Goldilocks zone is the area around a star where temperatures allow liquid water to exist.
Exoplanets in the Goldilocks zone are studied using transit spectroscopy, which analyses light passing through their atmosphere.
This analysis can reveal a water signature, indicating water may be present.
To support life, an exoplanet must have a water signature, be in the Goldilocks zone, and be large enough to support an atmosphere.

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練習問題

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1.Water has the ability to act as a solvent and dissolve many ionic and covalent compounds. Which of the following 3-carbon compounds will not dissolve in water?
Easy
- APropanoic acid
- BPropanol
- CGlycerol
- DPropane
2.The specific heat capacity of water is the highest of all liquids. Which of the following would be a correct definition of specific heat capacity?
Easy
- AThe heat required to change one mole of liquid into one mole of gas.
- BThe heat required to raise the temperature of 1 kg of liquid by 1°C.
- CThe heat required to change one mole of solid into one mole of liquid.
- DThe ability of a solid to transfer heat to a liquid.
3.Which of the following diagrams correctly shows the bonding between two water molecules?
Medium
- AA hydrogen bond between the δ+ hydrogen of one water molecule and the δ− oxygen of another.
- BA covalent bond between the oxygen of one water molecule and the hydrogen of another.
- CAn ionic bond between the oxygen of one water molecule and the hydrogen of another.
- DA hydrogen bond between the δ− oxygen of one water molecule and the δ− oxygen of another.
4.Which of the following are not examples of hydrogen bonding?
Medium
- ABase-pairing between two strands of DNA.
- BThe forces that hold water molecules together.
- CThe bond that joins one nucleotide to its neighbour in a strand of DNA.
- DInteractions between water and the polar R groups of certain amino acids.
5.Water (H₂O) is a polar molecule, whereas methane (CH₄) is nonpolar. Which of the properties of methane is explained by methane's lack of polarity?
Medium
- ALow molecular weight.
- BLow boiling point.
- CFlammability.
- DGreenhouse gas effect.
6.Which of the following observations is not explained by water's high latent heat of vaporisation and specific heat capacity?
Medium
- AIce is less dense than liquid water, so it floats on water.
- BWater exists in all three physical states (solid, liquid and gas) on Earth.
- CA small volume of water can dissipate a lot of heat from an organism.
- DA lot of heat energy is required to raise the temperature of water.
7.Which row of the table lists the four common metabolites in decreasing order of solubility in water?
Medium
- Aoxygen → sodium chloride → cholesterol → hydrophobic amino acid
- Bsodium chloride → oxygen → hydrophobic amino acid → cholesterol
- Chydrophobic amino acid → oxygen → sodium chloride → cholesterol
- Dsodium chloride → hydrophobic amino acid → oxygen → cholesterol
8.Which of the following properties of water are a result of intermolecular forces? (select all that apply)
Medium
- AHigh surface tension.
- BGood solvent.
- CCohesiveness.
- DHigh specific heat capacity.
- ELow viscosity.

もっと問題

9.Which of the following properties of water stops enzymes from being denatured during transpiration?
Hard
- AI only
- BI and II
- CII, III and IV
- DIII only
10.Sodium hydrogencarbonate (NaHCO₃) dissociates into sodium ions (Na⁺) and bicarbonate ions (HCO₃⁻) when dissolved in water. The bicarbonate ions create an alkaline environment suitable for certain enzymes, and in the blood they help buffer lactic acid during exercise. How does water help the production of bicarbonate ions?
Hard
- AThe oxygen atoms form ionic bonds with bicarbonate ions.
- BThe oxygen atoms in water attract bicarbonate ions.
- CThe hydrogen atoms form hydrogen bonds with bicarbonate ions.
- DThe hydrogen atoms in water attract bicarbonate ions.
11.Which of the following does not rely on the specific heat capacity of water?
Medium
- AHeat loss through evaporation of sweat
- BEnzyme function
- CAquatic ecosystems withstanding extreme temperature change
- DHumans living in sub-zero temperatures
12.Which of the following are roles of hydrogen bonding in biological molecules? (select all that apply)
Medium
- ABase-pairing between the two strands of DNA
- BHolding the two strands of the DNA double helix together
- CJoining one nucleotide to its neighbour within a single DNA strand
- DMaintaining the secondary and tertiary structure of proteins
- EForming the covalent bonds between hydrogen and oxygen within a water molecule
13.Which of the following statements about the solvent properties of water are correct? (select all that apply)
Medium
- APolar molecules and molecules with positive or negative charges can form hydrogen bonds with water and so generally dissolve
- BNon-polar molecules with no positive or negative charge cannot form hydrogen bonds with water and are generally hydrophobic
- CHydrophobic molecules tend to join together in groups because hydrogen bonds form between water particles but not with the non-polar molecule
- DAll metabolites are equally soluble in water because water is a universal solvent
- EFats are highly soluble in water because they contain many hydrogen bonds
14.Which of the following are examples of hydrogen bonding? (select all that apply)
Medium
- AThe forces that hold water molecules together
- BBase-pairing between two strands of DNA
- CInteractions between water and the polar R groups of certain amino acids
- DThe bond that joins one nucleotide to its neighbour in a strand of DNA
- EThe covalent bond between oxygen and hydrogen within a water molecule
15.Which of the following properties of water help to stop enzymes from being denatured during transpiration? (select all that apply)
Medium
- AWater retains a lot of heat
- BWater forms hydrogen bonds with other polar and nonpolar molecules
- CA lot of heat is required to evaporate water
- DWater is cohesive
- EWater has a low specific heat capacity
16.Match each property of water with the correct description.
Medium
- Cohesion
- Adhesion
- Specific heat capacity
- Thermal conductivity
- Viscosity
- Water particles sticking to each other
- Water particles sticking to other materials
- The energy required to raise the temperature of 1 kg of a substance by 1°C
- The ability of a substance to conduct heat
- The resistance of a fluid to flow
17.Match each biological role of hydrogen bonding with the correct example.
Medium
- Dissolving of solutes in water
- Cohesion and adhesion of water molecules
- Structure of proteins
- Tensile strength of collagen and cellulose
- Interactions during protein synthesis
- Polar molecules forming hydrogen bonds with water
- Water moving up the trunks of tall trees
- Secondary and tertiary levels of protein structure
- Hydrogen bonds between strands giving strength
- Interactions between mRNA and tRNA
18.Match each property of water with its advantage to living organisms.
Hard
- High specific heat capacity
- High latent heat of vaporisation
- Lower density of ice than liquid water
- High surface tension
- High thermal conductivity
- Maintains stable temperatures optimal for enzyme activity
- Allows effective cooling through sweating without losing too much water
- Ice floats and forms a habitat, insulating water below
- Allows small insects such as pond skaters to move across the surface
- Air is a good insulator for organisms in cold climates
19.Order the following metabolites from most soluble to least soluble in water.
Medium
- Sodium chloride
- Oxygen
- Cholesterol
- Hydrophobic amino acid
20.Order the following steps to show how water moves up a tall tree.
Medium
- Water evaporates from the leaves
- Cohesion between water molecules pulls water up the xylem
- Adhesion of water to the xylem walls helps water rise
- Water is drawn into the roots by osmosis
21.Order the following events in the origin of water on Earth according to the asteroid hypothesis.
Medium
- Earth formed and was too hot for water vapour to condense
- Asteroids and meteorites containing ice and organic materials impacted Earth
- Water vapour was released during the impact
- Water vapour was trapped by Earth's gravity and condensed to form liquid water
22.Other than the starting temperature of the water (70°C), which conditions must be kept the same for each cup throughout the experiment? (select all that apply)
Medium
- AVolume of water in each cup
- BType and thickness of the plastic cup
- CRoom temperature
- DTime intervals between temperature readings
- EColour of the tissue paper
23.A student places a drop of water on a clean glass slide and observes that the water spreads out and forms a meniscus where it meets the glass. Which property of water is responsible for this observation?
Medium
- AAdhesion between water molecules and the polar glass surface
- BCohesion between water molecules
- CHigh specific heat capacity of water
- DLow viscosity of water
24.A scientist measures the energy needed to raise the temperature of 1 kg of water by 1°C and obtains a value of 4200 J. What is this value called?
Medium
- ASpecific heat capacity
- BLatent heat of vaporisation
- CThermal conductivity
- DBuoyancy
25.A research team is studying a newly discovered exoplanet. Which of the following characteristics would suggest that the planet could support life? (select all that apply)
Hard
- AThe planet has a water signature in its atmosphere.
- BThe planet is located in the Goldilocks zone of its star.
- CThe planet is large enough to support an atmosphere.
- DThe planet has a surface temperature of 500°C.
- EThe planet has no atmosphere.
26.Water has a higher thermal conductivity than air, making air a good insulator for organisms in cold climates.
Easy
True or false?
27.Match each property of water with its correct consequence for living organisms.
Medium
- High specific heat capacity
- High latent heat of vaporisation
- Cohesion
- Adhesion
- Stable aquatic habitats with minimal temperature fluctuation
- Cooling effect during sweating in mammals
- Water columns can be pulled up xylem in tall trees
- Water can move through narrow capillary tubes in soil
28.Order the following steps to show how the asteroid hypothesis explains the origin of water on Earth.
Hard
- Asteroids and meteorites containing ice and organic materials impact Earth.
- Water vapour is released during the impact.
- Water vapour is trapped by Earth's gravity.
- Temperatures are low enough for water vapour to condense into liquid water.
- Liquid water is retained on the surface by gravity.
29.A team of astronomers uses transit spectroscopy to analyse the atmosphere of an exoplanet. They detect absorption patterns that indicate the presence of water vapour. What can they conclude about the planet?
Challenge
- AThe planet definitely supports life.
- BThe planet has a water signature and may be capable of supporting life if other conditions are met.
- CThe planet is located in the Goldilocks zone.
- DThe planet has liquid water on its surface.
30.A student investigated the thermal properties of water by pouring hot water at 70°C into two thin plastic cups and recording the temperature every 2 minutes for 10 minutes. The sides of one cup were covered with tissue paper soaked in hot water; the sides of the other cup were left uncovered. Which variable is the independent variable in this investigation?
Medium
- AThe presence of the wet tissue paper covering on the cup
- BThe temperature of the water in each cup
- CThe time interval between temperature readings
- DThe starting temperature of the water
31.In the cooling experiment, the cup covered with tissue paper soaked in hot water cools more slowly than the uncovered cup. Which property of water does this model?
Hard
- AWater's high specific heat capacity, as in sweating
- BWater's high latent heat of vaporisation, as in sweating
- CWater's low viscosity, as in blood flow
- DWater's high thermal conductivity, as in blubber insulation
32.A student sets up two thin plastic cups containing water at 70°C. One cup is covered with tissue paper soaked in hot water, and the other is left uncovered. The temperature is recorded every 2 minutes for 10 minutes. Which of the following must be controlled to make the comparison valid? (select all that apply)
Hard
- AThe volume of water in each cup
- BThe type and thickness of the plastic cup
- CThe starting temperature of the water
- DThe time interval between temperature readings
- EThe colour of the tissue paper
33.A student places a drop of water on a clean glass slide and observes a meniscus where the water meets the glass. Which property of water is responsible for this?
Medium
- ACohesion between water molecules
- BAdhesion between water molecules and the polar glass surface
- CSurface tension at the air-water interface
- DHigh specific heat capacity of water
34.Detergent pollution in a pond can harm pond skaters that rely on walking on the water surface. Which property of water is most directly disrupted by detergent?
Medium
- ACohesion between water molecules at the surface
- BAdhesion between water molecules and the pond bed
- CHigh specific heat capacity of water
- DLow viscosity of water
35.A student uses a respirometer to measure oxygen consumption by germinating seeds. The apparatus contains water to absorb carbon dioxide and to maintain humidity. Which properties of water are important for the reliable operation of this respirometer? (select all that apply)
Hard
- AWater is a solvent for the carbon dioxide produced
- BWater has a high specific heat capacity, minimising temperature fluctuations
- CWater has a high viscosity, slowing gas movement
- DWater has a high thermal conductivity, equalising temperature rapidly
- EWater is non-polar, allowing gases to dissolve
36.In an experiment to test the effect of a wet covering on the cooling of water, the cup covered with wet tissue paper will cool more slowly than the uncovered cup.
Easy
True or false?
37.A student investigates the rate of cooling of water in two cups: one covered with wet tissue paper and one uncovered. The temperature is recorded every 2 minutes for 10 minutes. Which of the following is the most appropriate control for this experiment?
Hard
- AA cup covered with dry tissue paper
- BA cup containing no water
- CA cup covered with wet tissue paper at a different starting temperature
- DA cup with twice the volume of water
38.A student uses a colorimeter to measure the absorbance of a coloured solution. The solution is made up in water. Which property of water makes it suitable as the solvent in this technique?
Medium
- AWater is a polar solvent that dissolves many ionic and polar solutes
- BWater has a high specific heat capacity
- CWater has a high surface tension
- DWater has a low viscosity
39.A student investigates the effect of temperature on the solubility of oxygen in water. They bubble oxygen through water at different temperatures and measure the dissolved oxygen concentration. Which of the following statements are correct? (select all that apply)
Hard
- AOxygen is less soluble at 37°C than at 20°C
- BOxygen is a non-polar molecule and is only sparingly soluble in water
- CThe presence of haemoglobin increases the oxygen-carrying capacity of blood
- DOxygen solubility in water increases as temperature increases
- EOxygen dissolves in water by forming hydrogen bonds with water molecules
40.Which term describes a molecule that has one end with a slight negative charge and one end with a slight positive charge?
Easy
- ADipole
- BHydrophobic
- CNon-polar
- DIsotope
41.Calculate the energy required to raise the temperature of 2 kg of water by 5°C. The specific heat capacity of water is 4200 J/kg/°C. Give your answer in joules.
Medium
42.Complete the sentence about water's properties.
Medium
The attraction between water molecules and other polar molecules, such as cellulose, is called ____, while the attraction between water molecules themselves is called ____.
43.Name the property of water that allows insects such as pond skaters to move across its surface.
Medium
44.Calculate the energy, in joules, required to raise the temperature of 0.5 kg of water by 4°C. The specific heat capacity of water is 4200 J/kg/°C.
Medium
45.How many times greater is the specific heat capacity of water (4200 J/kg/°C) than that of air (1000 J/kg/°C)? Give your answer to 1 decimal place.
Medium
46.The thermal conductivity of water is almost 30 times higher than that of air. If the thermal conductivity of air is 0.026 W/mK, what is the approximate thermal conductivity of water in W/mK? Give your answer to 2 decimal places.
Medium
47.A cell is 80% water by mass. If the cell has a total mass of 50 pg, what is the mass of water in the cell, in picograms (pg)?
Medium
48.Water makes up 71% of the Earth's surface. If the total surface area of Earth is 510 million km², what area is covered by water, in millions of km²? Give your answer to the nearest whole number.
Easy
49.Complete the sentence about water's solvent properties.
Easy
Water is known as the universal ____ because it can dissolve many ionic and covalent compounds, but non-polar molecules such as fats are ____.
50.Complete the sentences about hydrogen bonding in water.
Easy
In a water molecule, the oxygen atom attracts electrons more strongly than the hydrogen atoms, creating a ____ molecule. The weak attraction between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another is called a ____ bond.
51.Complete the sentence about the properties of water that allow transport in plants.
Medium
The attraction between water molecules is called ____, while the attraction between water molecules and the walls of the xylem is called ____. Together, these forces allow water to move up the xylem in a continuous column.
52.Complete the sentence about thermal properties of water.
Medium
The specific heat capacity of water is ____ J/kg/°C, which is much higher than that of air. This property is due to the many ____ bonds present in water.
53.Define the term hydrophilic.
Medium
54.Two of the properties of water are cohesion and adhesion. Explain how these properties are useful to living organisms.
Medium
55.Both water and methane are small molecules containing single covalent bonds between their atoms. State two differences between these two molecules that make their physical properties very different.
Medium
56.List two physical properties, associated with their state of matter, that differ between water and methane.
Medium
57.Describe the properties of water molecules that enable them to transport metabolites, using four named examples.
Hard
58.Calculate the energy required to raise the temperature of 2 kg of water by 5°C. The specific heat capacity of water is 4200 J/kg/°C. Give your answer in joules.
Medium
- A46200 J
- B37800 J
- C50400 J
- D42000 J
59.Calculate the energy, in joules, required to raise the temperature of 0.5 kg of water by 4°C. The specific heat capacity of water is 4200 J/kg/°C.
Medium
- A7560 J
- B9240 J
- C8400 J
- D10080 J
60.How many times greater is the specific heat capacity of water (4200 J/kg/°C) than that of air (1000 J/kg/°C)? Give your answer to 1 decimal place.
Medium
- A6.2
- B3.2
- C5.2
- D4.2
61.The thermal conductivity of water is almost 30 times higher than that of air. If the thermal conductivity of air is 0.026 W/mK, what is the approximate thermal conductivity of water in W/mK? Give your answer to 2 decimal places.
Medium
- A-0.22 W/mK
- B0.78 W/mK
- C2.78 W/mK
- D1.78 W/mK
62.A cell is 80% water by mass. If the cell has a total mass of 50 pg, what is the mass of water in the cell, in picograms (pg)?
Medium
- A40 pg
- B44 pg
- C36 pg
- D48 pg
63.Water makes up 71% of the Earth's surface. If the total surface area of Earth is 510 million km², what area is covered by water, in millions of km²? Give your answer to the nearest whole number.
Easy
- A326 million km²
- B434 million km²
- C362 million km²
- D398 million km²

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