IB A1.1 Water Past Paper PDF 2025

A1.1 Water This unit explores the essential role of water in sustaining life. Water's physical and chemical properties provide numerous benefits to aquatic, marine, and terrestrial organisms, serving as an excellent solvent for necessary compounds and facilitating com...

A1.1 Water This unit explores the essential role of water in sustaining life. Water's physical and chemical properties provide numerous benefits to aquatic, marine, and terrestrial organisms, serving as an excellent solvent for necessary compounds and facilitating communication in and out of cells. Despite its advantages, water also poses challenges for organisms living in it, such as its high viscosity, which requires adaptations in body shape and propulsion mechanisms. Ultimately, water remains crucial to all living things, having played a pivotal role in the evolution of life on Earth Theme: Unity and Diversity Level of Organisation: Molecules First Exams 2025  Objectives overview Students should appreciate that the first cells originated in water and that water remains the A1.1.1 SL Water as the medium for life medium in which most processes of life occur. Students should understand that polarity of covalent bonding within water molecules is due to Hydrogen bonds as a consequence of the polar covalent bonds within water unequal sharing of electrons and that hydrogen bonding due to this polarity occurs between A1.1.2 SL molecules water molecules. Students should be able to represent two or more water molecules and hydrogen bonds between them with the notation shown below to indicate polarity. Cohesion of water molecules due to hydrogen bonding and consequences for Include transport of water under tension in xylem and the use of water surfaces as habitats due A1.1.3 SL organisms to the effect known as surface tension. Adhesion of water to materials that are polar or charged and impacts for A1.1.4 SL organisms Include capillary action in soil and in plant cell walls. Emphasize that a wide variety of hydrophilic molecules dissolve in water and that most enzymes Solvent properties of water linked to its role as a medium for metabolism and A1.1.5 SL for transport in plants and animals catalyse reactions in aqueous solution. Students should also understand that the functions of some molecules in cells depend on them being hydrophobic and insoluble. Include buoyancy, viscosity, thermal conductivity and specific heat capacity. Contrast the Physical properties of water and the consequences for animals in aquatic physical properties of water with those of air and illustrate the consequences using examples of A1.1.6 SL habitats animals that live in water and in air or on land, such as the black-throated loon (Gavia arctica) and the ringed seal (Pusa hispida).  SL and HL Combined Content  IB Guiding Questions § What physical and chemical properties of water make it essential for life? § What are the challenges and opportunities of water as a habitat?  SL & HL Key Terms Hydrogen bonds Xylem Buoyancy Polar covalent bonds Surface tension Viscosity Cohesion Capillary action Thermal conductivity Adhesion Hydrophobic Specific heat capacity Hydrophilic Apoplast pathway Solvent Metabolism You wont understand some of the definitions, and that’s okay! We will be learning what each key word means throughout this unit J  What should I know before we begin? (chemistry link) Access this on kognity section A1. Unit and diversity: Molecules / A1.1 Water  Water is a polar molecule oxygen atoms = -ve hydrogen atoms = +ve δ- O Strong intramolecular covalent bond δ+ δ+ 104.5◦ H H Oppositely charged atoms attract each other forming intermolecular HYDROGEN BONDS δ- δ+ δ+ A1.1.1 Water as a Medium of Life Water is the Medium of Life Students should appreciate that the first cells originated in water and that water remains the medium in which most processes of life occur. Water is essential for life on Earth. The first cells are believed to have formed and evolved in the oceans. Cells require water as a solvent for biochemical reactions to occur. Water is the fluid that makes up all cells, organelles, and intercellular/tissue fluid. Water is necessary for the transport of substances into and out of cells. Water is essential to blood and many other body fluids. Water is the medium in which all organisms in oceans, lakes, and rivers live.  Water is the Medium of Life Students should appreciate that the first cells originated in water and that water remains the medium in which most processes of life occur. Life probably originated in water. Water is required for life as it is: The medium for metabolic reactions (Water is a solvent). A transport medium. Involved in many chemical reactions (Examples: hydrolysis reactions and photosynthesis.) Read the linked article on why water is necessary for life. WATCH: Full History of Earth in 10 minutes READ: Origins of Life: new evidence that first cells could have formed at the bottom of the ocean A1.1.2: Hydrogen bonds as a consequence of the polar covalent bonds within water molecules Students should understand that polarity of covalent bonding within water molecules is due to unequal sharing of electrons and that hydrogen bonding due to this polarity occurs between water molecules. Students should be able to represent two or more water molecules and hydrogen bonds between them with the notation shown below to indicate polarity. Water molecules https://www.biotopics.co.uk/jsmol/watermoving.html Water is a Polar Molecule Water is a polar covalent molecule. Two hydrogen atoms form covalent bonds with an oxygen atom. Explain why water forms hydrogen bonds.  Water is a Polar Molecule The oxygen atom has a more positive nucleus than the hydrogen atoms, and has a greater attraction for (the negatively charged) electrons. The electrons are shared unequally between the oxygen and hydrogen atoms in water molecules. Oxygen has a slightly negative charge (δ-) Hydrogen has a slightly positive charge (δ+) Polar Water Molecules form Hydrogen Bonds Water is a polar covalent molecule with weak charges at the poles. Water is attracted to other polar molecules forming hydrogen bonds. The hydrogen bonds that form between water molecules explain many of water’s properties. Note: If asked to draw two water molecules and a hydrogen bond, the hydrogen bond should be a dashed line. The covalent bonds between the oxygen and hydrogens in a water molecule should be solid lines.  Structure of Water Completing reading on the structure of water using kognity. Guidance: Draw two or more water molecules and hydrogen bonds between them with a notion indicating the polarity.  Water A1.1.3: Cohesion of water molecules due to hydrogen bonding and consequences for organisms Include transport of water under tension in xylem and the use of water surfaces as habitats due to the effect known as surface tension. A1.1.4: Adhesion of water to materials that are polar or charged and impacts for organisms Include capillary action in soil and in plant cell walls. Cohesion and Adhesion What does cohesion and adhesion mean? Water molecules are cohesive and adhesive due to their polarity. Watch the video then: Distinguish between cohesion and adhesion.  Cohesion and Adhesion Cohesion is attraction between the same kind of molecules, whereas adhesion is attraction between different kinds of molecules. Cohesive: Water molecules are attracted to other water molecules. The negatively charged oxygen of one water molecule is attracted to the positively charged hydrogens of another water molecule, forming a hydrogen bond. Adhesive: The polar water molecules are attracted to other polar molecules, forming hydrogen bonds. Cohesion and Surface Tension Surface tension is the property of a substance to resist an external force. A relatively strong surface tension forms at the surface of water due to the cohesive nature of water molecules. Surface tension allows animals like water striders to walk on water, and the surface of a lake becomes their habitat. Cohesion and Transport in the Xylem Xylem vessels are thin narrow tubes which transport water (and dissolved minerals) from the roots to the leaves of plants. The cohesive nature of polar water molecules allows water to form a continuous column of water molecules moving up the xylem/stem of plants.  Adhesion - Capillary Action in Soil Capillary action describes the ability of water to flow against gravity in a narrow space. Soil tends to be porous, as the clay and organic matter in the soil is polar. Water adheres to the polar soil particles and moves up the soil towards the roots of plants. Capillary action is greater in fine clay soils, and weaker in porous sandy soils.  Adhesion - Capillary Action in the Cell Wall The apoplast pathway allows water and dissolved nutrients to move through the cell walls of plants. Cellulose in the cell walls of plants is porous and polar. Water can move from cell to cell by capillary action, as the polar water adheres to the polar cellulose. A1.1.5: Solvent properties of water linked to its role as a medium for metabolism and for transport in plants and animals Complete kognity reading on the interactions with water Water is a Solvent Water is often called the universal solvent, as it allows a wide variety of hydrophilic substances to dissolve in it, because of its polarity. Hydrophilic substances are charged substances that mix and dissolve with water. Most metabolic reactions will only occur when the reactants are dissolved in water. Water is a polar solvent, and hydrophilic molecules dissolve in water easily. Many hydrophilic substances, such as ions, amino acids, and proteins, are essential for life and transported by water in plants and animals. u Hydrophobic substances, such as steroid hormones and wax cuticle, are insoluble in water. u The hydrophobic property of the wax cuticle on leaves prevents water from evaporating and keeps the leaves from dehydrating.  Water is a Transport Medium in Plants Water is the transport medium in plants and animals, as many polar molecules and ions dissolve in water. Polar (charged) organic particles dissolve in polar water, due to hydrogen bonding (eg glucose). Ions dissolve in water because of the polar water molecules surrounding the charged ions. Ionic minerals are dissolved in water as it is transported up the xylem of a plant. Organic molecules such as glucose and amino acids dissolve in water, and are transported through the phloem of plants.  Water is a Transport Medium in Animals Animals transport many substances around their bodies through the blood or hemolymph, including: Nutrients Hormones Oxygen Waste products of metabolism Carbon dioxide Antibodies Heat is also transported around the body in water.  Modes of Transport in Blood Substance Mode of transport Why it works Glucose Dissolved in Glucose is a polar molecule which dissolves in blood plasma water. Blood plasma is mostly water. Amino Acids Dissolved in Amino acids are polar molecules which dissolve in blood plasma water. Blood plasma is mostly water. Cholesterol and lipid molecules are nonpolar, and Cholesterol Lipoprotein will not dissolve in blood plasma, which is mostly and Lipids complexes water. Sodium Dissolved in Sodium chloride is an ionic compound which Chloride blood plasma dissolves in water. Blood plasma is mostly water. Oxygen Mostly attached Oxygen is nonpolar, and does not readily dissolve in to hemoglobin water.  Water is the Medium for Metabolism Metabolism is the complex network of interdependent and interacting chemical reactions occurring in living organisms. Enzymes are biological catalysts which speed up the rate of chemical reactions. Enzymes are involved in controlling metabolism. Metabolic reactions occur in aqueous solutions, with the reactants and enzymes dissolved in water.  Water Does Not Dissolve Hydrophobic Substances Hydrophobic substances are not charged and do not readily mix with water. Some molecules found in living organisms are hydrophobic, and do not dissolve in water. The functions of these substances depend on them being hydrophobic. Examples: Lipids are hydrophobic, allowing them to be used as an energy store without significantly changing the water potential of cells. Lipid hormones are hydrophobic, and are able to pass through the phospholipid bilayer of cell membranes.  Enzymes, which are biological catalysts, depend on water to function. Some proteins have both hydrophilic and hydrophobic sections that allow them to interact with both soluble and insoluble substances in the surrounding cell fluids. Fiedler et al., Cell Mol Life Sci, 2010, 67:1779-1798, © Springer Basel. A1.1.6: Physical properties of water and the consequences for animals in aquatic habitats Include buoyancy, viscosity, thermal conductivity and specific heat capacity. Contrast the physical properties of water with those of air and illustrate the consequences using examples of animals that live in water and in air or on land, such as the black-throated loon (Gavia arctica) and the ringed seal (Pusa hispida). A1.1.6 Physical properties of Water PROPERTY WATER AIR Objects in water float Objects in air have little upward BUOYANCY because water exerts an force upward force Water resists objects moving Air is less resistant to objects VISCOSITY through it moving through it Water can transfer heat Air does not transfer heat as THERMAL easily easily as water Water can absorb or give off Air changes temperature easily SPECIFIC HEAT heat with little change in and rapidly due to weather temperature events  Physical Properties of Water: Buoyancy Buoyancy is the upward force exerted by a fluid (liquid or gas) on an object immersed in the fluid. Liquid water is a denser fluid than air, providing greater buoyancy for aquatic animals and allowing them to float or swim more easily. This helps many aquatic animals conserve energy and allows them to stay afloat without expending a lot of effort. The ringed seal has a layer of blubber that provides buoyancy Ringed seal and helps it float on the surface of the water.  Physical Properties of Water: Viscosity Viscosity is the measure of a fluid's resistance to flow. The higher the viscosity, the more difficult it is for animals to move through the fluid. Water has a higher viscosity than air, and many aquatic animals, such as the black-throated loon, have a streamlined body shape which allows them to smoothly move through water. Black-Throated Loon  Physical Properties of Water: Thermal Conductivity Thermal conductivity is a measure of a material's ability to conduct heat. Water has a much higher thermal conductivity than air. Aquatic animals are more likely to lose heat to the environment, and must be adapted to reduce heat loss. Ringed seals have a layer of insulating blubber to prevent heat loss. Seals will huddle together out of the water (on land), decreasing overall exposed surface area and reducing heat loss. Black-throated loons are insulated by feathers which are coated with a hydrophobic oil to keep the feathers dry, which reduces heat loss.  Physical Properties of Water: Specific Heat Capacity Specific heat capacity is the energy required to raise the temperature of 1g of a substance by 1 K (or 1°C). Water has a very high specific heat capacity (4.186 J/g°C), as energy is required to break the hydrogen bonds between water molecules. The specific heat capacity of air is much lower than water. The temperature of large bodies of water is much more stable than air temperature, as the water can absorb or release much more heat energy without experiencing significant temperature change. Animals cells (which are primarily composed of water) are also resistant to temperature change, which helps maintain constant body temperature in endotherms such as birds and mammals. Physical Properties of Water - Ringed Seal Explain the consequences of the physical properties of water for the ringed seal (buoyancy, viscosity, thermal conductivity and specific heat capacity).  Physical Properties of Water - Ringed Seal Ringed seals (Pusa hispida) live in the arctic seas. Buoyancy: The ringed seal has a layer of blubber which allows it to remain buoyant while in the sea, reducing the energy required to swim in the sea. Viscosity: The streamlined body shape of the seals allow them to efficiently move through water. The flippers use drag to facilitate movement. Thermal conductivity: The seal’s layer of blubber insulates the animal when in water. Seals tend to huddle, reducing exposed surface area and heat loss when on land. Specific heat capacity: Ringed seals are endotherms and are adapted to maintain a constant body temperature. The high specific heat capacity of the water in their  Physical Properties of Water - Black-Throated Loon Explain the consequences of the physical properties of water for the black-throated loon (buoyancy, viscosity, thermal conductivity and specific heat capacity).  Physical Properties of Water - Black-Throated Loon Black-throated loons are migratory aquatic birds found in the Northern Hemisphere. Buoyancy: The black-throated loon is able to adjust its density by changing the volume of air in its air sacs. This allows the bird to control its depth when swimming in water. The black-throated loon is able to increase its buoyancy in air by increasing its surface area by spreading its wings. The bones of birds are hollow, further reducing their density and increasing the buoyancy of the black-throated loon, allowing it to fly.  Physical Properties of Water - Black-Throated Loon Viscosity: The streamlined shape of black-throated loons allow them to efficiently move through air and water. The webbed feet help the birds to move through water. Thermal conductivity: The black-throated loon is an endotherm that maintains a constant body temperature. The bird’s feathers trap air and provide insulation. The feathers are covered in a hydrophobic oil which keeps the feathers dry. Specific heat capacity: the high specific heat capacity helps the black-throated loon to regulate its body temperature, which is especially important when it is diving into cold water.  Review and Discuss: SL & HL Key Terms Xylem Hydrogen bonds Buoyancy Surface tension Polar covalent bonds Viscosity Capillary action Cohesion Thermal conductivity Hydrophobic Adhesion Specific heat capacity Hydrophilic Solvent Apoplast pathway Metabolism  A1.1 Water - IB Linking Questions How do the various intermolecular forces of attraction affect biological systems? What biological processes only happen at or near surfaces? Exam question Explain how the chemical bonding in water makes water a valuable coolant in living organisms (2 marks)