Water’s Chemical Power: Polarity, Dissolution, and Ions
Water is a polar molecule composed of two hydrogen atoms covalently bonded to one oxygen atom. The water molecule has a bent molecular geometry. The geometry of the molecule leads to a nonuniform distribution of charge throughout the molecule leaving the molecule with a positive end and a negative end, also known as poles. Oxygen has a stronger pull on the shared electrons in the bond with hydrogen, resulting in oxygen having a partial negative charge. This leaves the hydrogen atoms electron deficient which makes the hydrogen atoms in the molecule partially positive.1
The positive ends of the water molecule are attracted to the negative ends of adjacent water molecules. This attraction is called a hydrogen bond. Although it is not actually a covalent bond, it is a relatively strong intermolecular force that gives water many of its unique properties.
Even pure water contains small quantities of ions because water will auto-ionize. This means that one water molecule transfers a proton to another water molecule. This temporarily results in one hydronium (H3O+) ion and one hydroxide ion (OH-). This happens at relatively low quantities. Water at 25oC will have a concentration of hydronium ion, [H3O+], of 1.0x10-7 M.2
One important result of water’s polarity is its ability to act like a solvent. The polar nature of the water molecule allows it to dissolve compounds. An example of a substance able to dissolve in water is table salt, also known as sodium chloride. Sodium chloride is a lattice crystal of alternating positive sodium ions and negative chloride ions. As sodium chloride dissolves, the ions separate and the positive ends of the water molecule will surround the negative chloride ions, and the negative ends of the water molecule will surround the positive sodium ions. The ions can now move freely throughout the water. The resulting mixture, water and the dissolved ions, is known as an aqueous solution.1
Dissolved substances in an aqueous solution are called solutes. In the previous example, sodium chloride is the solute in the solution. Solutes that dissociate, or split up, into charged particles are also known as electrolytes. Electrolyte solutions are able to conduct electricity. Not every solution is able to conduct electricity. Sucrose, or sugar, readily dissolves in water, but it is not an electrolyte, and cannot conduct electricity. Measuring the conductivity of water can indicate the presence of dissolved ions but does not identify which ions are present.
Measuring What Is in Water
There are many ways to measure the composition of an aqueous solution, but two simple measurements that can be done in the classroom are pH and conductivity. pH is the concentration of hydronium ions (H3O+) which is related to how acidic or basic a solution is. pH can be measured using an electronic probe or using color changing pH paper.3 Conductivity can be measured using a simple conductivity tester or an electronic conductivity probe. In this unit, a simple LED conductivity tester is used because it gives students a clear visual indication of whether a solution conducts electricity. If the LED lights, the solution contains mobile charged particles that allow electric current to move through the solution. If the LED does not light, the solution either contains few dissolved ions or contains dissolved particles that are not charged, such as sugar molecules.1 More advanced conductivity probes can provide numerical data, but the LED tester is sufficient for helping students compare ionic and molecular solutions. These tests are useful for determining whether dissolved ions are likely present, but they do not identify which ions are present or whether the water is safe to drink.
The amount of solute in a solution can be variable and impacted by many factors; therefore, it is important to specify the relative quantities. This measurement is called concentration and there are many ways in which concentration can be measured. The choice of measurement depends on the scale and the substance being measured. Examples of common concentration measurements include molarity (M), milligrams per liter (mg/L), parts per million, billion, and trillion (ppm, ppb, ppt). This unit will primarily focus on mg/L, ppm, ppb, and ppt.
pH is a logarithmic measure related to the concentration of hydronium ions, H₃O⁺, in solution. A lower pH indicates a higher hydronium ion concentration, while a higher pH indicates a more basic or alkaline solution.
Local Drinking Water and Public Water Reports
The South-Central Connecticut Regional Water Authority (RWA) releases an annual water quality report to the public. This report functions as a local example of how water quality is measured, regulated, and communicated to a community. It provides information about where drinking water comes from, what substances are detected in the water, how those substances are measured, and whether the reported values meet state and federal standards. For this unit, the report is valuable because it gives students a concrete example of water quality being evaluated through repeated testing, concentration data, regulatory limits, and public communication.4
The report also connects drinking water to Earth systems. RWA water begins as precipitation that either flows through watersheds into reservoirs or soaks into the ground and becomes part of aquifers. The system that serves our local community includes ten active reservoirs and three aquifers, with most of the region’s water coming from reservoirs and a smaller portion pumped from wells. This makes the report a useful local source for understanding reservoirs, aquifers, groundwater, watersheds, and source-water protection. Drinking water is not separate from the environment; it is water that has moved through natural systems before being treated and distributed through public infrastructure.5
The RWA report also shows that drinking water is both natural and engineered. Source water is protected through watershed and aquifer management, treated through processes such as coagulation, filtration, disinfection, and corrosion control, and then distributed through pipes, pumping stations, and storage tanks. The report includes terminology such as Maximum Contaminant Level, Maximum Contaminant Level Goal, Action Level, milligrams per liter, parts per million, parts per billion, and parts per trillion. These terms help connect classroom chemistry to real-world water data. The report also reinforces an important scientific idea for students: detecting a substance in water does not automatically mean the water is unsafe. A meaningful water-quality claim requires knowing what was detected, at what concentration, compared to what standard, and with what limitations.
Natural Water Chemistry: Salinity, Geology, and Watersheds
Natural freshwater is not chemically pure water. In fact, the chemical contents of freshwater can vary greatly. The chemical composition of water depends on many factors, and geology is often the most important. Other sources of variation come from but are not limited to precipitation and human activities. Compared to salt water, fresh water will have much lower concentrations of dissolved ions. Fresh water should not be confused with pure water.
Salinity is the measurement of dissolved salt in water. Ocean water has a salinity of about 35 ppt, which means about 35 of 1,000 (3.5%) of the weight of seawater comes from the dissolved salts.6 Freshwater, on the other hand, has an average salinity of less than 1 ppt. In other words, ocean water is roughly 35 times saltier than freshwater. There are many dissolved ions in both saltwater and freshwater, but the dominant dissolved ions are sodium and chloride.7 Dissolved salts enter the ocean through weathering and runoff. Rain is slightly acidic and will erode rock and chemically dissolve soluble compounds. The dissolved solids then move through streams and rivers until it ultimately enters the ocean. As ocean water evaporates and more salt is continuously deposited into the oceans, the concentration of salt in ocean water increases.7
Water chemistry also varies because water does not all move through the environment in the same way. Surface water, groundwater, reservoir water, river water, and ocean water each have different chemical histories. Groundwater often spends more time in contact with soil, sediment, and bedrock than surface water, so it may dissolve more minerals before reaching a well, spring, stream, or reservoir. This is one reason why groundwater can have higher concentrations of dissolved ions than water that has moved quickly over the surface.9 The specific minerals present depend on local geology, which means that water from different places can have different chemical compositions even if it looks the same.
Hard water is a useful example of natural water chemistry. Water is described as hard when it contains relatively high concentrations of dissolved calcium and magnesium ions.10 These ions often enter water as it moves through rocks and sediments that contain soluble minerals. Hard water shows how geology can shape the properties of water. The effects of hard water are often noticed in everyday life through mineral deposits, soap scum, or changes in how soap lathers. In this way, hard water helps show that dissolved substances can affect the behavior of water even when those substances are invisible.
Watersheds provide a broader way to understand these differences in water chemistry. A watershed is the land area that drains into a common body of water, such as a stream, river, reservoir, harbor, or estuary. As water moves through a watershed, it interacts with soil, rock, vegetation, roads, buildings, storm drains, and human activities. Each of these interactions can affect what the water carries. A forested watershed may contribute to organic matter and naturally dissolved minerals, while an urban watershed may contribute salts, nutrients, oils, metals, or other substances from roads and developed land. A water sample can therefore be understood as a chemical record of the system through which it has moved.11
This idea is especially important in coastal Connecticut, where freshwater systems eventually connect to tidal rivers, harbors, and Long Island Sound. Rivers and streams carry freshwater and dissolved materials from inland watersheds toward brackish and saltwater environments. In places where freshwater mixes with seawater, salinity can change with tides, storms, seasons, and distance from the coast.12 Long Island Sound monitoring programs measure variables such as salinity, dissolved oxygen, pH, temperature, nutrients, and chlorophyll to understand how these changing conditions affect water quality and aquatic ecosystems.13 These local systems show that the distinction between freshwater and saltwater is a gradient shaped by movement, mixing, local geography, and biological activity.
Human Impacts, Living Systems, and Evidence-Based Decisions
Human activities can change water chemistry by adding substances to the natural movement of water through a watershed. Roads, lawns, farms, wastewater systems, pipes, parking lots, storm drains, and industrial areas can all influence what enters surface water and groundwater. Some sources are easy to identify, such as a discharge pipe entering a river. Others are more diffuse, such as stormwater runoff moving across roads and lawns before entering a drain, stream, reservoir, or harbor. This distinction between point and nonpoint sources is important because many water-quality problems are not caused by one obvious source, but by many small inputs across a landscape. 14
Road salt is one of the clearest examples of a familiar human activity changing freshwater chemistry. Sodium chloride is spread on roads to improve winter safety, but when snow and ice melt, the salt dissolves into sodium and chloride ions. These ions can move with runoff into storm drains, streams, rivers, reservoirs, and groundwater. Because dissolved ions affect conductivity, conductivity measurements can be used as evidence that salt or other ions are present in water. Chloride is also useful as an indicator because it tends to remain dissolved and can move through water systems over time. 15
Road salt also illustrates why environmental decisions often involve tradeoffs. Salt is useful because it helps keep roads safer during winter weather, but increased salt concentrations can affect drinking water sources, infrastructure, soil, plants, and aquatic organisms. This makes road salt a strong example of a substance that is beneficial in one context but harmful in another. Communities must balance safety, cost, feasibility, infrastructure concerns, and environmental impact when deciding how to manage winter roads. Possible solutions include reducing application rates, calibrating equipment, using brine more strategically, improving storage practices, and monitoring local waterways.15
Changes in water chemistry also matter because aquatic organisms are adapted to particular chemical conditions. Freshwater organisms are not only living in water; they are living in a chemical environment. Changes in salinity, pH, dissolved oxygen, nutrient levels, or pollutant concentrations can affect survival, reproduction, growth, and ecosystem stability. For example, increased dissolved salts can stress organisms adapted to low-salinity environments. Changes in pH can affect biological processes and the availability of certain substances in water. These chemical changes may be invisible, but they can still have biological consequences. 16
Nutrients provide another example of the connection between chemistry and living systems. Nitrogen and phosphorus are necessary for plant and algal growth, but excessive nutrient input can contribute to eutrophication. When nutrients stimulate heavy algal growth, the eventual decomposition of that organic matter can consume dissolved oxygen. Low dissolved oxygen can stress or kill aquatic organisms, especially fish and other animals that depend on oxygen dissolved in the water. This connection between nutrients, algal growth, decomposition, and oxygen depletion helps students see how a chemical change can become an ecosystem-level problem. 17
Evidence-based decision-making is also important when evaluating public claims about water. People often use words such as clean, pure, safe, natural, healthy, chemical, and contaminated without defining them scientifically. Alkaline water is a useful example because it involves a real chemical measurement, pH, but consumer claims may go beyond what that measurement can prove. Similarly, the detection of a substance in water does not automatically mean the water is unsafe, and the absence of visible pollution does not mean water is chemically pure. Evaluating claims about water requires evidence, careful language, and an understanding of what each type of measurement can and cannot show.
