Salt water plays a vital role in medicine, industry, research, and daily life. Its core properties stem from the ionic solution formed by the dissolution of sodium chloride (NaCl). Directly replacing salt water with water, while seemingly a simple change in composition, can trigger a series of chain reactions, potentially leading to functional failure or safety hazards.
Osmotic Pressure Imbalance: A "Life-or-Death Crisis" for Cells and Tissues
The most fundamental difference between salt water and water lies in osmotic pressure. Normal saline (0.9% NaCl solution) has an osmotic pressure equal to that of extracellular fluid in the human body and is considered an "isotonic solution." Pure water, on the other hand, has an osmotic pressure of zero, making it a "hypotonic solution." When water replaces salt water:
Cells absorb water and expand:
In a hypotonic environment, water molecules rapidly enter the cell through the cell membrane, causing the cell to expand. For example, red blood cells in pure water can rupture (hemolysis) due to excessive water absorption, releasing hemoglobin. Swelling nerve and muscle cells can compress surrounding tissues, causing dysfunction.
Tissue Edema and Damage:
If pure water is used for wound irrigation or nasal irrigation, electrolytes in body fluids will diffuse into the water due to the osmotic pressure difference, leading to local tissue dehydration and an increase in extracellular fluid volume, causing edema. Long-term use may delay wound healing and even cause tissue necrosis.
Medical Procedure Risks:
During intravenous infusion, if pure water is accidentally injected into a blood vessel, it can trigger hemodilution syndrome, causing red blood cell rupture and platelet aggregation, leading to thrombosis, shock, and even death. Historically, there have been cases of patients dying from the inadvertent administration of pure water.
Loss of Antibacterial Ability: Increased Risk of Infection
The antibacterial effect of salt water stems from two mechanisms:
Osmotic Inhibition of Bacteria:
High concentrations of salt water (such as 3%-5% NaCl) dehydrate bacterial cells and denature proteins, thereby inhibiting their growth and reproduction. For example, the ancient practice of salt water pickling exploited this principle. If water is used instead, bacteria can rapidly proliferate in an environment with a favorable osmotic pressure, increasing the risk of infection.
Ionic interference with metabolism:
The sodium and chloride ions in NaCl can destabilize bacterial cell membranes and interfere with enzyme activity. Pure water lacks these ions and cannot inhibit bacteria. In medical settings, such as when using pure water to clean open wounds, infection rates can be 3-5 times higher than with saline.
Changes in conductivity and corrosiveness: "Invisible killers" in industry and research
In industry and research, the conductivity and chemical properties of salt water are critical parameters. Substituting water for salt water can cause the following problems:
Sudden drop in conductivity:
The conductivity of salt water is 10⁶ times that of pure water (at 25°C, the conductivity of a 0.9% NaCl solution is approximately 15 mS/cm, while that of pure water is approximately 0.055 μS/cm). In processes such as electroplating and electrolysis, replacing salt water with water can dramatically increase resistance, resulting in reduced current efficiency and lower product yields. For example, in electrogalvanizing, using pure water can lead to uneven coating thickness and even prevent metal deposition.
Changes in Corrosiveness:
The Cl⁻ in salt water accelerates metal corrosion (for example, stainless steel corrodes 10 times faster in salt water than in pure water). Substituting it with pure water reduces corrosiveness, but the lack of corrosion-inhibiting ions (such as Na⁺) may lead to other problems. For example, in cooling systems, pure water may cause pitting corrosion, while salt water can mitigate corrosion by forming a protective film.
Uncontrolled Chemical Reactions:
In chemical experiments, salt water often serves as a solvent or catalyst. For example, in the Finkelstein reaction (halogen exchange reaction), NaCl promotes the dissolution of sodium iodide, increasing the reaction rate. Substituting it with water may prevent the reaction from proceeding or significantly reduce the yield.
Failure of Functional Scenarios: From everyday to professional use, a "total collapse"
Salt water has irreplaceable functions in specific scenarios, and replacing it with water will completely eliminate these functions:
Nasal irrigation and nebulization therapy:
Medical saline (such as 0.9% NaCl) moisturizes the nasal mucosa, thins mucus, and prevents irritation. Using pure water can cause mucosal dryness and bleeding due to osmotic pressure differences, and even trigger "rebound nasal congestion" (where congestion worsens after rinsing).
Salt spray testing:
In material corrosion resistance testing, salt spray testing requires a 5% NaCl solution to simulate marine environments. Substituting water for this solution prevents the formation of corrosive droplets, rendering the test results ineffective and potentially leading to premature product failure in actual use.
Cell culture and biological experiments:
Cell culture media require the addition of a certain concentration of NaCl to maintain osmotic pressure and ion balance. Using pure water to prepare the culture media can cause cell death due to osmotic pressure imbalance, rendering the experimental data completely unreliable.
Although water and salt water differ by only a single letter, they possess fundamental differences in osmotic pressure, antibacterial properties, conductivity, and functional properties. From the survival of cells at the cellular level to the efficiency and safety of industrial production to the accuracy of scientific research experiments, substituting water for this can have serious consequences. Therefore, in scenarios involving the human body, precision equipment, or critical processes, standard saline must be strictly used; in low-risk scenarios, if substitution is required, the osmotic pressure and ion concentration must be adjusted by adding electrolytes (such as NaCl, KCl) to ensure functionality and safety.
