Why does water flow from low to high concentration? Shouldn't it be the reverse?

Why does water flow from low to high concentration? Shouldn't it be the reverse?

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So my understanding of water flow is as follows:

Basically, water moves about randomly because it has thermal energy. It will occasionally hit other water molecules and "bounce" back. As such, in areas of high pressure, water bumps into other water molecules a lot. However, if a high pressure water tank were connected to a low pressure empty tank, some water would be hit in the direction of the empty tank, but it wouldn't get hit back. Over time, more and more water is hit over until the water pressure equalizes. A vacuum doesn't suck so much as not push back, if that makes sense. So my trouble with hypotonic/hypertonic solutions is as follows. In a hypotonic situation, the solute concentration outside the cell is lower than inside the cell. A hypertonic solution is the reverse. So a hypotonic solution has fewer water molecules, so you would expect water to flow out of the cell because it's not getting hit back as much. But it flows in. The reverse is true for hypertonic solutions. Why is this? Thanks!

A hypotonic solution has lower solute concentration outside the cell than inside the cell. In other words, the ratio of solute to water is higher inside the cell than outside. In order to equalize the concentrations, the solution inside the cell must be diluted, by drawing in water from outside the cell. A hypotonic solution has more water molecules per solute molecule than inside the cell, so water moves into the cell.

Your mistake is in thinking that a hypotonic solution has fewer water molecules than the cell. In a relative sense, it's the opposite - the hypotonic solution has a lower concentration than inside the cell, and therefore more water per solute than inside.

Diffusion and Osmosis

A water solution that contains nutrients, wastes, gases, salts and other substances surrounds cells. This is the external environment of a cell. The cell&rsquos outer surface of the plasma membrane is in contact with this external environment, while the inner surface is in contact with the cytoplasm. Thus, the plasma membrane controls what enters and leaves the cell.

The membrane permits the passage of some materials, but not all. The cell membrane is said to be selectively permeable. Small molecules, for example, may pass through the membrane. If no energy is required for substances to pass through the membrane, the process is called passive transport. We will discuss two examples of passive transport in this tutorial: diffusion and osmosis.

Although you may not know what diffusion is, you have experienced the process. Can you remember walking into the front door of your home and smelling a pleasant aroma coming from the kitchen? It was diffusion of molecules from the kitchen to the front door of the house that allowed you to detect the odors.

Diffusion is defined as the net movement of molecules from an area of greater concentration to an area of lesser concentration.

The molecules in a gas, a liquid or a solid are in constant motion due to their kinetic energy. Molecules are in constant movement and collide with each other. These collisions cause the molecules to move in random directions. Over time, however, more molecules will be propelled into the less concentrated area. Thus, the net movement of molecules is always from more tightly packed areas to less tightly packed areas. Many things can diffuse. Odors diffuse through the air, salt diffuses through water and nutrients diffuse from the blood to the body tissues.

This spread of particles through random motion from an area of high concentration to an area of lower concentration is known as diffusion. This unequal distribution of molecules is called a concentration gradient. Once the molecules become uniformly distributed, dynamic equilibrium exists. The equilibrium is said to be dynamic because molecules continue to move, but despite this change, there is no net change in concentration over time. Both living and nonliving systems experience the process of diffusion. In living systems, diffusion is responsible for the movement of a large number of substances, such as gases and small uncharged molecules, into and out of cells.

Figure (PageIndex<1>). (CC BY-NC-SA)


Osmosis is a specific type of diffusion it is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.

Semi-permeable membranes are very thin layers of material which allow some things to pass through them, but prevent other things from passing through. Cell membranes are an example of semi-permeable membranes. Cell membranes allow small molecules such as oxygen, water carbon dioxide and glucose to pass through, but do not allow larger molecules like sucrose, proteins and starch to enter the cell directly.

Figure (PageIndex<2>). (CC BY-NC-SA)

Example: If there was a semi-permeable membrane with more water molecules on one side as there were on the other, water molecules would flow from the side with a high concentration of water to the side with the lower concentration of water. This would continue until the concentration of water on both sides of the membrane were equal (dynamic equilibrium is established).

Figure (PageIndex<3>). (CC BY-NC-SA)

Osmotic Pressure
Adding sugars to water will result in a decrease in the water concentration because the sugar molecules displace the water molecules.

Figure (PageIndex<4>). osmotic pressure (CC BY-NC-SA LadyOfHats)

If the two containers are connected, but separated by a semi-permeable membrane, water molecules would flow from the area of high water concentration (the solution that does not contain any sugar) to the area of lower water concentration (the solution that contains sugar).

Figure (PageIndex<5>). osmotic pressure (CC BY-NC-SA LadyOfHats)

This movement of water would continue until the water concentration on both sides of the membrane is equal, and will result in a change in volume of the two sides. The side that contains sugar will end up with a larger volume.

Figure (PageIndex<6>). osmotic pressure (CC BY-NC-SA LadyOfHats)

Water solutions are very important in biology. When water is mixed with other molecules this mixture is called a solution. Water is the solvent and the dissolved substance is the solute. A solution is characterized by the solute. For example, water and sugar would be characterized as a sugar solution.

The classic example used to demonstrate osmosis and osmotic pressure is to immerse red blood cells into sugar solutions of various concentrations. There are three possible relationships that cells can encounter when placed into a sugar solution.

1. The concentration of solute in the solution can be equal to the concentration of solute in cells. In this situation the cell is in an isotonic solution (iso = equal or the same as normal). A red blood cell will retain its normal shape in this environment as the amount of water entering the cell is the same as the amount leaving the cell.

2. The concentration of solute in the solution can be greater than the concentration of solute in the cells. This cell is described as being in a hypertonic solution (hyper = greater than normal). In this situation, a red blood will appear to shrink as the water flows out of the cell and into the surrounding environment.

3. The concentration of solute in the solution can be less than the concentration of solute in the cells. This cell is in a hypotonic solution (hypo = less than normal). A red blood cell in this environment will become visibly swollen and potentially rupture as water rushes into the cell.

Part 1: Brownian Motion

In this part of the lab, you will use a microscope to observe Brownian motion in carmine red powder, which is a dye obtained from the pulverized guts of female cochineal beetles.


  • Glass slide
  • Toothpick
  • Carmine red powder
  • Coverslip
  • Tap water


  1. Obtain a microscope slide and place a drop of tap water on it.
  2. Using a toothpick, carefully add a very minuscule quantity of carmine red powder to the drop of water and add a coverslip.
  3. Observe under scanning, low, and then high power.

Lab Questions

  1. Describe the activity of the carmine red particles in water.
  2. If the slide were warmed up, would the rate of motion of the molecules speed up, slow down, or remain the same? Why?

Conductivity (Electrical Conductance) and Water

Water and electricity don't mix, right? Well actually, pure water is an excellent insulator and does not conduct electricity. The thing is, you won't find any pure water in nature, so don't mix electricity and water. Our Water Science School page will give you all the details.

Conductivity (Electrical Conductance) and Water

Multi-parameter monitor used to record water-quality measurements.

You're never too old to learn something new. All my life I've heard that water and electricity make a dangerous pair together. And pretty much all of the time that is true—mixing water and electricity, be it from a lightning bolt or electrical socket in the house, is a very dangerous thing to do. But what I learned from researching this topic was that pure water is actually an excellent insulator and does not conduct electricity. Water that would be considered "pure" would be distilled water (water condensed from steam) and deionized water (used in laboratories), although even water of this purity can contain ions.

But in our real lives, we normally do not come across any pure water. If you read our article about water being the "universal solvent" you know that water can dissolve more things than just about any other liquid. Water is a most excellent solvent. It doesn't matter if the water comes out of your kitchen faucet, is in a swimming pool or dog dish, comes out of the ground or falls from the sky, the water will contain significant amounts of dissolved substances, minerals, and chemicals. These things are the solutes dissolved in water. Don't worry, though—if you swallow a snowflake, it won't hurt you it may even contain some nice minerals your body needs to stay healthy.

Free ions in water conduct electricity

USGS employees electrofishing in the Frio River, Texas.

Water stops being an excellent insulator once it starts dissolving substances around it. Salts, such as common table salt (sodium chloride (NaCl)) is the one we know best. In chemical terms, salts are ionic compounds composed of cations (positively charged ions) and anions (negatively charged ions). In solution, these ions essentially cancel each other out so that the solution is electrically neutral (without a net charge). Even a small amount of ions in a water solution makes it able to conduct electricity (so definitely don't add salt to your "lightning-storm" bathwater). When water contains these ions it will conduct electricity, such as from a lightning bolt or a wire from the wall socket, as the electricity from the source will seek out oppositely-charged ions in the water. Too bad if there is a human body in the way.

Interestingly, if the water contains very large amounts of solutes and ions, then the water becomes such an efficient conductor of electricity that an electrical current may essentially ignore a human body in the water and stick to the better pathway to conduct itself—the masses of ions in the water. That is why the danger of electrocution in sea water is less than it would be in bathwater.

Lucky for hydrologists here at the USGS, water flowing in streams contains extensive amounts of dissolved salts. Otherwise, these two USGS hydrologists might be out of a job. Many water studies include investigating the fish that live in streams, and one way to collect fish for scientific study is to shoot an electrical current through the water to shock the fish ("zap 'em and bag 'em").


Humans cannot drink saline water, but, saline water can be made into freshwater, for which there are many uses. The process is called "desalination", and it is being used more and more around the world to provide people with needed freshwater.

Thirsty? How 'bout a cool, refreshing cup of seawater?

A floating solar still is used to desalinate small amounts of seawater, using evaporation and condensation.

No, don't take us literally! Humans cannot drink saline water. But, saline water can be made into freshwater, which is the purpose of this portable, inflatable solar still (it even wraps up into a tiny package). The process is called desalination, and it is being used more and more around the world to provide people with needed freshwater. Most of the United States has, or can gain access to, ample supplies of freshwater for drinking purposes. But, freshwater can be in short supply in many parts of the Nation and world. And, as the population continues to grow, shortages of freshwater will occur more often, if only in certain locations. In some areas, salt water (from the ocean, for instance) is being turned into freshwater for drinking.

The "simple" hurdle that must be overcome to turn seawater into freshwater is to remove the dissolved salt in seawater. That may seem as easy as just boiling some seawater in a pan, capturing the steam and condensing it back into water (distillation). Other methods are available but these current technological processes must be done on a large scale to be useful to large populations, and the current processes are expensive, energy-intensive, and involve large-scale facilities.

What makes water saline?

What do we mean by "saline water?" Water that is saline contains significant amounts (referred to as "concentrations") of dissolved salts. In this case, the concentration is the amount (by weight) of salt in water, as expressed in "parts per million" (ppm). If water has a concentration of 10,000 ppm of dissolved salts, then one percent of the weight of the water comes from dissolved salts.

Here are our parameters for saline water:

  • Freshwater - Less than 1,000 ppm
  • Slightly saline water - From 1,000 ppm to 3,000 ppm
  • Moderately saline water - From 3,000 ppm to 10,000 ppm
  • Highly saline water - From 10,000 ppm to 35,000 ppm

By the way, ocean water contains about 35,000 ppm of salt.

A view across a reverse osmosis desalination plant in Barcelona, Spain.

The worldwide need for freshwater

The scarcity of freshwater resources and the need for additional water supplies is already critical in many arid regions of the world and will be increasingly important in the future. Many arid areas simply do not have freshwater resources in the form of surface water such as rivers and lakes. They may have only limited underground water resources, some that are becoming more brackish as extraction of water from the aquifers continues. Solar desalination evaporation is used by nature to produce rain, which is the main source of freshwater on earth.

Another method: Reverse osmosis

Another way saline water is desalinized is by the "reverse osmosis" procedure. In most simplistic terms, water, containing dissolved salt molecules, is forced through a semipermiable membrane (essentially a filter), in which the larger salt molecules do not get through the membrane holes but the smaller water molecules do.

Reverse osmosis is an effective means to desalinate saline water, but it is more expensive than other methods. As prices come down in the future the use of reverse osmosis plants to desalinate large amounts of saline water should become more common.

Desalination is not modern science

Distillation desalination is one of mankind's earliest forms of water treatment, and it is still a popular treatment solution throughout the world today. In ancient times, many civilizations used this process on their ships to convert sea water into drinking water. Today, desalination plants are used to convert sea water to drinking water on ships and in many arid regions of the world, and to treat water in other areas that is fouled by natural and unnatural contaminants. Distillation is perhaps the one water treatment technology that most completely reduces the widest range of drinking water contaminants.

In nature, this basic process is responsible for the water (hydrologic) cycle. The sun supplies energy that causes water to evaporate from surface sources such as lakes, oceans, and streams. The water vapor eventually comes in contact with cooler air, where it re-condenses to form dew or rain. This process can be imitated artificially and more rapidly than in nature, using alternative sources of heating and cooling.

You can try this at home!

  • Dig a pit in the ground
  • Place a bowl at the bottom of the pit that will be used to catch the condensed water
  • Cover the pit loosley with a plastic sheet (you can use stones or other heavy objects to hold it in place over the pit)
  • Be sure that the lowest part of the plastic sheet hovers directly over the bowl
  • Leave your water "trap" overnight and water can be collected from the bowl in the morning

Your own personal desalination plant

Remember looking at the picture at the top of this page of a floating solar still? The same process that drives that device can also be applied if you find yourself in the desert in need of a drink of water.

The low-tech approach to accomplish this is to construct a "solar still" which uses heat from the sun to run a distillation process to cause dew to form on something like plastic sheeting. The diagram to the right illustrates this. Using seawater or plant material in the body of the distiller creates humid air, which, because of the enclosure created by the plastic sheet, is warmed by the sun. The humid air condenses water droplets on the underside of the plastic sheet, and because of surface tension, the water drops stick to the sheet and move downward into a trough, from which it can be consumed.

Some desalination facts

  • It is estimated that some 30% of the world's irrigated areas suffer from salinity problems and remediation is seen to be very costly.
  • According to the International Desalination Association, in June 2015, 18,426 desalination plants operated worldwide, producing 86.8 million cubic meters per day, providing water for 300 million people. This number increased from 78.4 million cubic meters in 2013, a 10.71% increase in 2 years.
  • The most important users of desalinated water are in the Middle East, (mainly Saudi Arabia, Kuwait, the United Arab Emirates, Qatar and Bahrain), which uses about 70% of worldwide capacity and in North Africa (mainly Libya and Algeria), which uses about 6% of worldwide capacity.
  • Among industrialized countries, the United States is one of the most important users of desalinated water, especially in California and parts of Florida. The cost of desalination has kept desalination from being used more often.

To further your knowledge about desalination/desalination plants please visit the links below.

*Some of this information came from the Water Education Foundation and from the Corpus Christi TAMU-CC Public Administration.

Why is osmotic pressure able to raise a water level against gravity?

My understanding of diffusion is that all the particles in a fluid randomly move around within the fluid volume. Eventually they are evenly spread out, reaching equilibrium. Models of such systems are defined by concentration gradients of the particles of interest.

I thought that the idea of the concentration gradients being the "driving force" of diffusion was just a convenient way to intuitively grasp the process of diffusion, and that such a force doesn't really exist, because the particles are randomly moving and their diffusion is statistical.

However, in the case of a semi-permeable membrane separating a high-salt concentration aqueous solution from a low concentration one, the diffusion of water across the membrane actually forces the solution levels on either side to be uneven, like in this illustration. I would have assumed that, if the diffusion of water across the membrane were just statistical chance, that the hydrostatic pressure would be able to keep the levels equal by pushing the water molecules back across the membrane once they started to raise the liquid level. Instead, the "osmotic pressure" must actually exist as a real pressure and balance the hydrostatic pressure of the raised water level.

What molecular interactions between the water and the solutes, or whatever else, causes the system to come to this equilibrium state?

Hormonal Influence on Reabsorption of Water

The renal medulla has a concentration gradient with a low osmolarity superficially and a high osmolarity at its deepest point. The kidneys have expended a large amount of cellular energy to create this gradient, but what do the nephrons do with this gradient? In the presence of hormones, the kidney is able to concentrate the filtrate to be 20 times more concentrated than the glomerular plasma and PCT filtrate.

The process of concentrating the filtrate occurs in the DCT and collecting ducts. Recall that the DCT and collecting ducts are lined with simple cuboidal epithelium with receptors for aldosterone and ADH, respectively. Solutes move across the membranes of the cells of the DCT and collecting ducts, which contain two distinct cell types, principal cells and intercalated cells. A principal cell possesses channels for the recovery or loss of sodium and potassium. An intercalated cell secretes or absorbs acid or bicarbonate. As in other portions of the nephron, there is an array of micromachines (pumps and channels) on display in the membranes of these cells.

Regulation of urine volume and osmolarity are major functions of the collecting ducts. By varying the amount of water that is recovered, the collecting ducts play a major role in maintaining the body’s normal osmolarity. If the blood becomes hyperosmotic, the collecting ducts recover more water to dilute the blood if the blood becomes hyposmotic, the collecting ducts recover less of the water, leading to concentration of the blood. Another way of saying this is: If plasma osmolarity rises, more water is recovered and urine volume decreases if plasma osmolarity decreases, less water is recovered and urine volume increases. This function is regulated by the posterior pituitary hormone ADH (vasopressin). With mild dehydration, plasma osmolarity rises slightly. This increase is detected by osmoreceptors in the hypothalamus, which stimulates the release of ADH from the posterior pituitary. If plasma osmolarity decreases slightly, the opposite occurs.

**EDITOR’S NOTE: Add figure like 25.19 from Marieb’s 10th edition to show production of concentrated and dilute urine**

When stimulated by ADH, the principal cells of the collecting duct will insert aquaporin channels proteins into their apical membranes. Recall that aquaporins allow water to pass from the duct lumen across the lipid-rich, hydrophobic cell membranes to travel through the cells and into the interstitial spaces where the water will be recovered by the vasa recta. As the ducts descend through the medulla, the osmolarity surrounding them increases (due to the countercurrent mechanisms described above). If aquaporin water channels are present, water will be osmotically pulled from the collecting duct into the surrounding interstitial space and into the peritubular capillaries. This process allows for the recovery of large amounts of water from the filtrate back into the blood, which produces a more concentrated urine. If less ADH is secreted, fewer aquaporin channels are inserted and less water is recovered, resulting in dilute urine. By altering the number of aquaporin channels, the volume of water recovered or lost is altered. This, in turn, regulates the blood osmolarity, blood pressure, and osmolarity of the urine.

**EDITOR’S NOTE: Add figure like 24.18 C and 24.19 to show aquaporin, Na+ channels, and Na+/K+ ATPase pump additions to DCT and CD. These figures are from McKinley 2nd ed.**

As Na + is pumped from the filtrate, water is passively recaptured for the circulation this preservation of vascular volume is critically important for the maintenance of a normal blood pressure. Aldosterone is secreted by the adrenal cortex in response to angiotensin II stimulation. As an extremely potent vasoconstrictor, angiotensin II functions immediately to increase blood pressure. By also stimulating aldosterone production, it provides a longer-lasting mechanism to support blood pressure by maintaining vascular volume (water recovery).

In addition to receptors for ADH, principal cells have receptors for the steroid hormone aldosterone. While ADH is primarily involved in the regulation of water recovery, aldosterone regulates Na + recovery. Aldosterone stimulates principal cells to manufacture luminal Na + and K + channels as well as Na + /K + ATPase pumps on the basal membrane of the cells of the DCT and collecting duct. When aldosterone output increases, more Na + is recovered from the filtrate and water follows the Na + passively. The movement of Na + out of the lumen of the collecting duct creates a negative charge that promotes the movement of Cl – out of the lumen into the interstitial space by a paracellular route across tight junctions. Peritubular capillaries (or vasa recta) receive the solutes and water, returning them to the circulation. As the pump recovers Na + for the body, it is also pumping K + into the filtrate, since the pump moves K + in the opposite direction.

Chapter Review

The kidney regulates water recovery and blood pressure by producing the enzyme renin. It is renin that starts a series of reactions, leading to the production of the vasoconstrictor angiotensin II and the salt-retaining steroid aldosterone. Water recovery is also powerfully and directly influenced by the hormone ADH. Even so, it only influences the last 10 percent of water available for recovery after filtration at the glomerulus, because 90 percent of water is recovered before reaching the collecting ducts. Depending on the body’s fluid status at any given time, the collecting ducts can recover none or almost all of the water reaching them.

The descending and ascending limbs of the loop of Henle consist of thick and thin segments. Absorption and secretion continue in the DCT but to a lesser extent than in the PCT. Each collecting duct collects forming urine from several nephrons and responds to the posterior pituitary hormone ADH by inserting aquaporin water channels into the cell membrane to fine tune water recovery.

The ascending loop is impervious to water but actively recovers Na + , reducing filtrate osmolarity to 50–100 mOsmol/kg. The descending and ascending loop and vasa recta form a countercurrent multiplier system to increase Na + concentration in the kidney medulla. The collecting ducts actively pump urea into the medulla, further contributing to the high osmotic environment. The vasa recta recover the solute and water in the medulla, returning them to the circulation. Nearly 90 percent of water is recovered before the forming urine reaches the DCT, which will recover another 10 percent.

Review Questions

1. Aquaporin channels are only found in the collecting duct.

3. The fine tuning of water recovery or disposal occurs in ________.

  1. the proximal convoluted tubule
  2. the collecting ducts
  3. the ascending loop of Henle
  4. the distal convoluted tubule

Critical Thinking Questions

1. Which vessels and what part of the nephron are involved in countercurrent multiplication?

2. Give the approximate osmolarity of fluid in the proximal convoluted tubule, deepest part of the loop of Henle, distal convoluted tubule, and the collecting ducts.

Why does water flow from low to high concentration? Shouldn't it be the reverse? - Biology

Percent Difference in Mass Based on Sucrose Solution Concentration

The process of osmosis was examined through this experiment using dialysis tubing and potato cores. By filling dialysis tubing with different concentrations of sucrose solution and leaving them in water over a period of time a pattern could be observed. Using this information, another experiment was conducted with potato cores submerged in sucrose solution to further examine osmosis. There were no direct correlations between the findings of both experiments. Therefore, the effect of osmosis is uncorrelated with the kind of solute, but only with the water content of the two solutions.

Both labs conducted tested the percent change in mass when osmosis occured. The first lab used dialysis tubes with a variety of sucrose concentrations. When placed in a beaker of water, the tubes were expected to swell with water. This is known as a hypotonic solution. The second lab tested the osmosis from a beaker of water to a small wedge of potato with different concentrations of water. The concentrations were unknown and it was up to us to infer which one was which.

Diffusion is the movement of particles from an area of high concentration to low concentration. A more specified form of diffusion is osmosis , what was primarily focused on in this lab, and it is the movement of water across a membrane, again from an area of high concentration to low concentration.

The main objective of this lab was to obtain a better understanding of osmosis by seeing it in action. Our hypothesis was that if a solute has a has a high concentration, it will gain more water than if the solute had a lower concentration.

Both of the experiments in this lab report were conducted at New Tech High @ Coppell in Mrs. Wootton's AP Biology class. Our experiment was completed with the use of the beakers,dialysis tubing, sucrose solvents and potatoes provided by Mrs. Wootton. As the data was coming in it was immediately inserted into our data tables to ensure that no data is missing. After the data was completed and submitted the class data was available to compare the results of various groups in the class.

Overall, for the first lab our hypothesis was correct, as we submerged the dialysis tubing filled with various solvent concentrations the water would go inside the bag. Our results from the Distilled Water and 0.4 mol seem to be our only outliers in terms of the mass decreasing. After pulling out the distilled water dialysis tubing we observed a slight error in the tying of the knot which may have corrupted our results and allowed water to pass through the top. Besides our two outliers the data almost forms a linear trend, as the sucrose increases then then so does the final mass of the bag. Our results may also be slightly skewed due to the time constraints that we were facing.

In the Potato Core experiment the majority of our trials were fairly consistent. Aligning with our predictions the potato in Beaker #4 gained weight of 0.24 g therefore deducting that Beaker #4 is filled with distilled water. Beaker #1 was the most significant change with a percent difference of 42.5 making it almost half of the potatoes mass. From the data obtained we can concur that the most likely result of the loss of mass is due to the water leaving the potato.

Osmosis is the diffusion of water, in which water moves to areas with high solute concentrations. This finding agrees with our hypothesis that the semi permeable bag with the 1 mol of sugar will have the most water movement, and will gradient down to the 0 mol (distilled water), which will have no water movement. All the water movement will be water going inside the bag.

Our results were not what we expected though. The highest percentage of mass change was in the distilled water. However, we realized that we had a few errors occur in tying our dialysis tubing correctly, which is what caused some of the results to be skewed.

The purpose of this lab was to test osmosis through two mediums. Although the results contained error the initial hypothesis remains true. The degree to which osmosis occurs is directly related to the molarity of the substances involved. Water molecules transfer through permeable membranes until both sides are equal, this means that the hypotonic to hypertonic flow is directly proportional to the mass increase.

How the Rate of Reaction Changes

In a typical chemical reaction, several substances react to form new products. The substances may be brought together as gases, liquids or in solution, and how much of each reactant is present affects how fast the reaction proceeds. Often there is more than enough of one reactant, and the rate of the reaction depends on the other reactants present. Sometimes the rate of reaction can depend on the concentration of all the reactants, and sometimes catalysts are present and help determine the speed of the reaction. Depending on the specific situation, changing the concentration of one reactant may have no effect.

For example, in the reaction between magnesium and hydrochloric acid, the magnesium is introduced as a solid while the hydrochloric acid is in solution. Typically the acid reacts with magnesium atoms from the metal, and as the metal is eaten away, the reaction proceeds. When more hydrochloric acid is in solution and the concentration is higher, more hydrochloric acid ions eat away at the metal and the reaction speeds up.

Similarly, when calcium carbonate reacts with hydrochloric acid, increasing the concentration of the acid speeds up the rate of reaction as long as enough calcium carbonate is present. The calcium carbonate is a white powder that mixes with water but does not dissolve. As it reacts with the hydrochloric acid, it forms soluble calcium chloride and carbon dioxide is given off. Increasing the concentration of calcium carbonate when there is already a lot in the solution will have no effect on the rate of reaction.

Sometimes a reaction depends on catalysts to proceed. In that case, changing the concentration of the catalyst can speed up or slow down the reaction. For example, enzymes speed up biological reactions, and their concentration affects the rate of reaction. On the other hand, if the enzyme is already fully used, changing the concentration of the other materials will have no effect.


In this experiment, a calibration curve was created by plotting absorbance vs. concentration in Excel. The calibration curve was constructed by measuring the absorbance rate of phosphate in five standard solutions.

The linear equation derived from the calibration curve was then manipulated and used to determine the concentration of phosphate in soda pop, and in an unknown water solution. The concentration of phosphate was experimentally determined to be 0.006834 M in Cola, and 1.41 x 10 -4 M in an unknown water sample.

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