The best way to visualize how water can have different densities is to look at the frozen form of water. Ice actually has a very different structure than liquid water, in that the molecules align themselves in a regular lattice rather than more randomly as in the liquid form. It happens that the lattice arrangement allows water molecules to be more spread out than in a liquid, and, thus, ice is less dense than water. Again, lucky for us, as we would not hear that delightful tinkle of ice cubes against the side of a glass if the ice in our ice tea sank to the bottom.
The density of ice is about 90 percent that of water, but that can vary because ice can contain air, too. That means that about 10 percent of an ice cube or iceberg will be above the water line. This property of water is critical for all life on earth. If water was most dense at the freezing point, then in winter the very cold water at the surface of lakes would sink, the lake could freeze from the bottom up. And, with water being such a good insulator due to its heat capacity , some frozen lakes might not totally thaw in summer.
The real-world explanation of water density is actually more complicated, as the density of water also varies with the amount of material that is dissolved in it. Water in nature contains minerals, gasses, salts, and even pesticides and bacteria, some of which are dissolved. As more material is dissolved in a gallon of water then that gallon will weigh more and be more dense— ocean water is denser than pure water.
We already said ice floats on water because it is less dense, but ice of a special kind can be denser than normal water. Heavy water, D 2 O instead of H 2 O, is water in which both hydrogen atoms have been replaced with deuterium, the isotope of hydrogen containing one proton and one neutron. Heavy water is indeed heavier than normal water which contains a tiny amount of heavy water molecules naturally , and heavy-water ice will sink in normal water. The instrument to measure the density of a liquid is called a hydrometer.
It is one of the simplest of scientific-measuring devices, and you can even make your own out of a plastic straws see links below. More often, though, it is made of glass and looks a lot like a thermometer. It consists of a cylindrical stem and a weighted bulb at the bottom to make it float upright. The hydrometer is gently lowered into the liquid to be measured until the hydrometer floats freely.
There are etched or marked lines on the device so the user can see how high or low the hydrometer is floating. In less dense liquids the hydrometer will float lower, while in more dense liquids it will float higher.
Since water is the "standard" by which other liquids are measured, the mark for water is probably labeled as "1. Hydrometers have many uses, not the least being to measure the salinity of water for science classes in schools. They are also used in the dairy industry to get estimates of the fat content of milk, as milk with higher fat content will be less dense than lower-fat milk. Studies have shown a direct relationship between metabolic rates and water temperature.
This occurs as many cellular enzymes are more active at higher temperatures This increase in metabolic rate can be handled by some species better than others. Increased metabolic function can be noticed in respiration rates and digestive responses in most species. Increased respiration rates at higher temperatures lead to increased oxygen consumption, which can be detrimental if rates remain raised for an extended period of time. Temperature fluctuations can also affect the behavior choices of aquatic organisms, such as moving to warmer or cooler water after feeding, predator-prey responses and resting or migrating routines Some species of sharks and stingrays will even seek out warmer waters when pregnant Plants are also affected by water temperature.
While some aquatic plants tolerate cooler waters, most prefer warmer temperatures While dormancy is appropriate for surviving a cold winter, warmer temperatures are required for most plants to flourish. Temperature can also inhibit plant respiration and photosynthesis In general, algal photosynthesis will increase with temperature, though different species will have different peak temperatures for optimum photosynthetic activity Above and below this temperature, photosynthesis will be reduced.
In addition to its effects on aquatic organisms, high water temperatures can increase the solubility and thus toxicity of certain compounds 1. These elements include heavy metals such as cadmium, zinc and lead as well as compounds like ammonia 19, This occurs because tissue permeability, metabolic rate and oxygen consumption all increase with increased water temperature Ammonia is known for its toxicity at high pH levels, but temperature can also influence acute and chronic criteria concentrations At low temperatures and a neutral pH, the following equation remains shifted to the left, producing the nontoxic ammonium ion:.
The solubility of oxygen and other gases will decrease as temperature increases 9. This means that colder lakes and streams can hold more dissolved oxygen than warmer waters. If water is too warm, it will not hold enough oxygen for aquatic organisms to survive. Water temperature can affect conductivity in two ways. As conductivity is measured by the electrical potential of ions in solution, it is affected by the concentration, charge and mobility of those ions Ionic mobility is dependent on viscosity, which is in turn dependent on temperature The more viscous it is, the less fluid it is; molasses and mercury are more viscous than water.
The inverse relationship between temperature and viscosity means that an increase in temperature will decrease viscosity A decrease in the viscosity of water increases the mobility of ions in water.
As such, an increase in temperature thus increases conductivity The second way that temperature can affect conductivity is through ionic concentration. Many salts are more soluble at higher temperatures As a salt dissolves, it breaks down into its respective ions. As warm water can dissolve several minerals and salts more easily than cold water, the ionic concentration is often higher 9.
TDS refers to all ion particles in solution that are smaller than 2 microns These salts and minerals enter the water from rocks and sediment in contact with it. As they dissolve and the ionic concentration increases, so will the conductivity of water. The rate at which conductivity increases is dependent on the salts present in solution In addition, there are a few salts that become less soluble at warmer temperatures, and thus will negatively affect conductivity The effect that temperature has on ORP values depends on the chemical species atoms, molecules and ions present in the solution Temperature-dependence data charts are usually available for calibration solutions, but not for field samples This lack of data is due to the difficulty in identifying and measuring every redoxing species that could be present in any given water source.
As these species are difficult to know and quantitatively define in environmental studies, most ORP electrodes will not automatically compensate for temperature. However, temperature can still alter a reading and should be recorded with each measurement considered when analyzing the data At a pH of 7, the hydrogen and hydroxyl ions have equal concentrations, 1 x M, keeping the solution neutral As the temperature increases or decreases, the ion concentrations will also shift, thus shifting the pH value Any change to a system at equilibrium, such as adding a reactant or altering the temperature, will shift the system until it reaches equilibrium again The equation:.
That means if the temperature of water increases, the equation will shift to the left to reach equilibrium again. A shift to the left decreases the ions in water, increasing the pH. Likewise, if the temperature were to decrease, the equation would shift to the right, increasing the ionic concentration and decreasing pH. However, that does not mean that temperature changes will make a solution more acidic or basic.
Because the ratio of hydrogen and hydroxyl ions remains the same, the acidity of water does not change with temperature Instead, the entire pH range shifts, so that neutral water will have a value other than 7. Water temperature and water density are directly related. As the temperature of water increases or decreases, it will alter the density of water. This is why ice expands and floats on water.
Pure water is also unique in that it achieves its maximum density, 1. The maximum density point is particularly important in freshwater. As surface water temperature approaches the maximum density temperature, it sinks and is replaced by warmer, lighter water This process continues until the water is uniformly cool.
The basic physics concept I hope almost every one might be aware of i. My question is why exactly at 4 degrees this phenomenon occur? Why not at some other temperature also what makes it to behave that way?. This shows that the expansion of liquid water takes place between 0 and 4 degrees C, and that when ice is formed from liquid water at exactly 0 degrees C, it expands.
This is because of the hydrogen bond in water molecules. In order to understand why it happens at 4 degree, you have to study the behavior of the hydrogen bond in water molecules. Water at 4 Celsius is more denser than any other temperature, because, water molecules jiggling and translational motion have greatly been hindered.
At 4 degree Celsius, water forms Hydrogen bonds, but time average picture shows that average number of water molecules for a given tiny volumes becomes larger than the zero degree Celsius, where H2O in ice forms nice symmetrical hexagonal structure. I hope this is helpful to get your head the question on water's density. Sign up to join this community. The best answers are voted up and rise to the top.
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