Water is most dense at temperatures near freezing. When water freezes, however, it expands, becoming less dense. If equal volumes of cold water (down to 4deg. F) and hot water are compared, cold water weighs more than hot water.

Salt water is more dense than fresh water Density = mass/volume. Increasing the mass by adding salt increases the density. Seawater is a little bit more dense than fresh water so it sinks beneath freshwater.

Atlantic

When water is cooled, the water molecules move slower and get closer together. This makes cold water more dense than room temperature water.

Heating a substance causes molecules to speed up and spread slightly further apart, occupying a larger volume that results in a decrease in density. Hot water is less dense and will float on room-temperature water. Cold water is more dense and will sink in room-temperature water.

Because there’s more space between the molecules, a volume of hot water has fewer molecules in it and weighs a little bit less than the same volume of cold water. So hot water is less dense than cold water.

It may not quench your thirst completely. Moreover, hot water can sometimes risk your esophageal health and can even harm your tongue. If you don’t like drinking hot water, mix both cold and hot water in equal amounts and guzzle it down at room temperature.

Any object or substance that is less dense than a fluid will float in that fluid, so hot water rises (floats) in colder water. When fluids are cooled, they contract and therefore become more dense. Any object or substance that is more dense than a fluid will sink in that fluid, so cold water sinks in warmer water.

Sugar dissolves faster in hot water than it does in cold water because hot water has more energy than cold water. As they move faster, they come into contact with the sugar more often, causing it to dissolve faster.

At higher temperatures, the average kinetic energy of particles is higher, and the particles in the substance are moving around more. In the cold water, the particles move slower and interact at a slower rate. Therefore, the sugar cube will dissolve the slowest in the cold water.

The rate of dissolving depends on the surface area (solute in solid state), temperature and amount of stirring.

In a solution, the solute is the substance that dissolves, and the solvent is the substance that does the dissolving. For a given solvent, some solutes have greater solubility than others. For example, sugar is much more soluble in water than is salt.

There is no obvious difference between the amount of salt that dissolves in the hot water compared to the cold water. This shows that temperature affects the dissolving of sugar more than it affects the dissolving of salt.

Cold water can hold more dissolved oxygen than warm water. In winter and early spring, when the water temperature is low, the dissolved oxygen concentration is high. In summer and fall, when the water temperature is high, the dissolved-oxygen concentration is often lower.

Cold water

Fast-moving water generally has more oxygen than still water, because the movement mixes the air into the water. Water with lots of aquatic plants have higher levels of dissolved oxygen, since submerged plants produce oxygen through photosynthesis.

The temperature and salinity of water influence how much oxygen it can hold. Freshwater can hold more dissolved oxygen than saltwater because saltwater has less space for oxygen molecules due to the sodium and chloride ions it contains.

Concentrations above this level can be harmful to aquatic life. Fish in waters containing excessive dissolved gases may suffer from “gas bubble disease”; however, this is a very rare occurrence. Oxygen levels that remain below 1-2 ppm for a few hours can result in large fish kills.

Water at lower temperatures should have higher mg/L of dissolved oxygen and higher %DO while warmer, polluted waters will have lower mg/L and %DO. Healthy water should generally have dissolved oxygen concentrations above 6.5-8 mg/L and between about 80-120 %.

coral reefs

Air and Water Temperature Increases Lower levels of dissolved oxygen due to the inverse relationship that exists between dissolved oxygen and temperature. As the temperature of the water increases, dissolved oxygen levels decrease.

Simple aeration or agitation can increase dissolved oxygen enough to prevent problems. Injecting air or, especially, pure oxygen can increase levels as well, but only as high as saturation levels. Paying attention to temperature can also help improve DO, as colder water can hold more oxygen.

Oxygen is added to water by: Re-aeration: Oxygen from air is dissolved in water at its surface, mostly through turbulence. Examples of this include: Water tumbling over rocks (rapids, waterfalls, riffles) Wave action Photosynthesis (during daylight) Plants produce oxygen when they photosynthesize.

Dissolved oxygen levels can be measured by a basic chemical analysis method (titration method), an electrochemical analysis method (diaphragm electrode method), and a photochemical analysis method (fluorescence method). The diaphragm electrode method is the most widely used method.

Most dissolved oxygen is introduced into water through aeration, flowing over rocks, or as a waste product of photosynthesis. It is generally considered that DO levels of at least 4-5 PPM are sufficient for most aquatic life, however, good fishing waters average about 9.0 PPM.

Normally, oxygen solubility is strongly dependent on (1) the amount of dissolved electrolyte salt(s) (decreases at higher concentration of electrolyte), (2) temperature (decreases at higher temperatures), and (3) pressure (increases at higher pressure).

Calculate DO value of the sample. Remember that in 200 mL sample, 1 mL of sodium thiosulfate of 0.025N equals to 1 mg/L dissolved oxygen: =>Dissolved oxygen (DO) (in mg/L) = mL of sodium thiosulfate (0.025N) consumed.