CHEMISTRY GLOSSARY

thermal resistance    →   toplinski otpor

Heat always flows from a higher to a lower temperature level. The driving force for the heat flux lies in the temperature difference ΔT between two temperature levels. Analogous to Ohm’s law, the following holds:

where H = dQ/dt is heat flux, measured in watts, ΔT is temperature difference across the thermal resistance, measured in kelvin, and R_th is thermal resistance, measured in K/W.

For example, suppose there were two houses with walls of equal thickness; one is made of glass and the other of asbestos. On a cold day, heat would pass through the glass house much faster. The thermal restistance of asbestos is then higher than of glass.

If the thermal Ohm’s law is divided by the heat capacity C, Newton’s law of cooling is obtained:

where dT/dt is rate of cooling or heating, measured in K s^-1, and C is heat capacity, measured in J K^-1.

thermometer    →   termometar

Thermometers are devices for measuring temperature. Linear and volume thermal expansion are macroscopic properties of matter, which can be easily measured, relative to measurements of microscopic properties, on the basis of which, temperature is defined. Thermometers based on thermal expansion are secondary instruments that is, they have to be calibrated in comparison to a standard thermometer. In a thermometer with liquid, mercury or alcohol is placed in a small glass container. If temperature increases, the liquid undergoes volume expansion and rises in a capillary. The level of the raised liquid is the measure of temperature. Mercury thermometers measure temperatures in the temperature range between -39 °C and 300 °C. Alcohol thermometers measure lower temperatures. Bimetal thermometers have a spiral spring, which consists of two metals with different coefficients of linear expansion. When temperature changes, metals undergo different change in length and the consequence twisting of the spring is transferred to a pointer, the deflection of which is the measure of temperature.

Torricelli, Evangelista    →   Torricelli, Evangelista

Evangelista Torricelli (1852-1908) is Italian physicist and mathematician. He became the first scientist to create a sustained vacuum and to discover the principle of a barometer. He filled a tube three feet long, and hermetically closed at one end, with mercury and set it vertically with the open end in a basin of mercury, taking care that no air-bubbles should get into the tube. The column of mercury invariably fell to about twenty-eight inches, leaving an empty space above its level. He discovered that the variation of the height of the mercury from day to day was caused by changes in the atmospheric pressure. He also constructed a number of large objectives and small, short focus, simple microscopes.

U-tube    →   U-cijev

U-tube filled with a sodium hydroxide is used for a cleaned and dried gas.

U-tube filled with a 1 mol dm^-3 aqueous potassium nitrate solution is used as a permanent salt bridge.

U-tube is a device used for water electrolysis, for laboratory hydrogen and oxygen obtaining.

U-tube manometer    →   U-manometar

U-tube manometer contains water or mercury in a U-shaped tube, and is usually used to measure gas pressure. One end of the U tube is exposed to the unknown pressure field (P) and the other end is connected to a reference pressure source (usually atmospheric pressure) (P_ref), shown in the schematic below.

If fluid C is the atmosphere, fluid B is the liquid in the U tube (e.g. water or mercury), and fluid A is a gas, then we can assume that ρB >> ρA, ρC. The pressure contributed by the weight of gas within the U tube can therefore be neglected. The gage pressure of the gas can be approximated by,

visible radiation    →   vidljivo zračenje

Human eye can only see electromagnetic radiation of wavelengths form 400 nm to 760 nm. This narrow part of electromagnetic spectrum is called visible radiation. Visible (white) light is a mixture of light of all kind of colours, it can be separated, with the help of a glass prism, into its component colours - visible light spectrum, and each colour corresponds to a certain area of wavelengths:

volumetric flask    →   odmjerna tikvica

Volumetric flasks are bottles made of glass, in a pear like in shape with long thin necks and flat bottoms. All come with a ground glass stopper for a tight seal. Volume marking is cut in glass with fluoride acid around the neck, so that parallax should be avoided (flask is put in front of the eyes so that one can see only a straight horizontal line). A volumetric flask is calibrated to contain (TC or In) the indicated volume of water at 20 °C when the bottom of the meniscus is adjusted to just rest on the center of the line marked on the neck of the flask. They are used for preparing the exactly known volume of sample solution and standard solutions of reagents. On each flask with volume designation a temperature on which the flask has been calibrated is designated.

wash bottle    →   boca štrcaljka

Plastic wash bottle is a squeeze bottle made of low density polyethylene (LDPE) whose contents can be forced out through a narrow hole at the top by squeezing the bottle.

Glass wash bottle is a bottle fitted with two glass tubes pass through the cap, so that on blowing into one of the tubes a stream of water issuing from the other may be directed upon anything to be washed or rinsed, as a precipitate upon a filter.

water gas    →   vodeni plin

Water gas (blue gas, synthesis gas) is a fuel gas used in industrial synthesis of organic chemicals, and in welding, glassmaking, and other high-temperature industrial applications. Water gas is made by passing steam over a bed of hot coal or coke. It mainly consists of carbon monoxide (CO) and hydrogen (H₂), contaminated with small amounts of CO₂, N₂, CH₄, and O₂.

Wilson’s chamber    →   Wilsonova komora

Wilson’s chamber is used for detection of radioactive radiation. Wilson’s chamber has a glass cylinder filled with air that has been saturated with water vapour. Radioactive radiation in its way ionises molecules of gas which then function as centres on which water vapour condenses into very small drops, thereupon showing Tyndall’s effect, i.e. is they are visible as a bright trail.