General Science

Duralumin is a strong, lightweight alloy composed primarily of aluminium (about 90-95%), with copper (3-5%) and magnesium (0.5-1%) as the main alloying elements. It was widely used in aircraft construction before modern aluminum alloys were developed. Bronze contains copper and tin, Magnalium contains aluminum and magnesium, and Monel contains nickel and copper.

Plants require sixteen essential elements for growth, but nitrogen (N), phosphorus (P), and potassium (K) are needed in the largest quantities - these are the primary macronutrients. Among all nutrients, nitrogen is typically required in the greatest amount as it's a crucial component of amino acids, proteins, chlorophyll, and nucleic acids. Calcium, ammonia (which provides nitrogen), and phosphorus are also important but not needed in quantities as large as nitrogen.

Ammonia (NH3) is one of the oldest and most widely used refrigerants due to its excellent thermodynamic properties and zero ozone depletion potential. It has high latent heat of vaporization, making it very efficient for heat absorption. Nitrogen and oxygen are not used as refrigerants in standard refrigeration systems, and chlorine is toxic and corrosive.

Brass is an alloy of copper and zinc. Bronze is primarily copper with tin. German silver (or nickel silver) contains copper, nickel, and zinc despite its name. Stainless steel is primarily iron (about 70-75%), chromium (12-30%), and nickel (0-20%) - it contains no copper, making it the correct answer.

Citrus fruits including lemons contain citric acid as their primary acid. Citric acid gives lemons their characteristic sour taste and can constitute up to 8% of the lemon's dry weight. Lactic acid is found in fermented milk products, tartaric acid in grapes, and acetic acid in vinegar.

When any solid material is heated to incandescence (glowing due to heat), it emits light according to blackbody radiation principles. The color depends on temperature - as temperature increases, the glow progresses from dull red to bright red, orange, yellow, white, and finally blue-white. A blue glass will still glow red first when heated because lower temperatures (around 700-800°C) produce red glow. The glass color only affects which wavelengths are transmitted in transmitted light, not the thermal emission color.

The hydrogenation of vegetable oils (converting unsaturated fats to saturated fats) is catalyzed by finely divided nickel metal at elevated temperatures and pressures. Nickel is an excellent hydrogenation catalyst due to its ability to absorb hydrogen and activate both hydrogen and the unsaturated bonds. Alumina, charcoal, and silica are not effective hydrogenation catalysts.

Polycarbonates, particularly materials like Lexan, are widely used in bulletproof glass and transparent armor applications due to their exceptional impact resistance and optical clarity. While polyamides like Kevlar are used in bulletproof fabrics, polycarbonates excel in rigid transparent armor. Polycarbonates can absorb tremendous impact energy without shattering and maintain structural integrity.

Oxygen is the most abundant element in Earth's crust, constituting approximately 46-47% by weight. This is primarily in the form of silicate minerals, oxides, and water. Silicon is the second most abundant at about 27-28%, followed by aluminum at about 8%. Carbon, nitrogen, and hydrogen are present in much smaller quantities in the crust despite being abundant in the atmosphere and living organisms.

In the human body, the major elements by mass are oxygen (65%), carbon (18.5%), hydrogen (9.5%), and nitrogen (3.2%). Iron, while essential for hemoglobin and oxygen transport, is a trace element present in only about 0.006% of body mass. Among the given options (oxygen, hydrogen, carbon, iron), iron is present in the smallest quantity.

Unripe fruits contain high concentrations of organic acids like citric acid (citrus), malic acid (apples), and oxalic acid. These acids decrease as fruits ripen and convert to sugars. Starch is converted to sugars during ripening, not acids.

Chromium has atomic weight ~52.00, which is less than iron's ~55.85. Nickel is ~58.69, zinc (typo 'Znc') is ~65.38, and copper is ~63.55 - all heavier than iron. Chromium is the only option lighter than iron.

Dmitri Ivanovich Mendeleev developed the Periodic Law in 1869, arranging elements by atomic weight and predicting undiscovered elements. Niels Bohr developed atomic structure theory, Huygens studied wave optics, and Newton formulated classical mechanics.

DDT (Dichlorodiphenyltrichloroethane) is a synthetic organochlorine insecticide used to control mosquitoes and agricultural pests. It is not a herbicide (kills weeds), fungicide (kills fungi), or rodenticide (kills rodents).

Mercury is the only metal that is liquid at standard temperature and pressure (melting point -38.8°C). It's a heavy, silvery liquid used in thermometers. Zinc, radium, and uranium are all solid metals at room temperature.

Urea (CO(NH2)2) contains approximately 46% nitrogen by mass, the highest among common nitrogen fertilizers. Calcium ammonium nitrate has ~27%, ammonium sulphate has ~21%, and calcium nitrate has ~15% nitrogen. Urea's high N content makes it efficient.

Acetylene (C2H2) is widely used in artificial fruit ripening, especially in developing countries. Calcium carbide (CaC2) reacts with moisture to release acetylene, which mimics ethylene's natural ripening action. Ethane and hydrogen don't trigger ripening; CO2 is sometimes used in controlled atmosphere storage but not for ripening.

Ammonium nitrate is a fertilizer compound that becomes a powerful explosive when mixed with diesel fuel or other fuel sources. This mixture, known as ANFO (Ammonium Nitrate Fuel Oil), has been widely used in mining and construction, and unfortunately also in terrorist attacks like the Oklahoma City bombing. Pure ammonium nitrate is stable, but the addition of fuel makes it highly explosive.

CFCs (Chlorofluorocarbons) were widely used in aerosol sprays, refrigerators, and air conditioning systems as refrigerants and propellants. However, they were never used in fire extinguishers - fire extinguishers use different agents like carbon dioxide, dry chemicals, foam, or halons (which are related but distinct from CFCs and were also phased out for environmental reasons).