General Science

Pearls are biological products formed within the soft tissue of living shelled mollusks. Chemically and structurally, they consist primarily of calcium carbonate in the form of aragonite platelets bound together by an organic matrix protein called conchiolin.

Diamond is the hardest known natural mineral on Earth, scoring 10 on the Mohs hardness scale. Its exceptional hardness comes from the strong covalent bonds between carbon atoms arranged in a tetrahedral lattice structure.

Potassium permanganate (KMnO4) is a powerful oxidizing agent that kills bacteria and other microorganisms in water through oxidation reactions. It oxidizes organic impurities and pathogens, making water safe for drinking. Options A and B are incorrect - it doesn't dissolve impurities or act as a sterilizing agent in the traditional sense.

Water has a unique density anomaly - it reaches maximum density at approximately 4°C. Above and below this temperature, water becomes less dense due to changes in molecular arrangement and hydrogen bonding. This property is crucial for aquatic life.

The average salinity of seawater is approximately 3.5%, meaning about 35 grams of dissolved salts per kilogram of seawater. This value varies slightly by location (near river mouths vs. open ocean), but 3.5% is the standard accepted average.

When iron rusts, it combines with oxygen from air to form iron oxide (Fe2O3). Since oxygen atoms add to the iron, the total mass increases. This follows the law of conservation of mass - the nail's weight increases by the mass of oxygen atoms that bond with iron.

Diamond's hardness comes from its 3D crystalline structure where each carbon atom is covalently bonded to four others in a tetrahedral arrangement. This creates an extremely rigid 3D network. In contrast, graphite has weak van der Waals forces between 2D layers, making it soft. Option C correctly identifies the crystalline structure.

Rocket propulsion works on Newton's third law and conservation of momentum. As rocket engine expels gases backward at high velocity, the rocket gains equal forward momentum. This momentum exchange allows rockets to work even in vacuum where there's nothing to push against.

Lead (Pb-208) is the most stable element because it has the highest binding energy per nucleon among all elements, and its particular isotope is doubly magic (82 protons, 126 neutrons). Uranium is radioactive and decays, hydrogen is reactive and forms compounds, and oxygen is highly reactive. Lead is the endpoint of many radioactive decay chains.

All acids contain hydrogen ions (H+) as the essential component that defines acidic behavior. When acids dissolve in water, they release H+ ions which are responsible for characteristic acidic properties like sour taste and ability to turn litmus red. Carbon, oxygen, and sulfur are not universal components of all acids.

Lithium is the lightest metal with an atomic mass of approximately 6.94 u and density of 0.534 g/cm³. It's the first element in Group 1 (alkali metals). Mercury is a liquid metal with high density, silver has atomic mass 107.87 u, and lead has atomic mass 207.2 u - all much heavier than lithium.

Air is a mixture of several gases, primarily nitrogen (78%), oxygen (21%), and small amounts of argon, carbon dioxide, water vapor, and trace gases. Each component retains its chemical properties. Air is not a compound (elements don't combine in fixed ratios), not an element (it's multiple elements), and not an electrolyte (it doesn't conduct electricity).

Red phosphorus is the stable allotrope used in safety matches because it's much less reactive than white phosphorus, which can ignite spontaneously in air. The striking surface on match boxes contains red phosphorus, which reacts with the match head when friction generates enough heat to start combustion.

Mass is a scalar quantity because it only has magnitude and no direction. Force, acceleration, and weight are all vector quantities as they have both magnitude and direction - weight always points toward Earth's center, force acts in the direction of application, and acceleration follows the direction of the net force.

Nichrome (nickel-chromium alloy) is used in heating elements because it has high electrical resistance and can withstand very high temperatures without oxidizing or degrading. Solder is for joining metals, alloy steel is for structural applications, and german silver is primarily used for decorative items and utensils.

Gobar gas (biogas from cow dung) is primarily methane (CH4), produced by anaerobic decomposition of organic matter. Methane makes up 50-75% of biogas, with the rest being CO2 and small amounts of other gases. Methane is the flammable component that allows biogas to be used as fuel.

Mercury is unique among metals in its ability to form amalgams (alloys) with other metals at room temperature due to its liquid state. This property is why mercury has been used in gold mining and dental fillings. Lead, tin, and zinc do not form amalgams with mercury.

Washing soda is sodium carbonate decahydrate (Na2CO3·10H2O), used in laundry and cleaning. Calcium carbonate is limestone/chalk, calcium bicarbonate is found in hard water, and sodium bicarbonate is baking soda - these are distinct compounds with different uses.

Urea's chemical name is carbamide because it contains a carbonyl group (C=O) bonded to two amino groups (-NH2). Its formula is CO(NH2)2. Aneurin is vitamin B1, and chloroethane is an organic solvent - neither are related to urea.

Lead-acid batteries use lead plates and sulfuric acid electrolyte. During discharge, both plates convert to lead sulfate, which reconverts during charging. Lead is ideal because it's cheap, has good electrochemical properties, and is dense enough to make compact batteries.

Sodium reacts violently with water and alcohol, so kerosene is used to store it safely. Kerosene is inert toward sodium and forms a protective layer that prevents contact with air moisture. Petrol (gasoline) is also used, but kerosene is more common in laboratory settings.

Plaster of Paris is calcium sulfate hemihydrate with formula 2CaSO4·H2O or CaSO4·0.5H2O. When mixed with water, it rehydrates to form gypsum (CaSO4·2H2O) and hardens. The hemihydrate form is what makes it useful for casts and molds.

Antiserum is blood serum containing polyclonal antibodies against specific antigens, produced by immunizing animals. It provides passive immunity. Antigens are what trigger antibody production, not what antiserum contains. Leucocytes (white blood cells) are cellular, not serum components.

Mica is an excellent thermal conductor but poor electrical conductor due to its layered silicate structure - heat transfers efficiently through the crystal lattice, but electrons cannot move freely. This unique combination makes it valuable as electrical insulator in high-heat applications like toasters.

The human cardiac cycle consists of one complete heartbeat (systole + diastole) and takes approximately 0.8 seconds at a normal resting heart rate of about 75 beats per minute (60/75 = 0.8 sec). 0.5s would mean 120 bpm (too fast), while 1.0-1.2s would mean only 50-60 bpm (unusually slow for a healthy adult at rest).