States of Matter

(i) Crystallization of sodium chloride - physical change (ii) Melting of ice - physical change (iii) Souring of milk - chemical change 

Methanol and water form a homogeneous mixture because they are completely miscible in all proportions, forming a single-phase solution. The other options are heterogeneous mixtures: starch and sugar (different particles), graphite and charcoal (distinct forms of carbon), calcium carbonate and calcium bicarbonate (separate solid compounds). Homogeneity means uniform composition throughout.

Water reaches its maximum density at 4°C. When heated from 0°C to 4°C, the volume decreases as density increases. From 4°C to 20°C, the volume increases normally as density decreases with heating. This anomalous expansion is unique to water.

According to the regelation phenomenon, increased pressure lowers the melting point of ice. When two ice blocks are pressed together, the contact point experiences high pressure, causing local melting. Water formed lubricates the surface, and when pressure is released, it refreezes, joining the blocks.

An emulsion is a colloid where one liquid is dispersed in another immiscible liquid. Examples include milk (fat in water), mayonnaise (oil in water), and butter (water in oil). A gas in liquid is foam, liquid in gas is aerosol, and gas in solid is solid foam.

Water is composed of two hydrogen atoms bonded to one oxygen atom, giving it the chemical formula H2O. This is a fundamental fact in chemistry that all students should memorize.

Potable water is water that is safe for human consumption because it has balanced minerals and is free from harmful contaminants. Tap water may contain chlorine or other additives, distilled water lacks minerals, and bottled water varies in quality - only potable water is guaranteed safe to drink.

Glowing of a platinum wire involves only a physical change - the wire becomes hot and emits light due to increased temperature, but its chemical composition remains unchanged. The other options involve chemical changes: combustion converts fuel to new substances, electrolysis breaks water into hydrogen and oxygen, and carbon dioxide reacts with lime water to form calcium carbonate.

Melting ice provides better cooling because it absorbs latent heat of fusion (334 J/g) during phase change from solid to liquid at 0°C. This energy absorption occurs at constant temperature, extracting significant heat from the body. Ice-cold water at 0°C cannot absorb this latent heat and only provides cooling through its specific heat capacity.

Boiling removes temporary hardness caused by bicarbonates (Ca(HCO3)2 and Mg(HCO3)2) by converting them to insoluble carbonates. Permanent hardness from chlorides and sulfates cannot be removed by boiling alone - it requires ion exchange or other treatment methods.

Pure water dissociates into equal concentrations of hydrogen ions (H+) and hydroxyl ions (OH-), making it neutral (pH 7). This balance means it's neither acidic (excess H+) nor alkaline (excess OH-). Boiling point, electron donation/acceptance, or proton donation/acceptance alone don't explain neutrality.

Water freezes from the top surface downward because of water's unique density anomaly - liquid water is densest at 4°C, while ice is less dense than liquid water. This means colder water rises to the surface, and ice floats. The surface reaches 0°C first and freezes, while deeper water remains liquid longer.

Heavy water (D2O) contains deuterium, an isotope of hydrogen that has twice the mass of regular hydrogen. This makes heavy water about 11% denser than ordinary water (H2O) - marginally heavier, not several times. Chemically, both are water with the same molecular structure and properties, but heavy water has different nuclear properties.

Heavy water is deuterium oxide (D2O), where hydrogen atoms are replaced by deuterium (hydrogen-2 isotope). It's called 'heavy' because deuterium has twice the mass of regular hydrogen. Tritium oxide would be even heavier, and the other options are unrelated to heavy water.

Solid carbon dioxide is called dry ice because it sublimates (transitions directly from solid to gas) at atmospheric pressure, which creates a cooling effect similar to regular ice but without melting into liquid. Dry ice is used primarily for cooling and preservation.