Answer:
1. The formula NH3 tells us that ammonia is made up of three hydrogen atoms bonded with one nitrogen atom.
2. Physical changes are reversible. Chemical changes lead to modification in chemical composition hence irreversible.
(a) Physical change. Copper wire can be unbent again.
(b) Chemical change. Burning of charcoal leads to the reaction of carbon with oxygen to release carbon dioxide in the air, which cannot be reversed.
(c) Chemical change. The presence of yeast leads to trapping of CO2 in the dough, which makes it rise. The sugar in the dough was consumed in the process, making it irreversible.
3.
(a) Ethanol is a liquid at -30 degC. It becomes vapor at +78 degC, and stays in liquid state between melting and boiling point values.
(b) Neon is a gas at 60 degC as its boiling point is -246 degC.
(c) Sulfur is a solid at 7 deg C. It starts melting only at +115 degC.
(d) In general, 25 degC is considered room temperature (rt). Substances with melting points less than 25 and boiling points greater than 25 will be liquid at rt. In the table, Bromine, Chlorine, Ethanol, Mercury, and Water will be liquids at room temperature.
4. Mass is an extensive property. It depends on the amount of matter in a sample and not on the type of matter present. e.g., 4 grams of salt and 4 grams of sugar are the same (will even look similar), but they are different substances. Hence, mass cannot be used for identification of a sample.
5. Physical changes can also lead to the evolution of a gas. For a change to be considered a chemical reaction, a new substance must be formed (in accordance with the law of conservation of mass). e.g., conversion of liquid water to water vapor is not a chemical change.
6. Gas molecules move freely in space. When stored in a sealed container, these molecules get restricted in that space and are unable to escape. Without a container, the molecules keep moving rapidly and occupy space with no boundaries.
In the case of a solid, the molecules have fixed positions. These can vibrate while staying in the same position but are unable to leave the fixed, compact space they occupy. Hence, they have a fixed shape and do not require a container.
