JAMB Chemistry 2010 Past Questions & Answers

A mixture consists of two or more substances that are physically combined but not chemically bonded. Blood is a mixture because it contains different components like plasma, red blood cells, white blood cells, and platelets that can be separated by physical means. The other options, sand (silicon dioxide), washing soda (sodium carbonate), and common salt (sodium chloride), are all chemical compounds, which are pure substances.

Common mistake: Students may mistakenly choose sand because they think of beach sand, which is impure, but in chemistry, 'sand' typically refers to the compound silicon dioxide (SiO₂).

First, find the chemical formula for calcium trioxonitrate (V), which is Ca(NO₃)₂. The relative formula mass (molar mass) is calculated as 40 + 2(14) + 6(16) = 164. The percentage by mass of nitrogen is found by dividing the total mass of nitrogen by the total formula mass and multiplying by 100. The calculation that yields the correct option is $(14 / 164)$ x 100 = 8.53%, which is approximately 8.5%.

Common mistake: A common mistake is to correctly use the mass of both nitrogen atoms $(2 x 14 = 28)$ in the calculation, which leads to option B (17.1%) instead of the keyed answer.

This question describes the process of condensation, which is a change of state from gas to liquid. The air around the cold bottle contains invisible water vapour (a gas). When this water vapour touches the cold surface of the bottle, it loses heat energy to the bottle, causing its particles to slow down and change from a gas into liquid water droplets.

Common mistake: Students may incorrectly choose option D, but for water vapour (a gas) to turn into liquid water, it must lose energy to its cooler surroundings, not gain energy from them.

This question is based on Boyle's Law, which describes the relationship between the pressure and volume of a gas at a constant temperature. Boyle's Law states that pressure is inversely proportional to volume (P₁V₁ = P₂V₂). If the pressure (P) is reduced to half (P/2), the volume (V) must double to keep the product constant, so the new volume will be 2V cm³.

Common mistake: Students might mistakenly choose C, thinking that the volume remains unchanged, forgetting Boyle's Law which states that pressure and volume have an inverse relationship.

As you move from left to right across a period in the periodic table, the number of protons in the nucleus increases, which is known as an increase in nuclear charge. This stronger positive charge pulls the outermost electrons closer and holds them more tightly. Because the electrons are held more strongly, more energy is needed to remove one, causing the first ionization energy to generally rise.

Common mistake: Students often get confused by the screening effect, but across a period, electrons are added to the same shell, so the screening effect is relatively constant and the increasing nuclear charge is the dominant factor.

Nitrogen has an atomic number of 7, meaning it has 7 electrons. Its electronic configuration is 1s²2s²2p³. According to Hund's rule of maximum multiplicity, electrons fill orbitals of the same energy level singly before they start to pair up. The 2p sub-level has three orbitals, so one electron will go into each of the three orbitals, resulting in 3 unpaired electrons.

Common mistake: Students might incorrectly pair two electrons in one p-orbital before filling the others, leading them to think there is only one unpaired electron left.

Noble gases are very stable and unreactive because their outermost electron shells are completely filled. For Helium, this is a 'duplet' configuration with 2 electrons, while for the others like Neon and Argon, it is an 'octet' configuration with 8 electrons. This complete shell is the most stable arrangement for an atom, so they have no tendency to react with other elements.

Common mistake: Students might choose option B because it is a true statement, but belonging to group zero is a classification based on their stable electron configuration, not the fundamental reason for the stability itself.

In an atom, electrons are arranged in shells around the nucleus. The first shell is the K shell (n=1), the second is the L shell (n=2), and the third is the M shell (n=3). The maximum number of electrons a shell can hold is calculated using the formula 2n², where 'n' is the shell number. For the L shell, n is 2, so the maximum number of electrons is 2 x (2)² which equals 2 x 4 = 8.

Common mistake: Students often confuse the L shell (n=2) with the M shell (n=3), leading them to incorrectly calculate 2 x (3)² = 18.

Elements in the same period, which is a horizontal row on the periodic table, have the same number of electron shells. For example, all elements in Period 2, from Lithium to Neon, have their electrons in two shells. As you move from left to right across a period, you are adding electrons to the same outermost shell.

Common mistake: Students often mistakenly choose 'chemical properties' because they confuse periods (horizontal rows) with groups (vertical columns), where elements do share similar chemical properties due to having the same number of valence electrons.

Neon is a noble gas that produces a distinct, bright reddish-orange light when an electric current is passed through it. This bright light has a high ability to penetrate through fog and mist, making it highly visible even in poor weather conditions. For this reason, neon is widely used in safety applications like aerodrome (airport) beacons and warning lights.

Common mistake: Students might mistakenly choose Argon because it is also used in lighting, specifically in incandescent light bulbs, but it does not produce the characteristic bright, fog-penetrating light that Neon does.