Science: Chemistry Essentials

An atom has 11 protons, 12 neutrons and 11 electrons. What is its mass number?

• A12
• B23✓
• C34
• D11

Answer: Mass number is protons + neutrons = 11 + 12 = 23. 11 is the atomic (proton) number, and electrons contribute almost nothing to the mass.

Which statement about the pH scale is correct?

• AA solution of pH 2 is acidic and a solution of pH 12 is alkaline✓
• BA solution of pH 2 is alkaline and a solution of pH 12 is acidic
• CA pH of 14 is neutral
• DPure water has a pH of 0

Answer: The scale runs from 0 (strongly acidic) through 7 (neutral, pure water) to 14 (strongly alkaline). So pH 2 is acidic and pH 12 is alkaline.

Balance the equation: __ Mg + O₂ → __ MgO. What are the missing numbers?

• AMg + O₂ → MgO₂
• BMg + O₂ → 2 MgO
• C2 Mg + O₂ → 2 MgO✓
• D2 Mg + 2 O₂ → 2 MgO

Answer: One O₂ molecule provides two oxygen atoms, so two MgO units form, which needs two Mg atoms: 2Mg + O₂ → 2MgO. MgO₂ is not the formula of magnesium oxide, and the other options do not balance.

In the reaction between an acid and an alkali, what products are formed?

• AA salt and water✓
• BCarbon dioxide and water
• CA salt and hydrogen
• DA metal and hydrogen

Answer: Neutralisation of an acid by an alkali produces a salt and water. A salt and hydrogen come from an acid reacting with a metal, and carbon dioxide is produced when an acid reacts with a carbonate.

Sodium (group 1) reacts with chlorine (group 7) to form sodium chloride. What type of bonding holds sodium chloride together?

• AHydrogen bonding between the atoms
• BIonic bonding between Na⁺ and Cl⁻ ions✓
• CCovalent bonding, with shared pairs of electrons
• DMetallic bonding, with delocalised electrons

Answer: Sodium gives up one electron to become Na⁺ and chlorine gains it to become Cl⁻; the electrostatic attraction between the oppositely charged ions is ionic bonding. Covalent bonding is between non-metals, and metallic bonding is within metals.

Which method would separate a mixture of sand and salt dissolved in water to recover both the sand and the salt?

• AEvaporate all of the water first, then filter the sand out of the dry residue that is left behind
• BDistil the mixture to collect the sand
• CUse chromatography to separate the sand and salt
• DFilter to remove the sand, then evaporate the water to leave the salt✓

Answer: Sand is insoluble, so filtration removes it; salt is dissolved, so evaporating the water leaves it behind. Evaporating first leaves both solids mixed, chromatography separates dissolved coloured substances, and distillation collects the liquid, not the sand.

Which of these is a compound?

• AHelium (He)
• BIron (Fe)
• CCarbon dioxide (CO₂)✓
• DOxygen (O₂)

Answer: A compound contains two or more different elements chemically bonded: CO₂ has carbon and oxygen. O₂ is a molecule of a single element, and Fe and He are elements.

What happens to the particles of a solid when it melts?

• AThey lose energy and move closer together into a more tightly packed arrangement
• BThey gain energy and move from fixed positions to sliding past one another✓
• CThey break apart into smaller particles that can move around more freely than before
• DThey change into different particles with new chemical properties as the solid turns to liquid

Answer: Melting is a physical change: the particles are the same but have enough energy to leave their fixed positions and move around each other as a liquid. Nothing new is formed.

In the periodic table, what do the elements in Group 1 have in common?

• AThey are all found as hard, dense solids in the middle transition block of the periodic table
• BThey each have seven electrons in their outer shell and gain one more when they react with metals
• CThey each have one electron in their outer shell and react vigorously with water✓
• DThey are all unreactive gases

Answer: Group 1, the alkali metals, all have a single outer electron, which they lose easily, making them very reactive — increasingly so down the group. Group 7 elements have seven outer electrons and Group 0 are the unreactive noble gases.

Hydrochloric acid reacts with magnesium. What are the products?

• AMagnesium oxide and hydrogen
• BMagnesium chloride and water
• CMagnesium chloride and hydrogen✓
• DMagnesium hydroxide and chlorine

Answer: Acid + metal → salt + hydrogen: 2HCl + Mg → MgCl₂ + H₂. Water is produced when an acid reacts with a base or alkali, not with a metal.

Which test identifies hydrogen gas?

• ADamp litmus paper is bleached white
• BLimewater turns cloudy
• CA lit splint gives a squeaky pop✓
• DA glowing splint relights

Answer: Hydrogen burns rapidly with a squeaky pop when a lit splint is held to it. A relighting glowing splint identifies oxygen, cloudy limewater identifies carbon dioxide, and bleached litmus identifies chlorine.

What is the chemical formula for water?

• AHO₂
• BOH
• CH₂O✓
• DH₂O₂

Answer: A water molecule has two hydrogen atoms bonded to one oxygen atom: H₂O. H₂O₂ is hydrogen peroxide, and OH on its own is the hydroxide ion.