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DSE Chemistry Recap – Introduction + Microscopic World I | Vanilla (2026)

Introduction + Microscopic World I

DSE Chemistry

Vanilla (2026)

For use on Friday, 4 September 2026

01

Classification of Matter

Elements, Compounds and Mixtures

Four samples, P, Q, R and S, are examined. The information obtained is shown in the table below.

Sample Experimental information
P It contains only one type of atom and cannot be decomposed into simpler substances by chemical methods.
Q It contains two elements chemically combined in a fixed mass ratio. Its properties are different from those of the constituent elements.
R It contains two substances in a variable proportion. The substances can be separated by fractional distillation.
S It contains iron powder, sodium chloride and sand.
  1. Classify each of P, Q and R as an element, a compound or a mixture. Justify each classification using the information given.
  2. Explain, in terms of particles, why a compound is considered a pure substance even though it contains more than one element.
  3. Carbon monoxide and carbon dioxide both contain carbon and oxygen. Explain why a sample containing both gases is a mixture rather than a single compound.
  4. Describe a sequence of physical procedures that can be used to separate S into iron, sodium chloride and sand. Your answer should state the relevant property used in each step.
  5. A student heats a mixture of iron powder and sulphur strongly. A black solid is formed. State one test or observation that could show that a new compound, rather than the original mixture, has been formed.
  6. Explain why fractional distillation can separate suitable liquid mixtures but cannot decompose a compound into its elements.
13 marks
02

Atomic Structure

Subatomic Particles, Isotopes and Relative Atomic Mass

Magnesium exists naturally as three isotopes. Data for the isotopes in a particular sample are shown below.

Isotope Isotopic mass Relative abundance / %
24Mg 23.985 78.99
25Mg 24.986 10.00
26Mg 25.983 11.01
  1. State the relative charge and relative mass of a proton, a neutron and an electron.
  2. For one atom of 25Mg, state the number of protons, neutrons and electrons. The atomic number of magnesium is 12.
  3. Write the electronic arrangement of:
    1. a magnesium atom;
    2. a magnesium ion, Mg2+.
  4. Explain why 24Mg, 25Mg and 26Mg are isotopes of the same element.
  5. Distinguish between the mass number of an isotope and its isotopic mass.
  6. Using all the data in the table, calculate the relative atomic mass of magnesium. Give your answer to four significant figures.
  7. Explain why the relative atomic mass obtained is not a whole number.
  8. State one chemical property that is expected to be nearly the same for all three magnesium isotopes, and explain your answer.
15 marks
03

The Periodic Table

Arrangement and Trends Across Period 3

Part of Period 3 of the Periodic Table is represented below. The elements are shown in order of increasing atomic number.

Element Na Mg Al Si P S Cl Ar
Atomic number 11 12 13 14 15 16 17 18
Electronic arrangement 2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8
  1. State the basis on which elements are arranged in the modern Periodic Table.
  2. Explain, using electronic arrangements, why all the elements shown are placed in the same period.
  3. Explain how the group number of each of Na, Mg and Cl is related to its electronic arrangement.
  4. State and explain the general change in atomic radius from sodium to chlorine across Period 3.
  5. Describe the general change in metallic character from sodium to chlorine across Period 3.
  6. Compare the tendency of sodium and chlorine atoms to form ions. In each case, state the ion formed and explain how its electronic arrangement is achieved.
  7. Silicon has properties intermediate between those of metals and non-metals. State the term used to describe this type of element.
  8. Explain why argon is chemically much less reactive than sodium and chlorine.
14 marks
04

Group I Elements

Properties, Reactions and Trends

Lithium, sodium and potassium are Group I elements. Equal-sized freshly cut pieces of the three metals are separately added to large troughs containing water at the same temperature.

Metal Electronic arrangement Selected observation with water
Lithium 2,1 Floats and moves slowly; bubbles of gas are produced.
Sodium 2,8,1 Melts into a silvery ball and moves rapidly on the surface.
Potassium 2,8,8,1 Reacts very vigorously and may burn with a lilac flame.
  1. Explain why lithium, sodium and potassium have similar chemical properties.
  2. Describe the trend in reactivity with water from lithium to potassium.
  3. Explain this trend in terms of:
    1. the number of occupied electron shells;
    2. the attraction between the nucleus and the outermost electron.
  4. Write a balanced chemical equation, including state symbols, for the reaction between sodium and water.
  5. State the expected observations when the solution formed from sodium and water is tested with:
    1. red litmus paper;
    2. phenolphthalein.
  6. Explain why the resulting solution gives these indicator changes.
  7. Predict two observations when rubidium is added to water, based on the trend in Group I.
  8. Group I metals are commonly stored under paraffin oil. Explain why this storage method is necessary.
  9. A student suggests using a large piece of potassium instead of a small piece to make the reaction easier to observe. Explain why this suggestion is unsafe.
15 marks

Question 1

  1. P is an element. It contains only one type of atom and cannot be decomposed into simpler substances by chemical methods.

    Q is a compound. It contains different elements chemically combined in a fixed mass ratio, and it has properties different from those of its constituent elements.

    R is a mixture. Its components are present in a variable proportion and can be separated by a physical method.
  2. A compound contains only one type of chemical substance with a fixed composition. All its particles have the same fixed combination of atoms. Although more than one element is present, the elements are chemically combined rather than simply mixed.
  3. Carbon monoxide and carbon dioxide are two different substances with different types of particles. Their relative amounts in the gas sample can vary, and they are not chemically combined with each other in one fixed ratio. Therefore, the sample is a mixture.
  4. A suitable sequence is:
    1. Use a magnet to remove the iron powder. This uses the magnetic property of iron.
    2. Add water to the remaining sodium chloride and sand. Sodium chloride dissolves in water, but sand does not.
    3. Filter the mixture. Sand remains as the residue, while sodium chloride solution passes through as the filtrate.
    4. Wash and dry the sand.
    5. Evaporate some water from the filtrate and allow the solution to cool to crystallize sodium chloride. Filter and dry the crystals.
  5. Any one suitable answer:
    • The black product is not attracted to a magnet in the same way as the original iron powder.
    • The original iron can be removed from the mixture by a magnet before heating, but the iron cannot be removed from the product by this physical method.
    • The product has properties different from both iron and sulphur.
  6. Fractional distillation separates liquids because they have different boiling points. It is a physical process and does not break chemical bonds. The elements in a compound are chemically bonded, so a chemical reaction is required to decompose the compound.
Element: contains only one type of atom.
Compound: contains elements chemically combined in a fixed ratio.
Mixture: contains substances not chemically combined; its composition can vary and its components can be separated by physical methods.

Question 2

  1. Particle Relative charge Relative mass
    Proton +1 1
    Neutron 0 1
    Electron −1 Approximately 1/1840
  2. For one neutral atom of 25Mg:
    • Number of protons = 12.
    • Number of neutrons = 25 − 12 = 13.
    • Number of electrons = 12.
    1. Magnesium atom: 2,8,2.
    2. Magnesium ion, Mg2+: 2,8. A magnesium atom loses its two outermost electrons when forming Mg2+.
  4. They have the same number of protons but different numbers of neutrons. All have atomic number 12, so they are atoms of magnesium. Their different neutron numbers give them different mass numbers.
  5. Mass number is the total number of protons and neutrons in the nucleus of a particular atom. It is always a whole number.

    Isotopic mass is the mass of an atom of a particular isotope relative to one-twelfth of the mass of a carbon-12 atom. It is determined experimentally and is not necessarily a whole number.
  6. Relative atomic mass = [(23.985 × 78.99) + (24.986 × 10.00) + (25.983 × 11.01)] ÷ 100
    = 24.305…
    Relative atomic mass of magnesium = 24.31 to four significant figures.
  7. Magnesium occurs naturally as a mixture of isotopes. Its relative atomic mass is the weighted average of the isotopic masses according to their relative abundances. A weighted average does not have to be a whole number.
  8. Any suitable magnesium reaction is acceptable, for example: all three isotopes react with oxygen to form magnesium oxide. Chemical properties depend mainly on electronic arrangement. All three isotopes have the same atomic number and hence the same electronic arrangement, 2,8,2.
The identity of an element is determined by its number of protons. Isotopes differ only in neutron number. Their chemical properties are nearly identical because neutral atoms of the isotopes have the same electronic arrangement.

Question 3

  1. Elements in the modern Periodic Table are arranged in order of increasing atomic number, or increasing number of protons.
  2. All the elements shown have three occupied electron shells. The period number corresponds to the number of occupied electron shells, so they are all in Period 3.
  3. For these main-group elements, the group number is related to the number of outermost-shell electrons:
    • Na has electronic arrangement 2,8,1 and one outermost electron, so it is in Group I.
    • Mg has electronic arrangement 2,8,2 and two outermost electrons, so it is in Group II.
    • Cl has electronic arrangement 2,8,7 and seven outermost electrons, so it is in Group VII.
  4. Atomic radius generally decreases from sodium to chlorine. Across the period, the number of protons increases while electrons are added to the same main electron shell. The increasing nuclear charge produces a stronger attraction for the electrons and pulls the electron cloud closer to the nucleus.
  5. Metallic character generally decreases across Period 3. Sodium, magnesium and aluminium are metals; silicon has intermediate properties; phosphorus, sulphur and chlorine are non-metals. The tendency to lose electrons decreases, while the tendency to gain or share electrons generally increases.
  6. A sodium atom has electronic arrangement 2,8,1. It loses one electron to form Na+, with electronic arrangement 2,8.
    Na ⟶ Na+ + e−
    A chlorine atom has electronic arrangement 2,8,7. It gains one electron to form Cl−, with electronic arrangement 2,8,8.
    Cl + e− ⟶ Cl−
    Both ions achieve a stable outermost electron shell.
  7. Silicon is described as a metalloid or semi-metal.
  8. Argon has electronic arrangement 2,8,8. Its outermost electron shell is completely filled. It has little tendency to gain, lose or share electrons and is therefore chemically very unreactive.

    Sodium readily loses one outermost electron, while chlorine readily gains one electron. Both can thereby attain stable electronic arrangements.
Across a period: atomic number increases, electrons enter the same main shell, atomic radius generally decreases and metallic character generally decreases.

Down a group: the elements have the same number of outermost-shell electrons and therefore show similar chemical properties.

Question 4

  1. Lithium, sodium and potassium all have one electron in their outermost electron shell. They tend to lose this electron to form ions with a charge of +1. Therefore, they undergo similar types of chemical reaction.
  2. Reactivity with water increases from lithium to sodium to potassium. Lithium reacts least vigorously, while potassium reacts most vigorously.
    1. Down Group I, the number of occupied electron shells increases. The outermost electron is farther from the nucleus and is more strongly shielded by inner-shell electrons.
    2. The attraction between the positively charged nucleus and the outermost electron therefore becomes weaker. The outermost electron is lost more easily, so reactivity increases down the group.
  4. 2Na(s) + 2H2O(l) ⟶ 2NaOH(aq) + H2(g)
    1. Red litmus paper turns blue.
    2. Phenolphthalein turns pink.
  6. Sodium reacts with water to form sodium hydroxide. Sodium hydroxide dissolves and provides OH−(aq) ions. The solution is alkaline, causing red litmus to turn blue and phenolphthalein to turn pink.
  7. Any two reasonable predictions:
    • Rubidium reacts more vigorously or explosively than potassium.
    • It moves very rapidly on the surface of the water.
    • The hydrogen produced may ignite immediately.
    • A flame may be observed.
    • Rubidium rapidly disappears and an alkaline solution forms.
  8. Group I metals react readily with oxygen and water vapour in the air. Paraffin oil prevents the metals from coming into contact with air and moisture, reducing the risk of unwanted reaction or fire.
  9. Potassium reacts very vigorously with water. A larger piece contains more potassium and produces more heat and hydrogen in a short time. The hydrogen may ignite, and the reaction may become explosive. A large piece could also eject hot alkaline solution and burning metal, causing serious injury.
Group I metals form M+ ions by losing one electron. Their reactivity increases down the group because the outermost electron becomes farther from the nucleus, more shielded and easier to remove.
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