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Chemistry entrance exam, 2017 E.C. (2025)

Real questions from the Grade 12 university entrance exam in Chemistry, as students sat it in 2017 E.C. Every question comes with the correct answer and a worked explanation.

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  • Natural science stream

    Who sits it

  • 2017 E.C. (2025)

    Exam year

  • 76 questions with answers

    In this set

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Answer each question yourself before opening the answer. The explanations point to the exact textbook section, so you know which page to reread when you miss one.

Question 1

Which of the following atomic models is NOT expressed correctly?

  1. A.Dalton's atomic model. billiard ball model
  2. B.Thomson's model of atom .Plum pudding model
  3. C.Thomson's model of atom .Planetary model
  4. D.Rutherford atomic model .Nuclear model
Show the answer and explanation

Answer: C Thomson's model of atom .Planetary model

Why

The planetary model, with electrons circling a central nucleus the way planets orbit the sun, came from Rutherford and was refined by Bohr. Thomson's picture had no nucleus at all: he described a ball of positive charge with electrons stuck through it, which is the plum pudding model. Pairing Thomson with the planetary model is therefore the mismatch. Option B names Thomson too, but plum pudding is exactly what he proposed, so that row is matched correctly.

Question 2

Suppose an unknown element does NOT deflect cathode rays in an electric field but has significant mass. Based on atomic models, which subatomic particle could it be composed of, and how would this impact the element's atomic structure?

  1. A.Protons; making the nucleus positively charged
  2. B.Neutrons; contributing to atomic mass without charge
  3. C.Electrons; moving freely in orbitals
  4. D.Positrons; leading to an unstable atom
Show the answer and explanation

Answer: B Neutrons; contributing to atomic mass without charge

Why

Cathode rays are beams of electrons, and a charged particle placed in an electric field is pushed to one side. This particle is not deflected, so it carries no charge. It still has real mass, and the only subatomic particle that is heavy and neutral is the neutron. It adds to the mass number without changing the atomic number. Option A is tempting because protons are heavy as well, but a proton is positive and the field would deflect it.

Question 3

A student is studying an atom with a mass number of 23 and an atomic number of 11. Based on nuclear composition, which of the following statements is correct?

  1. A.The nucleus contains 11 protons and 12 neutrons
  2. B.The nucleus contains 11 neutrons and 12 protons
  3. C.The nucleus contains only 11 protons
  4. D.The nucleus contains 23 neutrons and 11 protons
Show the answer and explanation

Answer: A The nucleus contains 11 protons and 12 neutrons

Why

The atomic number counts the protons, so this atom has 11 protons. The mass number counts protons plus neutrons together, so the neutrons are \( 23 - 11 = 12 \). The nucleus holds 11 protons and 12 neutrons. Option D is tempting if you read 23 as the number of neutrons, but 23 is the total of both kinds of particle, not the neutrons alone.

Question 4

A scientist shines red and blue light on a metal surface. The red light does not eject electrons, but the blue light does. What conclusion can be drawn?

  1. A.The red light has higher intensity than the red light
  2. B.The blue light has a higher frequency and sufficient energy
  3. C.The red light has more energy than the blue light
  4. D.The photoelectric effect depends only on intensity, not frequency
Show the answer and explanation

Answer: B The blue light has a higher frequency and sufficient energy

Why

In the photoelectric effect a single photon must carry at least the work function energy before an electron can leave the metal. Photon energy is \( E = h\nu \), so it is set by frequency, not by how bright the beam is. Blue light has the higher frequency, so its photons carry enough energy while red photons do not. Option D is tempting because a brighter beam does eject more electrons, but that only happens once the frequency is already above the threshold. Below it, no amount of intensity helps.

Question 5

The maximum kinetic energy of photoelectrons ejected from a metal when it is subjected to a radiation of frequency 3.0 × 10^15 Hz is 6.63 × 10^-19 J. The threshold frequency, ν₀ of the photoelectron of the metal is

  1. A.1.0×10^15 Hz
  2. B.2.0×10^-15 Hz
  3. C.3.0×10^15 Hz
  4. D.2.0×10^15 Hz
Show the answer and explanation

Answer: D 2.0×10^15 Hz

Why

Use \( h\nu = h\nu_0 + KE_{max} \). The incoming photon carries \( E = 6.63\times10^{-34}\times3.0\times10^{15} = 1.99\times10^{-18} \) J. Take away the kinetic energy: \( h\nu_0 = 1.99\times10^{-18} - 6.63\times10^{-19} = 1.33\times10^{-18} \) J. Then \( \nu_0 = 1.33\times10^{-18} \div 6.63\times10^{-34} = 2.0\times10^{15} \) Hz. Option B carries the right digits but a negative power of ten, and \( 2.0\times10^{-15} \) Hz is not a frequency any light source produces.

Question 6

Which of the following correctly explains Heisenberg's Uncertainty Principle?

  1. A.The exact position and momentum of a particle can be simultaneously determined.
  2. B.The more precisely the position of an electron is known, the less precisely its momentum is known.
  3. C.It suggests that electrons move in fixed circular orbits around the nucleus.
  4. D.It only applies to macroscopic objects and not quantum particles.
Show the answer and explanation

Answer: B The more precisely the position of an electron is known, the less precisely its momentum is known.

Why

Heisenberg's principle says the position and the momentum of a particle cannot both be pinned down at the same moment. The more tightly you fix where the electron is, the more its momentum is left uncertain, because \( \Delta x \cdot \Delta p \ge h/4\pi \). Option A states the opposite. Option D is tempting, but the limit applies to every object. It only matters for very light particles such as electrons, where the uncertainty is large compared with the values themselves.

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Chemistry papers from other years

2018 E.C. (2026)2016 E.C. (2024)2015 E.C. (2023)2014 E.C. (2022)2013 E.C. (2021)2005 E.C. (2013)

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Are these the real 2017 E.C. Chemistry entrance exam questions?

Yes. They come from the national university entrance exam in Chemistry that students sat in 2017 E.C. (2025). Our set holds 76 of its questions, each with its answer and an explanation that points back to the textbook.

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Can I download the Chemistry 2017 E.C. paper as a PDF?

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