Singapore G3 Science Chemistry and Biology free practice questions with answers

1. The table gives the melting points and boiling points of four substances. Which substance is a gas at 50 °C? [1 mark] [Melting and boiling points]

substance melting point / °C boiling point / °C P −210 −196 Q 232 2602 R 114 184 S −7 59

Correct answer: A — P

• A substance is a solid below its melting point, a liquid between its melting point and its boiling point, and a gas above its boiling point.

• At 50 °C, P is a gas.

• Why the other options are wrong:

• B (Q): at 50 °C it is a solid

• C (R): at 50 °C it is a solid

• D (S): at 50 °C it is a liquid

2. How many neutrons are there in one ion of ¹⁵₇N³−? [1 mark] [Atomic structure]

Correct answer: A — 8

• In the notation, the upper number is the nucleon number A = 15 and the lower number is the proton number Z = 7.

• Number of neutrons = A − Z = 15 − 7 = 8.

• Why the other options are wrong:

• B (10): a different particle count

• C (15): the nucleon number

• D (22): the two numbers were added

3. A particle contains 11 protons, 12 neutrons and 10 electrons. Which of the following is its correct symbol? [1 mark] [Atomic structure]

Correct answer: A — ²³₁₁Na+

• Protons = 11, so the element is Na (Z = 11).

• Nucleon number A = protons + neutrons = 11 + 12 = 23.

• Charge = protons − electrons = 11 − 10 = +1, so the particle is a cation.

• Why the other options are wrong:

• B (²³₁₂Mg+): the element is fixed by the number of protons

• C (²³₁₁Na−): the charge has the wrong sign

• D (²⁴₁₁Na+): the nucleon number is the number of protons plus neutrons