When a Sec 3 student tells me, “I know the formula, but I never know which number to use,” I ask them to show me a question they got wrong. Before we touch the calculator, I’ll ask, “What substance are you working with, and what is the question asking you to find?” If we can answer those two things, the next line usually gets easier.
I teach Secondary 3 Chemistry in Woodlands, near Admiralty MRT. I cap in-person classes at six students so I can check each student’s working and see whether the sticking point is a concept, a calculation or the wording of an explanation.
WhatsApp Mr. Jimmy about Sec 3 Chemistry
What we work on in Secondary 3 Chemistry
Bring a recent worksheet or Weighted Assessment if you have one. Tell me whether you take Pure Chemistry or Chemistry as part of Combined Science. The topics and depth vary, so I’ll use your schoolwork to decide where to start.
- Atomic structure and chemical bonding. Work out how many electrons an ion has, write formulas from ionic charges, and use particle models to explain properties.
- Chemical equations and the mole concept. Balance equations, convert grams to moles and use the coefficients to link amounts of different substances.
- Acids, bases, salts and practical questions. Predict products, connect observations to gas tests and explain why a step is needed in a preparation method.
Parents can check the Chemistry and Combined Science subject listings in the SEAB G3 syllabuses for school candidates.
Six common Sec 3 Chemistry mistakes and how to fix them
1. Changing the number of protons when an ion forms
A sodium atom has 11 protons and 11 electrons. A sodium ion, Na+, still has 11 protons. It has 10 electrons because it lost one. Write the proton count first, then use the charge to work out the electron count.
2. Writing an ionic formula without checking the total charge
Calcium ions carry a 2+ charge, and chloride ions carry a 1− charge. One calcium ion needs two chloride ions, giving CaCl2. Add the charges before you move on: +2 + 2(−1) = 0.
3. Changing subscripts to balance an equation
When magnesium reacts with oxygen, the product is magnesium oxide, MgO. Keep its formula and change the numbers in front: 2Mg(s) + O2(g) → 2MgO(s). Count two magnesium atoms and two oxygen atoms on each side. Changing MgO to MgO2 changes the substance.
4. Using a mole ratio on grams
In 2H2(g) + O2(g) → 2H2O(l), start with 4.0 g of hydrogen and excess oxygen. Divide by hydrogen’s molar mass, 2.0 g/mol, to get 2.0 mol of H2. The balanced equation gives a 1:1 mole ratio of hydrogen to water, so you form 2.0 mol of water, or 36 g. Keep the substance and unit beside each number.
5. Explaining melting by breaking the wrong bonds
When ice melts, the water molecules stay intact. The attractions between the molecules are partly overcome so the molecules can move past one another. If the question asks about a physical change in a simple molecular substance, check whether your answer talks about the molecules moving or about a new substance forming.
6. Writing an inference when asked for an observation
When a dilute acid reacts with a carbonate, “bubbles are seen” is an observation. “Carbon dioxide is produced” is an inference you can support with a test: the gas turns limewater milky. I ask students to write what they can see first, then what that tells them.
How I help students prepare for Chemistry assessments
After we correct a question, I’ll change one thing and ask the student to try again. If I give them grams of oxygen instead of grams of hydrogen, can they choose the new molar mass and mole ratio? If the acid reacts with a different substance, can they predict what they’ll see? That’s how I check whether the method has stuck. For more practice, see my Chemistry guide to acids, bases and salts.
Ask about Secondary 3 Chemistry tuition
Tell me which Chemistry course your child takes, what topic they’re studying and where they usually lose marks. I can suggest a starting point and share the current class options. You can also read our FAQs about class size and makeup lessons or find Jimmy Tuition on the map.