Week 15 · Term 1 review (Units 1–4) and Darslab’s own review tasks · 3 lessons
2
Review: reactions, equations and nutrients
Science Prep 2 Term 1, Units 1–4: Review
GoalReview chemical changes, equations, conservation of mass and the three food groups with new mixed tasks.
New words
chemical reaction · bonds break and new bonds form to make new substancesbalanced equation · the same number of atoms of each element on both sidescarbohydrate, fat, protein · the three food groups
Explanation
From Unit 2: a physical change makes no new substance, a chemical change does. Signs of a reaction are gas bubbles, a colour change, a precipitate, light, heat, a new smell, but each needs thinking. In a reaction atoms are rearranged, never made or lost, so mass is conserved in a closed system. To balance an equation, change coefficients only, never subscripts. Check by counting atoms. For nutrition: carbohydrates (sugars, starch, cellulose) are made of carbon, hydrogen and oxygen and give energy; fats have the same elements but give about twice as much energy per gram; proteins are chains of amino acids and also contain nitrogen. The lab tests give iodine with starch (dark blue), Benedict’s with glucose (colour change), Sudan IV with fat (red) and Biuret with protein (violet); we only read about them. For health questions, ask a doctor.
Food group
Elements
Test colour (told)
starch
C, H, O
iodine: dark blue
fat
C, H, O
Sudan IV: red
protein
C, H, O, N
Biuret: violet
Worked examples
2Ca + O2→2CaO
Balance P: Ca + O₂ → CaO. Left 1 Ca, 2 O. Put 2 before CaO: 2 Ca, 2 O on the right. Put 2 before Ca: 2 Ca on the left. Result: 2Ca + O₂ → 2CaO. In masses: 80 g Ca + 32 g O₂ → 112 g CaO. Half the amounts, 40 g Ca + 16 g O₂, give 56 g CaO.
Physical or chemical? Dissolving sugar in tea (physical); a nail rusting (chemical); water boiling (physical); milk going sour (chemical). The question to ask: has a new substance formed?
Match each food to its main nutrient: rice (carbohydrate), olive oil (fat), lentils (protein and carbohydrate), egg (protein), butter (fat).
Class activity
Equation workshop. Balance these five equations on paper and then check each by counting atoms: CO + O₂ → CO₂; Zn + HCl → ZnCl₂ + H₂; SO₂ + O₂ → SO₃; Cu + O₂ → CuO; Fe₂O₃ + C → Fe + CO₂. Use beads to build one of them. Then make a table of five foods from your kitchen with their main nutrient. Do not use any test chemicals.
Practice
1
Balance: CO + O₂ → CO₂
2CO + O₂ → 2CO₂.
2
Balance: Zn + HCl → ZnCl₂ + H₂
Zn + 2HCl → ZnCl₂ + H₂.
3
Balance: SO₂ + O₂ → SO₃
2SO₂ + O₂ → 2SO₃.
4
Balance: Cu + O₂ → CuO
2Cu + O₂ → 2CuO.
5
Balance: Fe₂O₃ + C → Fe + CO₂
2Fe₂O₃ + 3C → 4Fe + 3CO₂.
6
3 g of carbon burns completely with 8 g of oxygen in a closed flask. What mass of carbon dioxide forms? Which sign of a reaction would a bubbling fizz be, and why is bubbling alone not proof?
3 + 8 = 11 g. Bubbles are a gas sign, but boiling also makes bubbles, so we look for a new substance.
