Chemistry : common mistakes in P2 section A spm 2012 dont do it

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Example 1

Write the chemical formula of sulphur trioxide (Tuliskan formula kimia untuk sulfur dioxide) Q1 (a) (iii)

Students must write chemical formula and not chemical name.
Correct answer: SO₃

Example 2

Sulphuric acid produced is mixed with water in a beaker. State the observation if a piece of marble is dropped into the beaker. (Asid sulfurik yang terhasil dilarutkan ke dalam bikar berisi air. Nyatakan pemerhatian jika sebutir marmar dimasukkan ke dalam bikar itu.) Q1 (a) (vi)

Wrong answer: Carbon dioxide / Oxygen / Nitrogen gas is released.

Students are required to write the observation (through your naked eyes) not the type of gas or the name of the solutions. Students cannot see the gas molecule (unless students have Superman’s eye)
Correct answer: Bubbles of gas are released // Gas bubbles are released // Effervescence occurs in the beaker
(Also accept: Beaker becomes warm // mass of marble decrease)

Example 3

State one use of ammonium sulphate in daily life. (Nyatakan satu kegunaan ammonium sulfat dalam kehidupan seharian.) Q1 (c) (ii)

Wrong answer: fertilizer / fertiliser

Students answer is not complete and did not state the use.
Correct answer: As fertilisers / Used as a fertiliser / Ammonium sulphate is used as fertiliser in daily life.

Example 4

State the melting point of substance P. (Nyatakan takat lebur bahan P.) Q2 (a) (ii)

Wrong answer: 83 / 83˚

Without measuring unit, the answer is considering wrong response. The measuring unit must include in the answer and the measuring unit must be correct.
Correct answer: 83˚C

Example 5

State the physical state of P from time t₁ to t₂. (Nyatakan keadaan fizikal P dari masa t₁ to t₂) Q2 (a) (iii)

Wrong answer: solid

There are two physical states present from t₁ to t₂.
Correct answer: solid and liquid

Example 6

Explain why the temperature remains constant from t₁ to t₂. (Terangkan kenapa suhu tidak berubah dari masa t₁ to t₂.) Q2 (a) (iv)

Wrong answer: To overcome the force of attraction between atoms.

From the prediction (also one of the PEKA experiment in Form 4), solid P is naphthalene so it should be molecules not atoms.
Correct answer: is used to overcomes the forces of attraction between the molecules / particles P // It is because the heat energy absorbed is used to overcomes the force of attraction between particle so that solid P can melt into liquid state. // The heat absorbed by particles of substance P is used to overcome the forces holding the particles in fixed positions.

Example 7

Write the formulae for all anions that are present in sodium chloride solution. (Tuliskan formula bagi semua anion yang hadir dalam larutan natrium klorida.) Q3 (a) (ii)

Wrong answer: hydroxide ions, chloride ion

The question is asked for the formulae not chemical name, so students are required to read the question carefully.
Correct answer: Cl^- , OH^-

Example 8

Describe a test to indentify the gas collected at the cathode. (Huraikan satu ujian untuk mengenalpasti gas yang terkumpul di katod) Q3 (b) (iii)

Wrong answer: A burning / glowing wooden splinter is inserted into the test tube.

To test hydrogen gas – a lighted wooden splinter is used.
To test oxygen gas – a glowing wooden splinter is used.
To test chlorine gas – a damp blue litmus paper is used and litmus paper will be decolourised.
Another mistake is ‘inserted’. Students should use place or put into the test tube.
Correct answer:
1) Place a lighted wooden splinter into the test tube // A lighted wooden splint is placed into the test tube of the cathode
2) A ‘pop’ sound is produce / can be heard // The presence of the gas will cause the lighted wooden splinter to burn with a ‘pop’ sound.

Example 9

Give a reason for your answer in (c) (i). (Beri alas an bagi jawapan anda di (c) (i).) Q3 (c) (ii)

Wrong answer: Because hydroxide ion is more reactive.

Students should give explanation by discussing the position in electrochemistry series.
Correct answer: Because hydroxide ion is located lower than chloride ion in electrochemistry series.

Example 10

Name a substance that can replace bromine water. (Namakan satu bahan lain yang boleh menggantikan air bromine.) Q4 (a) (iv)

Wrong answer: KMnO₄ / Iodine water

Students must name the substance not give chemical formula. Besides that, the solution must be present.
Correct answer: Acidified potassium manganate(VII) solution // Acidified potassium dichromate(VI) solution // Chlorine water.

Example 11

What is the function of dilute sulphuric acid? (Apakah fungsi asid sulfurik cair?) Q5 (a)

Wrong answer: to separate the two solutions.

This is not the main function of dilute sulphuric acid and zero mark will be given.
Correct answer: To complete the electric circuit // To complete the flow of electric current throughout the circuit // To allow movement of ions.

Example 12

State the observation at carbon electrode X. (Nyatakan pemerhatian di elektrod karbon X.) Q5 (b) (i)

Wrong answer: solution turns yellow.

For all the observation (colour changes), students must state the colour of the solution before reaction and after the reaction or ‘the xxx colour of the solution turns / changes  yyy colour’. In simple words, it must have before changes and after changes.
Correct answer: Green colour solution turns to brown / yellow.

Example 13

State the observation at carbon electrode Y. (Nyatakan pemerhatian di elektrod karbon Y.) Q5 (c) (i)

Wrong answer: It decolourise / It turns colourless.

Similar to example 12, students must describe the colour before changes.
Correct answer: Purple colour solution is decolourised / turns colourless // The purple acidified potassium manganate(VII) solution turns colourless.

Example 14

What is alkali X?

Wrong answer: Potassium hydroxide, KOH

For this question, chemical formula and/or chemical name are accepted. Only concentrated sodium hydroxide will be use in the production of soaps and glycerol.
Correct answer: Concentrated sodium hydroxide / Concentrated NaOH.

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