Cleansing Agents – SPM 2008


Diagram shows the apparatus used to investigate how to remove an oily stain from a cloth and the observation made from that investigation.

(i) Compare and explain the effectiveness of the cleaning action between

  • experiment I and III
  • experiment II and IV.

(ii) identify the cleaning agents, A and B. State which cleaning   agent is more effective.

Suggested answer

(i)    Experiment I and III

  • Both the cleaning agents A and B are effective in soft water. Soft water does not consist of calcium and magnesium ions.
  • Both dissolves in soft water.
  • They are able to lower the surface tension of water. The water wets the surface of the cloth thoroughly.

         Experiment II and IV

  • Cleaning agent A is not effective in hard water. Hard water consists of calcium and magnesium ions.
  • These ions react with the cleaning agent A to form an insoluble precipitate (scum). Formation of scum reduces the number of cleaning agent A molecules available for cleaning.
  • Cleaning agent B is effective both in soft water and hard water.
  • Cleaning agent B does not form precipitate (scum) in hard water.  Cleaning agent B form soluble salt in hard water. The cleaning agent B molecules are available for cleaning.

(ii) Cleaning agent A is soap; Cleaning agent B is detergent.

Detergent is more effective than soap.


Elelctrolytic Cell and Chemical Cells

In an electrolytic cell,
  • Anion in the electrolyte move to  anode(connected to the positve terminal of the battery).
  • Anions is selectively discharged by losing electrons(Depends at the concentration of ions in the electrolyte). In other words, anion is oxidized at anode. If metal electrodes are used instead of carbon electrodes, metal is oxidized to its metal ions by losing electrons.
  • Cations in the electrolyte move to cathode(connected to the negative terminal of the battery).
  • Cations which is at the lower postion in the Electrochemical Series is selectively discharged by gaining electrons. In other words, cations are reduced.

oxidation occurs at anode while
Reduction occurs at cathode.

In a chemical cell
  •  Electrode which is more electropositive is oxidized to its ions by loses electrons, and acts as negative terminal.
  • Electrons move through the external circuit and received by positive terminal.
  • Cations in the electrolytes get attracted to the positive terminal and reduced to metal by gaining electrons.
Oxidation occurs at negative terminal.
Reduction occurs at positive terminal.


Control Iron Rusting by Using More Electropositive Metal

The hull of the Titanic has rusted since it sank in 1912.

Ship hull is protected by rusting by bolting magnesium bar onto it.  How does magnesium bar protect the ship from rusting? Explain.                                                                 [2 marks]
  • Magnesium is more electropositive than iron. 
  • Magnesium loses electrons more easily than iron.
  • Mg --> Mg2+ + 2e-
  • Electrons move to iron. Therefore iron is protected by rusting

The same method is used controlling rusting of underground water pipes or steel utility pole. Blocks/stacks of magnesium is connected to the water pipes/steel utility pole by wires. Magnesium is  replaced from time to time when all magnesium corrodes.   This method is more known as sacrificial protection.