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.