CIE iGCSE Co-Ordinated Science B4. Biological molecules Exam Style Questions Paper 2
▶️ Answer/Explanation
Each food test uses a specific reagent that gives a distinctive colour change.
Benedict’s solution tests for reducing sugars, ethanol for fats, and iodine for starch.
The biuret test is the correct reagent for protein, giving a purple/violet colour when protein is present.
✅ Answer: (B)
Benedict’s solution tests for reducing sugars, ethanol for fats, and iodine for starch.
The biuret test is the correct reagent for protein, giving a purple/violet colour when protein is present.
✅ Answer: (B)
▶️ Answer/Explanation
The biuret test detects peptide bonds in proteins. A positive result is indicated by a colour change from blue (the original reagent) to purple/violet. Thus, the correct observation is purple.
✅ Answer: (D)
✅ Answer: (D)
▶️ Answer/Explanation
Proteins are composed of amino acids, which contain the elements carbon, hydrogen, oxygen, and nitrogen. The presence of nitrogen in the molecular formula is key to identifying an amino acid. Among the given options, only \(\mathrm{C_2H_5NO_2}\) contains nitrogen, which is the amino acid glycine. The other molecules are carbohydrates and fats which lack nitrogen.
✅ Answer: (A)
✅ Answer: (A)
▶️ Answer/Explanation
Iodine turning orange-brown (negative) shows no starch. Red precipitate with Benedict’s shows reducing sugars are present. Lilac with biuret shows protein is present. Clear solution with ethanol shows no fat is present.
✅ Answer: (B)
✅ Answer: (B)
▶️ Answer/Explanation
Carbohydrates, fats, and oils all contain only carbon (C), hydrogen (H), and oxygen (O); they do not contain nitrogen.
Proteins are built from amino acids which all contain an amino group \((-NH_2)\), meaning they contain carbon, hydrogen, oxygen, and nitrogen (and sometimes sulfur).
✅ Answer: (D)
Proteins are built from amino acids which all contain an amino group \((-NH_2)\), meaning they contain carbon, hydrogen, oxygen, and nitrogen (and sometimes sulfur).
✅ Answer: (D)