Final answer:
A flame ionization detector can detect methane, ethanol, and acetone, as they produce ions when pyrolyzed in a flame, but it cannot effectively detect water. So options (1), (2) & (3) are correct.
Step-by-step explanation:
Among the molecules listed, a flame ionization detector (FID) would be able to detect methane (CH4), ethanol (C2H5OH), and acetone (CH3COCH3) because these substances produce ions when they are pyrolyzed in a flame, which is the principle of detection in FID. However, water (H2O) does not effectively produce ions in a flame and therefore would typically not be detected by an FID.
Methane (CH4): The FID can detect methane, as it is a hydrocarbon that readily ionizes in the hydrogen flame, producing a measurable signal.
Ethanol (C2H5OH): Ethanol can be detected by the FID as well. Similar to methane, it is a hydrocarbon-based compound that ionizes in the flame.
Acetone (CH3COCH3): Acetone can also be detected by the FID. It contains carbon-hydrogen bonds that can undergo ionization in the hydrogen flame.
Water (H2O): The FID is not suitable for detecting water because it is primarily designed for organic compounds with carbon-hydrogen bonds. Water does not contain such bonds and does not ionize effectively in the flame used by the FID.
In summary, the FID would detect methane, ethanol, and acetone among the given options due to their organic nature and ability to ionize in the hydrogen flame.