Short Answer 
The clue refers to a colorless liquid hydrocarbon, specifically an alkane. The answer is heptane, which is the seventh member of the alkane series and aligns with the description provided.
Step 1: Identify the Clue
The crossword clue you are solving is for a specific type of hydrocarbon. The clue is ‘a colorless liquid hydrocarbon of the alkane series.’ This hints at a compound that is part of a broader category of organic chemistry known as alkanes, which are hydrocarbons composed solely of *single bonds* between carbon atoms.
Step 2: Understand Alkanes
Alkanes are characterized by having only single bonds linking their carbon atom chains. While the first four alkanes (methane, ethane, propane, and butane) are gases at room temperature, alkanes become liquid as the carbon chain length increases. For instance, pentane is already a liquid, and this trend continues through higher alkanes, including heptane.
Step 3: Conclusion – The Answer
From the understanding of the alkane series, the colorless liquid hydrocarbon that fits the provided crossword clue is heptane. Heptane is the seventh member of the alkane series, and its characteristics align perfectly with the description given in the clue.