Given a scientific passage and an answer, generate a question for the given answer.

Passage: Population density is the average number of individuals in a population per unit of area or volume. For example, a population of 100 insects that live in an area of 100 square meters has a density of 1 insect per square meter. If the same population lives in an area of only 1 square meter, what is its density? Which population is more crowded? How might crowding affect the health of a population?. Answer: population density
What is the term for the average number of individuals in a population per unit of area or volume?

Passage: Zinc reacting with hydrochloric acid produces bubbles of hydrogen gas. Answer: hydrogen
Zinc reacting with hydrochloric acid produces bubbles of which gas?

Passage: Alkenes can have different shapes. They can form straight chains, branched chains, or rings. Alkenes with the same atoms but different shapes are called isomers. Smaller alkenes have relatively high boiling and melting points, so they are gases at room temperature. Larger alkenes have lower boiling and melting points, so they are liquids or waxy solids at room temperature. Answer: isomers
Alkenes with the same atoms but different shapes are called what?