Which of the following substances are mixtures rather than pure compounds?

Introduction / Context:
In basic chemistry, it is important to distinguish between pure substances and mixtures. This question checks whether you can identify which everyday materials are composed of a single chemical species and which ones contain a combination of different substances. Understanding this distinction helps in topics like separation techniques, physical properties, and industrial applications of materials.

Given Data / Assumptions:

• We are given four options: distilled water, gasoline, liquefied petroleum gas, and pure oxygen gas.
• We assume standard laboratory or household conditions.
• We use the common chemistry definition of a mixture as a physical combination of two or more substances that are not chemically bonded.
• We treat gasoline and liquefied petroleum gas as they exist in commercial use.

Concept / Approach:
A pure substance contains only one type of particle and has a fixed composition, such as a single compound or single element. A mixture contains more than one type of particle and may have variable composition. Gasoline and liquefied petroleum gas are well known to be blends of different hydrocarbons, so they are mixtures. Distilled water is purified water, which is essentially the compound H2O. Pure oxygen gas is the element O2. Both of these are considered pure substances when no other dissolved or mixed species are present.

Step-by-Step Solution:
Step 1: Recall that distilled water is water that has been boiled and condensed to remove dissolved salts and impurities. It is therefore essentially pure H2O, a pure compound. Step 2: Gasoline (petrol) is a commercial fuel obtained from petroleum refining. It contains a complex mixture of many different hydrocarbon molecules with varying chain lengths and structures. Step 3: Liquefied petroleum gas (LPG) is mainly a mixture of propane, butane, and sometimes other light hydrocarbons, stored under pressure in liquid form. Step 4: Pure oxygen gas is a single element in molecular form (O2). When no other gases are present, it is treated as a pure substance. Step 5: From these facts, we see that gasoline and liquefied petroleum gas are mixtures, while distilled water and pure oxygen are pure substances.

Verification / Alternative check:
A quick way to verify is to ask whether composition can vary from sample to sample. In gasoline, the exact proportion of different hydrocarbons can change between refineries or even batches, so it is a mixture. The same is true for liquefied petroleum gas, where the propane and butane ratio can vary. Distilled water is always H2O with negligible impurities, and pure oxygen is always O2. Since both gasoline and liquefied petroleum gas fit the definition of mixtures, the option that combines them is the correct one.

Why Other Options Are Wrong:
Distilled water: This is a pure compound, water, with dissolved impurities removed, so it is not a mixture in this context.
Gasoline (petrol): This alone is a correct example of a mixture, but the option that includes both gasoline and liquefied petroleum gas is more complete.
Liquefied petroleum gas (LPG): This alone is also a mixture, but again the combined option is designed to be the best choice.
Pure oxygen gas: This is a single element and is not a mixture when supplied as pure O2.

Common Pitfalls:
Students sometimes think that anything in a bottle or cylinder is a mixture and anything that looks clear is pure, which is not a reliable test. Others may forget that fuels are almost always blends of several hydrocarbons. Another common mistake is to assume that because water is common in nature with dissolved salts, all water samples are mixtures, but distilled water is carefully purified. It is important to focus on the chemical composition, not just appearance or use.

Final Answer:
Both gasoline and liquefied petroleum gas are mixtures, so the correct option is the combined one.