Which of the following is not a pure substance?

The correct answer is option 3. Sugar solution.

A pure substance is a material that has a constant composition and distinct properties. Let's examine each option to understand why a sugar solution is considered not a pure substance:

1. Copper:

Copper is an element, and if it is in a pure form without any impurities, it is a pure substance. It consists of only one type of atom (Cu).

2. Gold:

Similar to copper, gold is an element. In its pure form without impurities, it is a pure substance. It consists of only one type of atom (Au).

3. Sugar Solution:

A sugar solution is a mixture composed of sugar (a compound) dissolved in water (a compound). Since it is a mixture of different substances and does not have a constant composition throughout, it is considered not a pure substance. The properties of a sugar solution can vary depending on the ratio of sugar to water.

4. Water:

Water, in its pure form (without impurities), is a compound and, therefore, a pure substance. It consists of two hydrogen atoms and one oxygen atom (H₂O).

In summary, while copper, gold, and pure water are examples of pure substances, a sugar solution is a mixture and not considered a pure substance due to its variable composition. The properties of a sugar solution can change depending on the amount of sugar dissolved in water.