Question

which of the following observations would indicate that a material is a pure substance?
it does not dissolve in water.
it changes color after heating.
it cannot be physically broken down into different types of atoms.
it can be physically broken down into different types of molecules.

Explanation:

To determine if a material is a pure substance, we analyze each option:

• Option 1: Solubility in water (or lack thereof) doesn't determine if a material is pure. Mixtures (e.g., sand and water) or pure substances (e.g., oil) can be insoluble.
• Option 2: Color change on heating can be a physical or chemical change, but this doesn't indicate purity. Mixtures or pure substances can change color upon heating.
• Option 3: A pure substance (element or compound) has a fixed composition. Elements are made of one type of atom and can't be physically broken into different atoms. Compounds are made of molecules (or formula units) and can be chemically broken, but physical means (like mechanical separation) can't break a pure substance into different atom types. Mixtures can have different atom types that might be physically separable, but pure substances can't be physically broken into different atom types.
• Option 4: Physical breakdown into different molecules is possible for mixtures (e.g., separating components of a mixture of gases) or for compounds (but that's a chemical change, not physical). So this doesn't indicate purity.

So the observation that a material cannot be physically broken down into different types of atoms indicates it's a pure substance (specifically an element, a type of pure substance).

Answer:

It cannot be physically broken down into different types of atoms.