Types of Pure Substances – Long Answer Questions

Medium Level (Application & Explanation)

Answer:

A pure substance has a constant composition and distinct properties.
It is well defined. It does not change from sample to sample.
Pure substances are of two types: Elements and Compounds.
An element has only one kind of atom. Example: Hydrogen (H), Oxygen (O), Gold (Au).
A compound has two or more elements chemically combined in a fixed proportion.
Examples: Water (H₂O), Carbon dioxide (CO₂), Sodium chloride (NaCl).
Elements use symbols. Compounds use formulas to show their composition.

Answer:

An element is a pure substance that cannot be broken into simpler substances by chemical means.
It has only one type of atom.
Each element has a unique symbol like H, O, Au.
Elements are listed in the Periodic Table as metals, non-metals, or metalloids.
Example: Gold (Au) is very non-reactive. It does not rust or tarnish.
Other examples are Hydrogen (H) and Oxygen (O).
Any pure metallic or non-metallic substance in its original form is an element.

Answer:

A compound is a pure substance made when two or more elements combine chemically.
The elements are present in a fixed proportion by mass or atoms.
Compounds have new properties different from the elements that form them.
They are written using chemical formulas like H₂O, CO₂, NaCl.
Compounds can be ionic (like NaCl) or covalent (like H₂O).
They can be broken down only by chemical reactions, not by physical methods.
Thus, a compound shows identity and consistency in composition.

Answer:

In a compound, elements are chemically bonded.
New bonds lead to new properties.
Example: Sodium (reactive metal) and Chlorine (poisonous gas) form NaCl (table salt), which is safe to eat.
Example: Hydrogen (flammable gas) and Oxygen (supports burning) form water (H₂O), which puts out fire.
So, compounds do not behave like their individual elements.
The fixed proportion and chemical combination cause this change.
This is why compounds are unique substances.

Answer:

Composition: Element has one type of atom; compound has two or more types.
Separation: Element cannot be broken by chemical means; compound can be broken chemically.
Properties: Elements retain their own properties; compounds have new properties.
Formula/Symbol: Elements use symbols (H, O, Au); compounds use formulas (H₂O, CO₂, NaCl).
Density: Elements have fixed density; compounds’ density can vary across different compounds.
Examples: Elements—H, O, Fe, Au; Compounds—H₂O, CO₂, NaCl.
Both are pure substances with consistent composition.

Answer:

Oxygen gas (O) is an element. It has only oxygen atoms. It has a symbol O and is in the Periodic Table.
Gold foil (Au) is an element. It has only gold atoms. It is very non-reactive.
Water (H₂O) is a compound. It has hydrogen and oxygen in a fixed proportion.
Carbon dioxide (CO₂) is a compound. It has carbon and oxygen in a fixed ratio.
Table salt (NaCl) is a compound. It has sodium and chlorine.
Elements use symbols. Compounds use formulas that show ratios.
Thus, oxygen and gold are elements, while water, carbon dioxide, and salt are compounds.

Answer:

Water (H₂O) is a compound with chemically combined elements.
Filtration and evaporation are physical methods. They do not break chemical bonds.
So, these methods cannot split water into H and O.
Compounds can be broken only by chemical reactions.
We need a chemical change (like decomposition) to separate the elements.
This shows that compounds have fixed composition and need chemical means for separation.
Thus, physical methods fail for compounds.

Answer:

Water forms only when hydrogen and oxygen combine in a fixed proportion of 2:1 (H:O).
If the ratio is not 2:1, the extra gas remains as a mixture, not part of water.
A mixture does not have chemical bonding like a compound.
A compound needs a chemical reaction and a fixed ratio to form.
Water’s formula (H₂O) proves its constant composition.
Without the correct ratio, you do not get pure H₂O.
So, the idea of fixed proportion decides whether a compound forms.

Answer:

Yes, it is the same compound in all three states.
The chemical formula remains H₂O in solid, liquid, and gas.
The composition is constant, so it is a pure substance.
Only physical state changes. The chemical identity does not change.
Properties like shape and flow may change, but composition stays the same.
This shows that state changes are physical, not chemical.
The identity of water remains H₂O in every state.

Answer:

Disagree. Gold (Au) is an element that is very non-reactive.
It does not rust or tarnish easily. It often stays elemental.
Not all metals react in the same way. Reactivity depends on the element’s nature.
A compound forms only when elements chemically combine.
Gold’s low reactivity makes such combination less likely in normal conditions.
So, being a metal does not mean it will easily form compounds.
Gold is a good example of a stable element.