7. Which of the following changes gives no evidence that a chemical reaction has taken place?

A cube of solid forms a puddle of liquid.
A certain liquid is added to a solid, and bubbles of gas form.
Two liquids are mixed, and a precipitate forms.
Heating a blue solid turns the solid white.

9. Which of the following represents a compound?
Na
He
H2O
Li

No.

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To determine which of the given options does not indicate evidence of a chemical reaction, we need to understand what constitutes a chemical reaction.

A chemical reaction involves the formation or breaking of chemical bonds, resulting in the formation of new substances with different properties. Some observable signs of a chemical reaction include the release of gas, formation of a precipitate, color change, and the absorption or release of heat.

Let's analyze each option:

7. A cube of solid forms a puddle of liquid.
This change suggests a phase change where a solid converts into a liquid. It does not provide any evidence for a chemical reaction as no new substances are formed.

A certain liquid is added to a solid, and bubbles of gas form.
This change indicates the production of gas, which is often an indication of a chemical reaction. The reacting substances may have combined to form a new compound, releasing gas in the process.

Two liquids are mixed, and a precipitate forms.
The formation of a precipitate indicates a chemical reaction where two reactant compounds undergo a reaction to create a solid product that falls out of the solution.

Heating a blue solid turns the solid white.
A color change, especially when accompanied by heating, is often a sign of a chemical reaction. The transformation from blue to white suggests a chemical change.

Therefore, the option that gives no evidence of a chemical reaction is:
7. A cube of solid forms a puddle of liquid.

Moving on to the next question:

9. Which of the following represents a compound?
A compound is a substance composed of two or more different elements chemically bonded together in a fixed ratio.

Na: This represents an element, sodium.

He: This represents an element, helium.

H2O: This represents the chemical formula for water, which is a compound. H2O consists of two elements, hydrogen and oxygen, bonded together.

Li: This represents an element, lithium.

Therefore, the option that represents a compound is:
H2O.

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