Question

1. which of the following is the best example of a chemical change? ice melting into liquid water. sugar dissolving in a glass of water. a metal nail rusting when exposed to air and moisture. water boiling and turning into steam. 2. which observation is the strongest indicator that a chemical reaction has likely occurred? a gas is produced when two solutions are mixed. the size of a substance is reduced after cutting it. a substance changes from a solid to a liquid. a substance feels warmer after being placed in sunlight.

Explanation:

Step1: Define chemical and physical changes

A chemical change results in a new substance with different properties. A physical change only changes the state or form of a substance without forming a new substance.

Step2: Analyze each option in question 1

• Ice melting into liquid water is a phase - change (solid to liquid), a physical change.
• Sugar dissolving in water is a physical change as sugar molecules disperse in water but no new substance is formed.
• A metal nail rusting is a chemical change. Iron reacts with oxygen and water to form iron oxide (rust), a new substance.
• Water boiling and turning into steam is a phase - change (liquid to gas), a physical change.

Step3: Define indicators of chemical reactions

Indicators of a chemical reaction include production of gas, formation of a precipitate, color change, temperature change due to heat release or absorption, and formation of a new substance.

Step4: Analyze each option in question 2

• Producing a gas when two solutions are mixed is a strong indicator of a chemical reaction as a new substance (the gas) is formed.
• Reducing the size of a substance by cutting is a physical change, not a chemical reaction indicator.
• A substance changing from solid to liquid is a physical phase - change, not a chemical reaction indicator.
• A substance feeling warmer after being placed in sunlight is a result of heat absorption, not a chemical reaction indicator.

Answer:

1. A metal nail rusting when exposed to air and moisture.
2. A gas is produced when two solutions are mixed.