Question

1. which of the following processes absorbs energy from the surroundings?

a) burning charcoal
b ) lighting a match
c) melting ice
d) respiration

1. which of these changes is reversible and physical?

a) cooking rice
b ) rusting iron
c) burning wood
d) condensation of water vapor

1. what type of chemical reaction is represented by:

\\(2\text{h}_2 + \text{o}_2 \
ightarrow 2\text{h}_2\text{o}\\)
a) decomposition
b ) single replacement
c) double replacement
d) combination (synthesis)

1. what do we call the substances present \before\ a chemical reaction starts?

a) catalysts
b ) indicators
c) products
d) reactants

Explanation:

🆕 New Concept Discovered: Endothermic vs. Exothermic Processes · 🆕 New: Physical vs. Chemical Changes

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Step 1: Analyze Question 8 (Energy Changes)

We need to find the process that absorbs energy from the surroundings (an endothermic process).

• Burning charcoal (A) and Lighting a match (B) are combustion reactions. They release heat energy to the surroundings (exothermic).
• Melting ice (C) is a phase change from solid to liquid. To break the intermolecular bonds holding the ice together, the system must absorb heat energy from its surroundings.
• Respiration (D) is a metabolic process that breaks down glucose to release energy for cells (exothermic).

Therefore, melting ice is the process that absorbs energy.

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Step 2: Analyze Question 9 (Physical vs. Chemical Changes)

We need to find a change that is both reversible (can be undone to get the original substance back) and physical (does not alter the chemical identity of the substance).

• Cooking rice (A), Rusting iron (B), and Burning wood (C) are chemical changes. They create new chemical substances and cannot be easily reversed.
• Condensation of water vapor (D) is a phase change from gas to liquid. The chemical structure of the water molecules remains \( \text{H}_2\text{O} \) (physical change), and it can easily be reversed by boiling or evaporating the water back into vapor.

Therefore, condensation of water vapor is a reversible physical change.

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Step 3: Analyze Question 10 (Types of Chemical Reactions)

We need to classify the chemical equation:

$$ 2\text{H}_2 + \text{O}_2 ightarrow 2\text{H}_2\text{O} $$

• In this reaction, two simpler reactant substances (\( \text{H}_2 \) and \( \text{O}_2 \)) combine to form a single, more complex product (\( \text{H}_2\text{O} \)).
• This fits the general form:

$$ \text{A} + \text{B} ightarrow \text{AB} $$

• This type of reaction is called a combination or synthesis reaction.

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Step 4: Analyze Question 11 (Reaction Terminology)

We need to identify the term for substances present before a chemical reaction starts.

• Reactants are the starting substances written on the left side of a chemical equation.
• Products are the substances formed after the reaction takes place, written on the right side.
• Catalysts speed up a reaction but are not consumed by it.
• Indicators change color to show the presence of a substance or a change in pH.

Therefore, the starting substances are called reactants.

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Answer:

• Question 8: C) Melting ice
• Question 9: D) Condensation of water vapor
• Question 10: D) Combination (synthesis)
• Question 11: D) Reactants