Question

1. which process shows a physical change only?

a) burning coal
b ) rusting of iron
c) frying an egg
d) boiling water into steam

1. which of the following is an endothermic process?

a) photosynthesis in plants
b )fireworks explosion
c) burning of paper
d) candle burning

1. when hydrochloric acid reacts with sodium hydroxide, what substance is produced along with water?

a) oxygen
b ) salt (nacl)
c) hydrogen gas
d) carbon dioxide

1. which is the best evidence of a chemical reaction when two clear solutions are mixed?

a) a solid precipitate forms
b )liquid level decreases
c) solution turns clear
d) liquid evaporates

Explanation:

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

Step 1: Analyze Question 4 (Physical vs. Chemical Changes)

A physical change alters the form or state of a substance without changing its chemical identity (no new substances are made). A chemical change transforms a substance into one or more entirely new substances with different chemical properties.

• A) Burning coal: Combustion reaction that produces carbon dioxide and ash (Chemical).
• B) Rusting of iron: Iron reacts with oxygen and moisture to form iron oxide (Chemical).
• C) Frying an egg: Heat denatures and permanently changes the structure of proteins (Chemical).
• D) Boiling water into steam: Liquid water changes state to gaseous water (\(\text{H}_2\text{O}\) remains \(\text{H}_2\text{O}\)) (Physical).

Therefore, the process showing a physical change only is D.

Step 2: Analyze Question 5 (Endothermic vs. Exothermic Processes)

An endothermic process absorbs heat/energy from its surroundings to proceed. An exothermic process releases heat/energy into its surroundings.

• A) Photosynthesis in plants: Plants absorb light energy from the sun to convert carbon dioxide and water into glucose and oxygen (Endothermic).
• B) Fireworks explosion: Releases massive amounts of heat, light, and sound (Exothermic).
• C) Burning of paper: Combustion reaction that releases heat and light (Exothermic).
• D) Candle burning: Combustion reaction that releases heat and light (Exothermic).

Therefore, the endothermic process is A.

Step 3: Analyze Question 6 (Acid-Base Neutralization)

When an acid reacts with a base, they undergo a neutralization reaction to produce a salt and water:

$$\text{Acid} + \text{Base} ightarrow \text{Salt} + \text{Water}$$

Here, hydrochloric acid (\(\text{HCl}\)) reacts with sodium hydroxide (\(\text{NaOH}\)):

$$\text{HCl} + \text{NaOH} ightarrow \text{NaCl} + \text{H}_2\text{O}$$

The substance produced along with water is sodium chloride, commonly known as salt (\(\text{NaCl}\)).

Therefore, the correct option is B.

Step 4: Analyze Question 7 (Evidence of a Chemical Reaction)

When two clear liquid solutions are mixed, several indicators can point to a chemical reaction occurring. A key piece of evidence is the formation of an insoluble solid from the reaction of dissolved ions, known as a precipitate.

• A) A solid precipitate forms: Direct evidence of a new insoluble substance being created (Chemical reaction).
• B) Liquid level decreases: Usually due to evaporation, which is a physical change.
• C) Solution turns clear: If they were already clear solutions, remaining clear does not indicate a reaction.
• D) Liquid evaporates: A physical phase change from liquid to gas.

Therefore, the best evidence of a chemical reaction is A.

Answer:

• Question 4: D) Boiling water into steam
• Question 5: A) Photosynthesis in plants
• Question 6: B) Salt (NaCl)
• Question 7: A) A solid precipitate forms