Question

part e - critical thinking

1. a cold pack becomes icy after being squeezed. is this reaction endothermic or exothermic? how do you know?
2. a cube of aluminum (mass = 54 g, volume = 20 cm³) is placed in water (density = 1 g/cm³). will it sink or float? show your calculations.
3. which type of chemical reaction is shown? cao + h₂o → ca(oh)₂ explain your reasoning.
4. compare the ph of lemon juice, soap, and pure water. classify each as acid, base, or neutral.
5. a student mixes vinegar and baking soda in a balloon attached to a flask. the balloon inflates. what evidence shows a chemical reaction occurred?

Explanation:

Step1: Determine endothermic or exothermic for cold - pack

An endothermic reaction absorbs heat from the surroundings, making the surroundings feel cold. Since the cold - pack becomes icy (cold to the touch) after being squeezed, heat is absorbed from the surroundings. So it is an endothermic reaction.

Step2: Determine if aluminum cube sinks or floats

Calculate the density of the aluminum cube using the formula $
ho=\frac{m}{V}$, where $m = 54\ g$ and $V = 20\ cm^{3}$. So $
ho=\frac{54}{20}=2.7\ g/cm^{3}$. Since the density of water is $1\ g/cm^{3}$ and the density of aluminum ($2.7\ g/cm^{3}$) is greater than the density of water, the aluminum cube will sink.

Step3: Identify chemical reaction type

In the reaction $CaO + H_{2}O
ightarrow Ca(OH)_{2}$, two substances ($CaO$ and $H_{2}O$) combine to form a single product ($Ca(OH)_{2}$). This is a synthesis reaction.

Step4: Compare pH and classify

Lemon juice is acidic with a pH less than 7. Soap is basic with a pH greater than 7. Pure water is neutral with a pH of 7.

Step5: Identify evidence of chemical reaction

When vinegar (acetic acid) and baking soda (sodium bicarbonate) react, they produce carbon dioxide gas. The inflation of the balloon attached to the flask is evidence that a gas was produced, which is a sign of a chemical reaction.

Answer:

1. Endothermic. Heat is absorbed as the pack becomes cold.
2. Sink. Density of aluminum is $2.7\ g/cm^{3}$ which is greater than water's density of $1\ g/cm^{3}$.
3. Synthesis reaction. Two substances combine to form one product.
4. Lemon juice: acidic, pH < 7; Soap: basic, pH > 7; Pure water: neutral, pH = 7.
5. The balloon inflates due to the production of carbon - dioxide gas, which is evidence of a chemical reaction.