Chapter 3, Problem 49AP

Classify each of the following as aphysicalorchemicalchange or property.

1. Milk curdles if a few drops of lemon juice are added to it.

Butter turns rancid if it is left exposed at room temperature.

Salad dressing separates into layers after standing.

Milk of magnesia neutralizes stomach acid.

The steel in a car has rust spots.

A person is asphyxiated by breathing carbon monoxide.

Sulfuric acid spilled on a laboratory notebook page causes the paper to char and disintegrate.

Sweat cools the body as the sweat evaporates from the skin.

Aspirin reduces fever.

Oil feels slippery.

Alcohol hums, forming carbon dioxide and water.

Interpretation Introduction

(a)

Interpretation:

Classify the following statement as a physical or chemical change or property.

1. Milk curdles if a few drop of lemon juice are added to it.

Concept Introduction:

Chemical change occurs in a chemical reaction where there is a change in the chemical composition of the substance. In a physical change, there is no change in the chemical composition of the substance but there is a change in the odor, color, melting point etc. of the substance.

Answer to Problem 49AP

Milk curdles if a few drops of lemon juice are added to it: chemical change.

Explanation of Solution

On adding the acid in lemon juice, the pH of milk drops, which causes the protein casein molecules to attract one another and form a clump. This curdling of milk is a chemical change which is accelerated in at warmer temperatures than at cold temperatures.

Interpretation Introduction

(b)

Interpretation:

Classify the following statement as a physical or chemical change or property.

Butter turns rancid if it is left exposed at room temperature.

Concept Introduction:

Chemical change occurs in a chemical reaction where there is a change in the chemical composition of the substance. In a physical change, there is no change in the chemical composition of the substance but there is a change in the odor, color, melting point etc. of the substance.

Answer to Problem 49AP

Butter turns rancid if it is left exposed at room temperature: chemical change.

Explanation of Solution

Butter has fat in it and when fat undergoes oxidation reaction in air it produces a gas that is unpleasant smelling. This how butter turns rancid if it is left exposed at room temperature, which is a chemical change.

Interpretation Introduction

(c)

Interpretation:

Classify the following statement as a physical or chemical change or property.

Salad dressing separates into layers after standing.

Concept Introduction:

Chemical change occurs in a chemical reaction where there is a change in the chemical composition of the substance. In a physical change, there is no change in the chemical composition of the substance but there is a change in the odor, color, melting point etc. of the substance.

Answer to Problem 49AP

Salad dressing separates into layers after standing: physical change .

Explanation of Solution

Salad dressing is made up on oil, vinegar, salt, pepper and water. Oil and water are immiscible; hence salad dressing separates into layers after standing. Salad dressing is a mixture and it separating into layers after standing is a physical change. There is no change in the chemical composition of the components of a salad dressing.

Interpretation Introduction

(d)

Interpretation:

Classify the following statement as a physical or chemical change or property.

Milk of magnesia neutralizes stomach acid.

Concept Introduction:

Chemical change occurs in a chemical reaction where there is a change in the chemical composition of the substance. In a physical change, there is no change in the chemical composition of the substance but there is a change in the odor, color, melting point etc. of the substance.

Answer to Problem 49AP

Milk of magnesia neutralizes stomach acid: chemical change.

Explanation of Solution

Milk of magnesia is Mg(OH)₂, a base, which reacts with HCl in stomach to form a salt MgCl₂ Here there is formation of a new product. Hence, milk of magnesia neutralizing stomach acid is a chemical change.

Interpretation Introduction

(e)

Interpretation:

Classify the following statement as a physical or chemical change or property.

The steel in a car has rust spots.

Concept Introduction:

Chemical change occurs in a chemical reaction where there is a change in the chemical composition of the substance. In a physical change, there is no change in the chemical composition of the substance but there is a change in the odor, color, melting point etc. of the substance.

Answer to Problem 49AP

The steel in a car has rust spots: chemical change.

Explanation of Solution

Rusting is a chemical redox reaction. Iron in the presence of oxygen in the air and water or air moisture gets converted to hydrated iron(III) oxides Fe₂ O.

Interpretation Introduction

(f)

Interpretation:

Classify the following statement as a physical or chemical change or property.

A person is asphyxiated by breathing carbon monoxide.

Concept Introduction:

Chemical change occurs in a chemical reaction where there is a change in the chemical composition of the substance. In a physical change, there is no change in the chemical composition of the substance but there is a change in the odor, color, melting point etc. of the substance.

Answer to Problem 49AP

A person is asphyxiated by breathing carbon monoxide: chemical change.

Explanation of Solution

Carbon monoxide binds strongly and irreversibly to the heme protein in hemoglobin. As a result hemoglobin can no longer carry oxygen in the blood. So in the person of carbon monoxide hemoglobin undergoes a chemical change. Therefore, a person is asphyxiated by breathing carbon monoxide is a chemical change.

Interpretation Introduction

(g)

Interpretation:

Classify the following statement as a physical or chemical change or property.

Sulfuric acid spilled on a laboratory notebook page causes the paper to char and disintegrate.

Concept Introduction:

Chemical change occurs in a chemical reaction where there is a change in the chemical composition of the substance. In a physical change, there is no change in the chemical composition of the substance but there is a change in the odor, color, melting point etc. of the substance.

Answer to Problem 49AP

Spilled acid burns a hole in cotton jeans: chemical change.

Explanation of Solution

Acids are corrosive and they can break the carbon carbon covalent bonds in cotton fabrics that jeans are made up of. So acids buring a hole in cotton jeans is a chemical change.

Interpretation Introduction

(h)

Interpretation:

Classify the following statement as a physical or chemical change or property.

Sweat cools the body as the sweat evaporates from the skin.

Concept Introduction:

Chemical change occurs in a chemical reaction where there is a change in the chemical composition of the substance. In a physical change, there is no change in the chemical composition of the substance but there is a change in the odor, color, melting point etc. of the substance.

Answer to Problem 49AP

Sweat cools the body as the sweat evaporates from the skin: physical change.

Explanation of Solution

Sweat is in the liquid state. In order to evaporate or get transformed into vapor phase, sweat absorbs heat from the surrounding area on the skin, thereby cooling the whole body.

The transformation of sweat from liquid to gas phase is a physical change.

Interpretation Introduction

(i)

Interpretation:

Classify the following statement as a physical or chemical change or property.

Aspirin reduces fever.

Concept Introduction:

Chemical change occurs in a chemical reaction where there is a change in the chemical composition of the substance. In a physical change, there is no change in the chemical composition of the substance but there is a change in the odor, color, melting point etc. of the substance.

Answer to Problem 49AP

Aspirin reduces fever: chemical change.

Explanation of Solution

Aspirin binds to an enzyme COX and inhibits it. Thus preventing COX from producing a high level of prostaglandin, which is an inflammatory agent. Prostaglandin levels are high during and aspirin works to reduce the level of prostaglandin thus reducing pain and inflammation during fever.

Interpretation Introduction

(j)

Interpretation:

Classify the following statement as a physical or chemical change or property.

Oil feels slippery.

Concept Introduction:

Chemical change occurs in a chemical reaction where there is a change in the chemical composition of the substance. In a physical change, there is no change in the chemical composition of the substance but there is a change in the odor, color, melting point etc. of the substance.

Answer to Problem 49AP

Oil feels slippery: physical change.

Explanation of Solution

Slipperiness of oil is its physical property. There is no change in the chemical composition of the oil.

Interpretation Introduction

(k)

Interpretation:

Classify the following statement as a physical or chemical change or property.

Alcohol burns, forming carbon dioxide and water.

Concept Introduction:

Chemical change occurs in a chemical reaction where there is a change in the chemical composition of the substance. In a physical change, there is no change in the chemical composition of the substance but there is a change in the odor, color, melting point etc. of the substance.

Answer to Problem 49AP

Alcohol burns, forming carbon dioxide and water: chemical change.

Explanation of Solution

Combustion of alcohol is a chemical change because after burning alcohol no longer exists but all of it gets converted to carbon dioxide and water.

C₂ H₅ OH + 3 O₂ = 2 CO₂ + 3 H₂ O.