8th Grade Physical Science Chapter 2 Review

An apple turning brown is an example of a chemical change because it involves a reaction known as oxidation. When the apple's flesh is exposed to oxygen in the air, enzymes in the apple begin to break down the chemicals in the apple, resulting in a change in color from white to brown. This change is irreversible and involves a chemical reaction, making it a clear example of a chemical change.

Salt is the most soluble substance in water because it is an ionic compound composed of positive and negative ions. When salt is added to water, the water molecules surround the ions and pull them apart, dissolving the salt. This process is known as hydration. The strong attraction between the ions and the polar water molecules allows for a high degree of solubility, making salt dissolve easily in water. In contrast, wood, sand, and iron are not soluble in water as they are not composed of ions and do not have the same polar properties.

The term used to describe the temperature at which a solid becomes a liquid is the melting point.

The correct answer is "reacts with HCI." This is a chemical property because it describes how the substance reacts with another substance, in this case, hydrochloric acid (HCI). Chemical properties relate to the substance's ability to undergo chemical reactions and change its composition. In this case, the substance shows a chemical reaction with HCI, indicating its reactivity and providing information about its chemical behavior.

Cutting a piece of paper, crushing salt into smaller pieces, and sharpening a pencil are all examples of physical changes. In these processes, the substances undergo a change in their physical appearance or form, but their chemical composition remains the same. The paper is still made of cellulose, the salt is still composed of sodium and chlorine ions, and the pencil is still made of graphite. Therefore, these changes are classified as physical changes rather than chemical changes, which involve a rearrangement of atoms and a formation of new substances.

A raw egg will float in concentrated salt water but sink in tap water because the density of the egg is less than that of the salt water but more than that of the tap water.

The student is using density to classify the objects. Density is the measure of how much mass is contained within a given volume. Objects with higher density will sink in water, while objects with lower density will float. Therefore, by dividing the objects into two groups based on their ability to float in water, the student is essentially classifying them based on their density.

The stomach acids play a crucial role in digestion by breaking down the food into nutrients. These acids, such as hydrochloric acid, help to dissolve the food particles and break them down into smaller molecules. This process is mainly a chemical change as it involves the breakdown of complex molecules into simpler ones through chemical reactions. Once the food is broken down into nutrients, it can be easily absorbed by the body for energy and other essential functions.

The ability of a substance to undergo a chemical reaction with another element is an example of a chemical property of matter. Chemical properties describe how a substance interacts with other substances to form new substances. In this case, the substance's reaction with another element indicates its chemical reactivity and ability to form new compounds.

Metals have the most efficient thermal conductivity compared to semimetals, nonmetals, and gases. This is because metals have a large number of free electrons that can easily move and transfer heat energy through the material. These free electrons can rapidly carry and distribute heat, making metals excellent conductors of heat. In contrast, semimetals, nonmetals, and gases have fewer free electrons or lack the necessary atomic structure to efficiently transfer heat, resulting in lower thermal conductivity.

During the process of digestion, proteins are broken down into smaller molecules through chemical changes. This involves the action of enzymes, such as proteases, which break the peptide bonds between amino acids in proteins. These smaller molecules, such as amino acids, can then be absorbed by the body and used for various purposes, including the synthesis of new proteins. Cell division, nuclear reactions, and natural selection are not directly involved in the process of protein digestion.

These changes are classified as chemical changes because new substances are formed. In each of the given examples, the original material undergoes a chemical reaction and transforms into a different substance. The iron reacts with the oxygen and moisture in the air to form rust. The firewood undergoes combustion and is converted into smoke and ashes. The copper penny reacts with the vinegar to form gas bubbles. In all of these cases, the composition of the original material changes, resulting in the formation of new substances.

When the chip of limestone is placed in hydrochloric acid solution, a chemical reaction occurs. The hydrochloric acid reacts with the limestone, causing it to break down and release carbon dioxide gas. This release of gas is evidence that a chemical change has occurred, as the reactants (limestone and hydrochloric acid) have been transformed into new substances (carbon dioxide gas and other compounds). This is different from a physical change, where the substance may change form or state but the chemical composition remains the same.

To calculate the density of an apple, one must know the mass and volume of the apple. Density is defined as the mass of an object divided by its volume. In this case, the mass of the apple and its volume are required to determine its density. The mass refers to the amount of matter in the apple, while the volume represents the space occupied by the apple. By dividing the mass by the volume, the density of the apple can be calculated.

The given answer is correct because it describes a physical change rather than a chemical reaction. Breaking sodium chloride into smaller pieces does not involve any chemical reactions or the formation of new substances. It is simply a change in the physical state of the substance.

Photosynthesis is a chemical reaction because new substances are formed. In this process, carbon dioxide and water react to produce sugar and oxygen. The reactants (carbon dioxide and water) combine and undergo a chemical change to form the products (sugar and oxygen). This transformation results in the formation of new substances, which is a characteristic of a chemical reaction.

The fact that water can separate into hydrogen and oxygen is an example of a chemical property because it involves a chemical reaction. When water undergoes electrolysis, it breaks down into its constituent elements, hydrogen and oxygen. This process requires a chemical change and is not simply a physical change like freezing or boiling. Chemical properties describe the behavior of substances in terms of their ability to undergo chemical reactions and form new substances.

Experiment 2 shows a chemical change because a new substance was produced. This can be inferred from the fact that bubbles were produced, the liquid became cloudy, and small, white solid particles appeared. These observations suggest a chemical reaction occurred, resulting in the formation of a new substance. In contrast, Experiment 1 only resulted in the production of bubbles, which is indicative of a physical change rather than a chemical change.

A physical change refers to a change in the physical properties of a substance without altering its chemical composition. Shredding a sheet of paper is a physical change because it only changes the size and shape of the paper, but the paper itself remains the same substance. Burning wood, changing colors of a copper roof, and rust forming on an iron fence are all chemical changes as they involve a transformation in the chemical composition of the materials involved.

Because the object must be less dense than the water to float in it

Salt decreases the freezing point of water because it lowers the temperature at which water molecules can bond together and form ice. This is why salt is commonly used to melt ice on roads and sidewalks before a snowstorm. By lowering the freezing point, the salt prevents the water from freezing and forming dangerous ice patches, making it easier and safer for people to walk or drive on the surfaces.

The observation of the lithium floating and making a buzzing sound as gas bubbles were forming around it indicates that a chemical change took place. This is because the formation of gas bubbles suggests a chemical reaction between the lithium and water, resulting in the release of gas. The floating and buzzing sound can also be attributed to the chemical reaction. Therefore, the observation supports the conclusion that a chemical change occurred.

The correct answer is density. Density is a property that describes the mass of a substance per unit volume. Since the two solid samples are of the same substance, it is likely that they have the same density. Mass, shape, and volume are not necessarily properties that would be the same for two solid samples of the same substance.

Metals typically have the greatest density compared to nonmetals, gases, and liquids. This is because metals have closely packed atoms or ions, resulting in a higher mass per unit volume. In contrast, nonmetals, gases, and liquids have less closely packed particles, leading to lower densities. Therefore, metals are known for their high density compared to other types of substances.

The most likely explanation for why the rock sank and the apple floated is that the density of the apple was less than the water in the lake. Density is a measure of how much mass is contained in a given volume. If an object is less dense than the fluid it is placed in, it will float. In this case, the apple had a lower density than the water, allowing it to float, while the rock had a higher density, causing it to sink.

Burning a log of wood is an irreversible change because it involves a chemical reaction that cannot be easily reversed. The wood undergoes combustion, releasing heat, light, and gases, and transforming into ash. Once the wood is burned, it cannot be restored to its original form. On the other hand, shredding paper, dissolving salt in water, and melting an ice cube are all reversible changes as they can be undone by reversing the physical processes involved.

Baking the dough in the oven involves a chemical change. During the baking process, the heat causes the dough to undergo a series of chemical reactions. The yeast in the dough produces carbon dioxide gas, which causes the dough to rise. Additionally, the heat causes the proteins in the flour to denature and form a network, giving the bread its structure and texture. These chemical changes result in the transformation of the dough into bread.

When salt (NaCI) is dissolved into water, it undergoes a physical change. This is because the chemical composition of salt remains the same before and after dissolving. The sodium ions (Na+) and chloride ions (CI-) are simply separated and surrounded by water molecules, forming a solution. No new substances are formed, and the salt can be recovered by evaporating the water. Therefore, this process is an example of a physical change rather than a chemical change.

Ocean water moves around the earth because of density differences. Density is affected by various factors such as temperature, salinity, and pressure. When water becomes denser, it sinks and moves towards areas of lower density, creating a current. This movement of water is essential for redistributing heat, nutrients, and oxygen, and plays a crucial role in regulating Earth's climate and supporting marine life. Gyres, on the other hand, are large circular ocean currents that are driven by a combination of factors including wind patterns and the rotation of the Earth. While gyres can influence the movement of ocean water, it is primarily density differences that drive the overall circulation.

The small pieces of a brick would have the same density as the original brick. Density is a physical property that describes the amount of mass in a given volume. Since the small pieces come from the same brick, they would have the same mass as the original brick, but their volume would be smaller due to being cut into smaller pieces. Therefore, the density of the small pieces would remain the same as the original brick.

Water boiling is an example of a physical change because it involves a change in the physical state of water from a liquid to a gas. The chemical composition of water remains the same during boiling, only the arrangement and movement of water molecules change. In contrast, iron rusting, paper burning, and silver tarnishing are examples of chemical changes because they involve the formation of new substances with different chemical properties.

Physical changes are reversible with some materials because physical changes involve a change in the physical properties of a substance without altering its chemical composition. These changes can be reversed by applying certain conditions or using specific materials. For example, melting ice can be reversed by cooling it, or evaporating water can be reversed by condensing it. However, not all physical changes are reversible with all materials. Some physical changes, such as breaking glass or burning paper, are irreversible and cannot be undone. Therefore, the statement "reversible with some materials" accurately describes the nature of physical changes.

When vinegar and baking soda are mixed, a gas forms. This is an indication of a chemical change because the reactants (vinegar and baking soda) combine to form a new substance (gas). Chemical changes involve the rearrangement of atoms and the formation of new bonds, resulting in the creation of different substances with different properties. In this case, the gas formation indicates a chemical reaction between the vinegar and baking soda.

When water undergoes a physical change, such as melting or boiling, the molecules do not form new chemical bonds. Physical changes only involve a change in the state or form of a substance, without altering its chemical composition. In the case of water, its molecules may gain or lose energy, move more rapidly, or become less closely packed, but they do not form new chemical bonds.

When investigating a chemical reaction, it is expected that the products will have different properties than the reactants separately. This is because during a chemical reaction, the bonds between atoms are broken and new bonds are formed, resulting in the rearrangement of atoms to form new molecules. This rearrangement leads to a change in the chemical composition and structure of the substances involved, resulting in different properties for the products compared to the reactants.

Burning is a process that involves a chemical reaction where atoms rearrange to form new compounds. During burning, the fuel combines with oxygen to produce heat, light, and various byproducts. This rearrangement of atoms is a fundamental characteristic of chemical reactions, making burning the correct answer to the question. Cooling, crushing, and evaporating are physical processes that do not involve the rearrangement of atoms.

Coppers and other metals are often used in making pots and pans because they have high conductivity, which is a physical property. This means that they can efficiently transfer heat from the stove to the food being cooked, allowing for even and quick cooking. Chemical properties, on the other hand, describe the behavior of substances in chemical reactions, which is not relevant in this context. Insulation, whether physical or chemical, would not be desirable in pots and pans as it would hinder heat transfer.

Reactivity is not a physical property that a substance can be classified by. Physical properties are characteristics that can be observed or measured without changing the composition of the substance. Hardness, thermal conductivity, and density are all physical properties that can be measured or observed. Reactivity, on the other hand, refers to how a substance reacts with other substances, which involves a chemical change rather than a physical characteristic.

To find the density of a substance, Victor needs to know its mass and volume. He already knows the mass of the unknown liquid, which is 42 grams. Therefore, he must find the liquid's volume in milliliters in order to calculate its density.