Understanding Chemical Changes and Properties: A Quiz

Iron rusting is a chemical change because it involves a reaction between iron and oxygen in the presence of moisture, resulting in the formation of iron oxide (rust). This process alters the chemical composition of the original material, creating a new substance with different properties. In contrast, ice melting, paper tearing, and water boiling are physical changes, as they do not change the chemical identity of the substances involved.

Color can vary widely among different samples of the same substance due to factors like impurities, lighting conditions, or physical states. Unlike density, reactivity, and flammability, which are intrinsic properties that remain consistent regardless of external conditions, color is subjective and can be influenced by environmental factors, making it a less reliable characteristic for substance identification.

When two or more elements chemically combine, they form a compound. This process involves the formation of chemical bonds, resulting in a substance that has distinct properties different from the individual elements. For example, when hydrogen and oxygen combine, they create water, which has unique characteristics not found in hydrogen or oxygen alone. In contrast, mixtures and solutions involve physical combinations without the formation of new substances.

The Tyndall effect occurs when light is scattered by particles in a colloid or suspension. In the case of a colloid like milk, the fat globules are large enough to scatter light, making the beam visible. In contrast, a solution of salt water is homogeneous and does not scatter light, while a pure substance lacks the necessary particles to exhibit this effect. A suspension of sand may scatter light, but it typically settles over time and is not considered a stable colloid. Thus, milk is the best example of a mixture that shows the Tyndall effect.

Distillation is a separation process that relies on differences in boiling points to separate components in a mixture. When a liquid mixture is heated, the substance with the lowest boiling point vaporizes first. This vapor is then collected and condensed back into a liquid, effectively separating it from the other components. This technique is commonly used in purifying liquids and separating liquid mixtures, making it an essential method in chemistry and various industrial applications.

A substance that maintains a constant composition and properties is classified as either an element or a compound. Elements are pure substances made up of only one type of atom, while compounds consist of two or more different atoms chemically bonded together in fixed ratios. In contrast, mixtures, colloids, and suspensions can vary in composition and properties, as they involve combinations of different substances. Thus, only elements and compounds exhibit uniform characteristics throughout.

The law of conservation of mass states that in a closed system, the total mass remains constant during a chemical reaction. This means that the mass of the reactants equals the mass of the products, indicating that matter is neither created nor destroyed. Instead, atoms are rearranged to form new substances, but the overall mass does not change. Thus, the mass before and after the reaction is the same, reinforcing the idea that mass stays constant in reactions.

Melting ice is a physical change because it involves a transition from solid to liquid without altering the chemical composition of water. This process is reversible; when the temperature drops, the liquid water can freeze back into solid ice. Unlike burning wood, rusting metal, and digesting food, which involve chemical changes that produce new substances, melting ice simply changes state, making it a clear example of a reversible physical change.