Question

State whether each of the following represents a chemical change or a physical change: (a) A few grams of sucrose (table sugar) are placed in a small beaker of deionized water; the sugar crystals "disappear," and the liquid in the beaker remains clear and colorless. (b) A copper statue, over time, turns green. (c) When a teaspoon of baking soda (sodium bicarbonate) is placed into a few ounces of vinegar (acetic acid), volumes of bubbles (effervescence) are produced. (d) When a few grams of a blue crystalline solid are placed into a beaker of deionized water, the crystals "disappear" and the liquid becomes clear and blue in color. (e) In the lab, a student mixes \(2 \mathrm{~mL}\) of sodium hydroxide with \(2 \mathrm{~mL}\) of hydrochloric acid in a test tube. He notices that the test tube is very warm to the touch. (f) A woman visits a hairdresser and has her hair colored a darker shade of brown. After several weeks the hair, even though washed several times, does not change back to the original color.

Short answer

(a) Physical change, (b) Chemical change, (c) Chemical change, (d) Physical change if composition remains unchanged, (e) Chemical change, (f) Chemical change.

Step-by-step solution

Analyzing sucrose in water (a)

Dissolving sucrose in water is a physical change as it involves a change in state from solid to solution without changing the chemical composition. The molecules of sucrose spread evenly throughout the water, but they remain the same chemically.

Observing changes in a copper statue (b)

The green color on the copper statue is due to a process called patination, which involves the formation of copper carbonate as a result of a reaction between copper and the atmospheric elements such as oxygen. This is a chemical change.

Reacting baking soda with vinegar (c)

The reaction between baking soda and vinegar produces carbon dioxide gas bubbles, which is a chemical change. The original substances react chemically to form new products.

Dissolving a blue crystalline solid in water (d)

When the blue crystals dissolve in water and produce a blue solution, it is a physical change if the dissolving process does not alter the chemical composition of the blue substance.

Mixing sodium hydroxide and hydrochloric acid (e)

The reaction between sodium hydroxide and hydrochloric acid is an exothermic chemical change, resulting in the formation of water and sodium chloride with a release of heat.

Hair coloring (f)

Coloring hair is a chemical change. The dye reacts with the hair, creating a new, stable color that does not wash out easily, indicating that a chemical reaction has occurred.

Key concepts

Dissolving Sugar in Water

When sugar is dissolved in water, it may seem like the sugar has disappeared, but really it's a physical change known as dissolution. The solid sugar breaks down into smaller components that spread out evenly in the water, but the chemical structure of the sugar molecules doesn't change. Solubility is key here, as it measures how well a solute like sugar can dissolve in a solvent like water under certain conditions. To enhance understanding, remember that solubility depends on factors such as temperature and the nature of the solute and solvent.

Students can explore this further by performing the sugar dissolving experiment themselves and observing that the water remains clear, highlighting that the sugar molecules are evenly distributed without any chemical reaction taking place.

Copper Patination

The process of copper turning green, known as patination, is indeed a dazzling example of a chemical change. Over time, copper reacts with oxygen, water, and carbon dioxide in the atmosphere to form a green layer called patina, composed primarily of copper carbonate. This layer is not just aesthetically pleasing on rooftops and statues but also serves to protect the metal underneath from further corrosion.

To dive deeper, students might compare this to the rusting of iron, another chemical change, to understand how different materials interact with their environment.

Baking Soda and Vinegar Reaction

The interaction between baking soda and vinegar is a classic chemical reaction that fascinates students of all ages. As they react, they produce carbon dioxide gas—a telltale sign of a chemical change. The baking soda (a base) and vinegar (an acid) react to form new chemicals, including water and carbonic acid, which quickly breaks down into carbon dioxide and water.

Experiments with varying amounts of these substances can show how the quantities of reactants affect the amount of gas produced and provide a practical interpretation of the law of conservation of mass.

Solubility of Solids

Solubility of solids like the blue crystalline substance in water indicates the ability of the solute to dissolve and spread out evenly throughout the solvent. This physical change suggests that the intrinsic properties of the substance remain unchanged. It’s the intermolecular forces between the water molecules and the solid that allow for this process without altering the chemical composition.

Hands-on experimentation with different solids and temperatures will elucidate how solubility is influenced by these external conditions, emphasizing the concept of saturated and unsaturated solutions.

Exothermic Reaction

Exothermic reactions, such as the reaction between sodium hydroxide and hydrochloric acid, release energy in the form of heat, signifying a chemical change. It's a highly spontaneous and natural occurrence in chemistry where the energy released can be felt as warmth or seen as light in some reactions. This forms the basis of thermochemistry, a branch that deals with the energy changes during chemical reactions.

Students observing this reaction can be introduced to energy diagrams and learn about activation energy, reactant and product energy levels, as well as the concept of enthalpy.

Hair Coloring Chemistry

Chemical changes are not just limited to the lab—they're part of our daily lives. Hair coloring is a perfect example. The dyes react with the hair to create new chemical bonds, resulting in a color change that doesn't wash out easily. This enduring alteration is due to the chemical reactions between the dye components and the proteins in the hair, leading to a lasting transformation.

Exploring hair anatomy and the chemistry of hair coloring products could not only enrich students' understanding but also demonstrate the practical application of chemistry in personal care.