Question

Determine whether each of the following describes a physical change or a chemical change: (a) The helium gas inside a balloon tends to leak out after a few hours. (b) A flashlight beam slowly gets dimmer and finally goes out. (c) Frozen orange juice is reconstituted by adding water to it. (d) The growth of plants depends on the sun's energy in a process called photosynthesis. (e) A spoonful of salt dissolves in a bowl of soup.

Short answer

(a) Physical change, (b) Chemical change, (c) Physical change, (d) Chemical change, (e) Physical change.

Step-by-step solution

Determine Description for (a)

For (a), helium gas leaking from a balloon is considered a physical change because the helium gas molecules are still helium after they move out of the balloon. This change involves a physical movement of molecules without altering their chemical structure.

Analyze Description for (b)

For (b), as a flashlight beam fades and goes out, it is indicative of a chemical change. This is because the batteries within the flashlight undergo a chemical reaction that converts chemical energy into electrical energy, and as the chemicals are used up, the reaction slows down and stops.

Interpret Description for (c)

In (c), mixing frozen orange juice with water is a physical change. The juice and water are physically combined, but the chemical composition of each substance remains unchanged.

Understand Description for (d)

For (d), photosynthesis in plants is a chemical change. During photosynthesis, plants use sunlight to convert carbon dioxide and water into glucose and oxygen, resulting in the formation of new substances through a chemical process.

Evaluate Description for (e)

In (e), dissolving salt in water is a physical change. The salt crystals physically disperse and become distributed evenly within the water, but its chemical structure as sodium chloride remains unchanged.

Key concepts

Physical Change

A physical change occurs when the form or appearance of a substance changes, but its chemical composition remains the same.
It's like reshuffling the cards in a deck – the order changes, but they’re still the same cards. This makes physical changes usually reversible.

• For example, the helium escaping from a balloon is a physical change. The helium molecules may move from inside to outside the balloon, but they remain as helium.
• Similarly, when frozen orange juice is mixed with water, the liquids blend, yet each keeps its original chemical identity.
• When salt dissolves in water, it simply disperses into smaller particles without changing chemically.

These examples demonstrate that physical changes involve state changes or mixtures without altering molecular structure.

Chemical Change

A chemical change, or chemical reaction, transforms one substance into one or more new substances through the reorganization of atoms. This transformation involves breaking and forming bonds, often resulting in substances with different chemical properties from the original ones.

• A flashlight growing dim is a sign of a chemical change, as the battery chemicals react and convert into different substances.
• Another common example is rusting iron, where iron reacts with oxygen to form iron oxide (rust).

Chemical changes are often irreversible under normal conditions, and they usually involve energy changes, such as releasing or absorbing heat or light.

Photosynthesis

Photosynthesis is a fascinating chemical change that allows plants to transform carbon dioxide and water into glucose and oxygen using sunlight.
This process is essential for plant growth and provides the oxygen we breathe.

• The sunlight absorbed by chlorophyll is the energy source for this reaction.
• Through photosynthesis, plants not only sustain themselves but also support the larger ecosystem by producing oxygen and serving as food for herbivores.

In essence, photosynthesis is nature’s way of turning light energy into chemical energy that fuels life on Earth.

Chemical Reaction

Chemical reactions are processes in which chemical bonds between atoms are broken and new ones are formed.
These reactions can change the identity and properties of the initial substances.

• Chemical reactions include combustion, corrosion, and the food digestion in our bodies.
• They can be exothermic, releasing heat, or endothermic, absorbing heat during the reaction.

Understanding chemical reactions helps us in multiple fields, from cooking to manufacturing and environmental science. By learning how to control and utilize these reactions, we can improve and innovate in technology and everyday life.