Physical and Chemical Changes

A set of practice flashcards covering physical and chemical changes, their classifications, indicators, and common examples from the notes.

What is a physical change?

A change in which no new substance is formed; the composition remains the same and the change is usually reversible by physical methods. It may involve changes in state, shape, or size (e.g., ice melting, sugar dissolving).

What is a chemical change?

A permanent change in which one or more new substances with different properties are formed; the change is not easily reversible by simple physical methods; energy may be absorbed or released.

How can you tell that a chemical change has occurred?

Usually indicated by a change in colour, evolution of gas, energy changes (heat, light, or sound), or the formation of a new substance.

What is evaporation?

A slow change of a liquid to its vapour at any temperature below the boiling point, occurring from the surface of the liquid.

What is boiling?

The rapid change of a liquid to vapour at its boiling point, occurring throughout the liquid with heat supplied.

What is condensation?

The process in which a vapour or gas changes into a liquid.

What is sublimation?

The process of a solid turning directly into a gas on heating, without passing through a liquid; the solid formed after cooling is called a sublimate (e.g., ammonium chloride).

What is freezing point?

The temperature at which a liquid turns into a solid; pure substances have definite freezing points (e.g., water freezes at 0°C).

What is melting point?

The temperature at which a solid starts turning into a liquid; solids have definite melting points (e.g., ice melts at 0°C).

What is the boiling point?

The temperature at which a liquid boils; for pure water, it's 100°C at standard pressure; all pure liquids have definite boiling points.

What is interconversion of states of matter?

The change of a substance from one state to another (solid ↔ liquid ↔ gas) without changing its chemical composition, e.g., ice to water to steam.

What is a reversible change?

A change that can be reversed by changing the conditions; no new substance is formed; examples include melting ice, dissolving sugar in water, bending a rubber band.

What is an irreversible change?

A change that cannot be reversed by simple means; a new substance is formed or transformation is permanent (e.g., burning paper, rusting iron).

What is a slow change?

A change that takes a long time to complete (hours to years), e.g., growth of a plant, evaporation of water, fossil fuel formation.

What is a fast change?

A change that occurs in a very short time, e.g., burning of paper, bursting of a cracker, lighting a bulb.

What is a periodic change?

Changes that occur at regular intervals; e.g., day and night, seasons, full moon, tides.

What is a non-periodic change?

Changes that do not occur at regular intervals; e.g., landslides, earthquakes, epidemics, fever, sneeze.

What is natural change vs man-made change?

Natural changes occur in nature (e.g., day/night). Man-made changes are caused by humans (e.g., making steel, kite, flatbread).

What are desirable and undesirable changes?

Some changes are desirable at one time and undesirable at another (e.g., growth of crops desirable; cutting trees harmful; burning fuel desirable for cooking but polluting).

What is respiration in chemical changes?

A chemical process in living beings where oxygen reacts with digested food to release energy, producing carbon dioxide and water (glucose + oxygen → carbon dioxide + water + energy).

What is digestion of food in chemical changes?

The process by which the food in our body breaks into simpler compounds; a chemical change.

What is the formation of curd from milk?

A chemical and irreversible change where milk becomes curd with different properties; cannot be reversed to milk.

What is heating of iron and sulfur?

A chemical change forming iron sulfide with different properties; iron sulfide is a black solid, not magnetic, unlike iron.

What is rusting of iron?

A chemical change where iron forms iron oxide (rust) and cannot be reversed to iron.

What is a simultaneous physical and chemical change?

When a substance undergoes both a physical and a chemical change at the same time, e.g., candle: melting of wax (physical) and burning of wax producing water vapour and carbon dioxide (chemical).

What are reactants and products in a chemical change?

Reactants are the starting substances; products are the new substances formed during the chemical change.

Is dissolving salt in water a physical or chemical change?

A physical change; salt dissolves in water to form a solution with no new substance formed and the salt retains its properties.

What is sublimation and what is a sublimate?

Sublimation is solid-to-gas transformation without a liquid stage; the solid formed after cooling the vapour is called a sublimate.

Why do changes in temperature involve energy changes in changes of state?

In physical changes, energy is often absorbed or released (e.g., melting absorbs heat; freezing releases heat; evaporation absorbs heat; condensation releases heat).