Which of the changes are chemical changes?
a) Sugar crystals are ground to a fine powder.
b) A candle is burned
c) Two clear colorless salt solutions are mixed and a bright orange precipitate forms.
d) Candle wax is melted
e) A silver teapot turns black
a) Sugar crystals are ground to a fine powder: This change is a physical change because the substance remains the same (sugar) even though its physical appearance is altered.
b) A candle is burned: This change is a chemical change because the candle wax reacts with oxygen in the air to produce carbon dioxide, water vapor, and heat.
c) Two clear colorless salt solutions are mixed and a bright orange precipitate forms: This change is a chemical change because a new substance with a different color (the bright orange precipitate) is formed as a result of a chemical reaction between the two salt solutions.
d) Candle wax is melted: This change is a physical change because the state of the substance changes from solid to liquid (without a change in its chemical composition).
e) A silver teapot turns black: This change is a chemical change because the silver teapot is undergoing a chemical reaction with substances in the environment (such as sulfur compounds) that causes it to tarnish and turn black.
To determine which of the changes are chemical changes, let's review the characteristics of a chemical change. Chemical changes involve the transformation of one or more substances into new substances with different chemical properties. These changes are typically irreversible and involve the breaking and forming of chemical bonds.
Now, let's examine each of the changes listed:
a) Sugar crystals are ground to a fine powder.
This is a physical change because grinding sugar crystals results in smaller particles, but the chemical composition of sugar remains the same.
b) A candle is burned.
Burning a candle is a chemical change. As the candle burns, the wax reacts with oxygen in the air, forming new substances like carbon dioxide, water vapor, and various combustion byproducts. The chemical composition of the candle wax changes during this process.
c) Two clear colorless salt solutions are mixed, and a bright orange precipitate forms.
This is a chemical change. When two clear colorless salt solutions are mixed, a chemical reaction occurs, resulting in the formation of a new substance (the bright orange precipitate). The chemical composition of the reactants changes to form the precipitate.
d) Candle wax is melted.
Melting candle wax is a physical change. The wax changes from a solid to a liquid state due to the increase in temperature, but there is no change in its chemical composition.
e) A silver teapot turns black.
This is a chemical change. The blackening of the silver teapot indicates a chemical reaction, likely the formation of silver sulfide. This is a result of the silver reacting with sulfur compounds present in the air, leading to a change in the chemical composition of the silver.
In summary, the changes that are chemical changes are:
- A candle is burned
- Two clear colorless salt solutions are mixed, and a bright orange precipitate forms
- A silver teapot turns black