Change of State
What is the process called when a solid turns into a liquid?
Melting.
You put water in a freezer and it becomes ice. Is this change reversible or irreversible?
Reversible
Melted chocolate is left to cool and becomes solid again. What type of change is this?
Reversible
What is the reddish-brown substance that forms on iron called?
Rust.
Ice cream melts on a hot day. Is this a physical or chemical change?
Physical change.
What is the process called when a liquid turns into a solid?
Freezing.
Why can melting ice be considered a reversible change?
The liquid water can be frozen again to form ice.
Is cooking an egg a reversible or irreversible change?
Irreversible.
Name the two main things needed for iron to rust.
Oxygen and water (moisture).
A puddle disappears on a sunny day. What change of state has occurred?
Evaporation.
What is the process called when liquid water changes into water vapour?
Evaporation
Salt dissolves in water. Describe one way to recover the salt.
Evaporate the water so the salt crystals remain.
What is one sign that a chemical reaction may have occurred?
For example, gas bubbles form, a new colour appears, heat or light is produced, or a new substance forms.
Is rusting a reversible or irreversible change?
Irreversible under ordinary conditions.
A student tears paper into tiny pieces. Is this reversible or irreversible, and why?
It is difficult to reverse because the original sheet cannot easily be restored, although tearing does not create a new substance.
What is the process called when water vapour cools and turns into liquid water?
Condensation.
What type of change occurs when wood burns and forms ash?
An irreversible chemical change.
Why can't a cooked pancake simply be changed back into its original raw batter?
Cooking causes chemical changes that form new substances and cannot easily be undone.
Why might an iron nail rust faster when left outside in the rain than when kept in a dry room?
It has more contact with moisture and oxygen, which help rusting occur.
Two identical iron nails are placed in separate jars. One jar contains dry air, while the other contains water and air. Which nail is more likely to rust, and why?
The nail in the jar with water and air, because rusting requires both moisture and oxygen.
Water is heated until it becomes steam. The steam then cools and forms droplets. Name both changes of state in order.
Evaporation, followed by condensation.
A student melts a candle and allows the wax to harden again. Explain why this is generally considered reversible.
The wax changes state but remains wax, so it can melt and harden again.
A student mixes two substances and gas bubbles form. Does this prove that a chemical reaction occurred? Explain your answer.
Not necessarily. Gas bubbles can indicate a chemical reaction, but they could also result from boiling or another physical process. More evidence is needed.
A student wants to stop an iron gate from rusting. Name one method they could use and explain how it helps.
Paint or coat the gate to keep water and oxygen away from the iron; galvanising with zinc is another method.
A student says, “Every change that cannot easily be undone is a chemical reaction.” Explain why this statement is incorrect.
Some physical changes, such as tearing paper, are difficult to reverse but do not create a new substance. Chemical reactions form new substances.