Something new forms
Blow into water, then into lime water, and watch what is made.
Water and lime water
Blow air from your breath through a straw into a glass of water: you see bubbles. Now do the same in lime water (clear, like water).
What do you think will happen in the lime water?
What this lesson covers
The idea
A chemical change is one in which one or more new substances are formed through a chemical reaction, which can be represented in short form as a chemical equation.
Water and lime water
Blow air from your breath through a straw into a glass of water: you see bubbles. Now do the same in lime water (clear, like water).
What do you think will happen in the lime water?
- Only bubbles, just like water
- The liquid will turn milky
- The liquid will turn blue
Two tumblers
Blow into A and into B. Then leave B alone for a while.
Chemical change
In A there were bubbles only. In B the lime water turned milky, and later a white substance settled at the bottom. Something new had formed.
Changes in which one or more new substances are formed are called chemical changes. New substances are formed through a process called a chemical reaction.
The carbon dioxide in the air you breathe out reacts with lime water and forms a new white substance, calcium carbonate, which does not dissolve in water. A little water forms too.
A chemical reaction can be written in short form as a chemical equation:
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Read the arrow as "forms". On the left are the substances that react. On the right are the new substances formed.
Notes
A chemical change is one in which one or more new substances are formed through a chemical reaction. A chemical reaction can be shown in short form as a chemical equation.
Check yourself
Tumbler A had water and tumbler B had lime water. In which one did a chemical change happen?
Which sign in tumbler B showed that a chemical change had taken place?
In a chemical change, what is formed through a chemical reaction?
In Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water, which is the new white substance that settles?
- Tumbler A. Only bubbles came out. The water stayed the same.
- Tumbler B — correct. Yes! A new white substance formed there.
- Both tumblers. Only one of them made a new substance.
- Bubbles rose in the liquid. Bubbles rose in water too, where nothing new formed.
- The glass was see-through. That tells us nothing about a new substance.
- A new white solid formed — correct. Yes! A new substance means a chemical change.
- Only a different shape and size. That describes a physical change.
- One or more new substances — correct. Yes!
- No new substance at all. In a chemical change new substances do form.
- Calcium carbonate — correct. Yes! It is formed from the reaction and does not dissolve in water.
- Calcium hydroxide. This is the lime water you started with.
- Carbon dioxide. This is the gas from your breath that reacted.