Science 8th grade
UNIT 9: Material changes 9.6 Detecting chemical reactions
Science 8th grade
UNIT 9: Material changes 9.6 Detecting chemical reactions
In a chemical reaction, new products are formed from the reactants. How can you tell this has happened?
There are a few clues you can look out for that might mean a reaction has taken place.
A colour change
Gently heating black copper oxide with sulfuric acid produces a blue solution of copper sulfate.
copper oxide + sulfuric acid → copper sulfate + water
(black) (blue)
A gas is given off
When magnesium is placed in hydrochloric acid, bubbles of hydrogen gas are given off.
magnesium + hydrochloric acid → magnesium chloride + hydrogen
Heat is produced
When potassium is placed in water, hydrogen gas is given off. The reaction produces so much heat that the gas burns.
The word equation for this reaction is:
potassium + water → potassium hydroxide + hydrogen
A change in pH
If you add potassium to water that contains Universal Indicator, you see the indicator change colour from green to purple. This shows that the solution has become alkaline. The pH has changed.
When an acid is used to neutralise an alkali, there is also a change in pH. This is a type of reaction called a neutralisation reaction.
The neutralisation reaction between sodium hydroxide and hydrochloric acid can be described by the word equation:
sodium hydroxide + hydrochloric acid → sodium chloride + water
A precipitate is formed
If you mix solutions of silver nitrate and calcium chloride, a chemical reaction takes place. In the reaction, an insoluble solid is formed. This is called a precipitate. The solid is silver chloride.
silver nitrate + calcium chloride → silver chloride + calcium nitrate
You might have used limewater to detect carbon dioxide. Limewater is a solution of calcium hydroxide Carbon dioxide makes limewater turn cloudy because a precipitate of calcium carbonate forms.
calcium hydroxide + carbon dioxide → calcium carbonate + water
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Questions
1) Name two things you might look for to see if a chemical reaction has taken place.
2) Potassium is called an alkali metal. Suggest a reason for this.
3) What is the name of the compound formed when oxygen reacts with magnesium?
$A+I$: 4) Suggest what happens when carbon burns in oxygen. Write a word equation for this reaction.
Activity 9.6 (Has a chemical reaction taken place?)
SE: You will be given a series of experiments to do. Your aim is to carry them out according to the instructions, and to observe and record what happens. Pay attention to any safety instructions.
For each experiment decide if there is a chemical change or a physical change. Give a reason for your choice.