Detecting chemical reactions
In this topic you will:
- learn about different ways of telling if a chemical reaction has happened
- test gases given off in reactions so that you can identify them
- carry out practical tasks safely.
Key words
- cloudy
- glowing
- precipitate
What happens in a chemical reaction
In a chemical reaction, new products are formed from the reactants. How can you tell this has happened? There are some clues you can look out for that might mean a reaction has taken place.
A gas is given off
One of the most useful clues to help decide if a chemical reaction has taken place is whether a gas is given off. Consider these three reactions.
Reaction 1
You have seen the reaction of magnesium with acid and seen bubbles of the gas hydrogen given off in Topic 8.1. You learnt how to test for hydrogen.
magnesium + hydrochloric acid → magnesium chloride + hydrogen
Common Mistake
Don’t assume that fizzing or bubbling always means a chemical reaction. Sometimes it’s just air being released or a physical change — look for other clues too, like new substances or temperature change.
Quick Fact
Hydrogen gas makes a ‘pop’ sound when tested with a lit splint — this is one of the simplest tests to confirm gas production during a reaction.
Reaction 2
When baking powder and vinegar react, a gas is also given off. This gas is carbon dioxide. You can test for carbon dioxide by using limewater.
When limewater mixes with the carbon dioxide, the limewater turns cloudy.
Reaction 3
When a piece of apple is placed in hydrogen peroxide it bubbles. A gas is given off. This gas is oxygen. To test for oxygen you use a glowing splint. When the glowing splint is placed in the mouth of the test tube, it will relight if the gas is oxygen.
Other chemical reaction clues
Reactant ‘disappears’
When magnesium ribbon reacts with acid, hydrogen is produced and the magnesium ribbon ‘disappears’. The magnesium is used up in the reaction; it combines with the chlorine from the hydrochloric acid to form magnesium chloride.
Colour change
Gently heating black copper oxide with sulfuric acid produces a blue solution of copper sulfate.
copper oxide (black) + sulfuric acid → copper sulfate + water
Safety
If you try this in the laboratory, make sure you do not boil the blue liquid, as harmful fumes will be given off.
Heat is produced
When potassium is placed in water, hydrogen gas is given off. The reaction produces so much heat the hydrogen burns.
Safety
This reaction should only be done by a teacher.
When you added zinc to hydrochloric acid, hydrogen gas was given off and the test tube felt hot.
Change in pH
When you neutralise an alkali, there is a change in pH. It is called a neutralisation reaction.
sodium hydroxide + hydrochloric acid → sodium chloride + water
Common Mistake
Some students confuse color changes caused by heating (like the blue copper sulfate) with temperature change from an exothermic reaction. Always check whether heat is actually being produced or if you're just seeing a visual clue.
Quick Fact
The pH scale ranges from 0 to 14. A neutral solution has a pH of 7. When acids and alkalis react, the pH moves closer to 7 as neutralisation takes place.
A precipitate is formed
If you mix solutions of silver nitrate and calcium chloride, a chemical reaction takes place. When the two solutions (liquids) are mixed, a solid is formed. This solid is called a precipitate. In this example, the solid is silver chloride.
silver nitrate + calcium chloride → silver chloride + calcium nitrate
When you tested for carbon dioxide gas, you used limewater. Limewater is a solution of calcium hydroxide. You saw that the limewater turned cloudy when carbon dioxide was bubbled into it. This is because a precipitate of calcium carbonate formed. You added a gas to a liquid, and a solid was formed.
calcium hydroxide + carbon dioxide → calcium carbonate + water
Think Like a Scientist
You will carry out a series of experiments to decide whether a chemical reaction has taken place in each case. Follow the steps below carefully and record your observations.
Step 1: Read the instructions for each of the eight experiments below (A–H).
Step 2: For each one, carry out a risk assessment and think about safety precautions.
Step 3: Conduct each experiment in any order. Carefully observe what happens.
Step 4: After each experiment, record what you observed and whether you think a chemical reaction occurred. Explain your reasoning.
Show Answer
Signs like color change, temperature change, gas production (bubbles), or formation of a new solid suggest a chemical reaction has occurred.
Show Answer
A risk assessment helps identify possible dangers and allows you to take precautions, such as wearing safety glasses or using tongs when heating substances.
Show Answer
Experiment D (melting chocolate) likely involves only a physical change, as no new substance is formed—it simply changes state from solid to liquid.