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Signs of Reaction

In this worksheet, students will answer questions on chemical reactions and what to look for to indicate that a reaction is taking place.

'Signs of Reaction' worksheet

Key stage:  KS 3

Curriculum topic:   Chemistry: Chemical Reactions

Curriculum subtopic:   Types of Chemical Reactions

Difficulty level:  

Worksheet Overview

QUESTION 1 of 10

When a chemical reaction takes place, there are several changes that may occur. These visible changes are called the indicators of a chemical reaction. There are several indicators of reaction:

  

colour change when chemical is added to a flask of solution

Colour Change

 

A change in colour is a sign of a chemical reaction. For example, frying an egg is a chemical reaction with a colour change - colourless to white.

 

 

TReaction giving off gas

Gas Given Off  

 

During a chemical reaction, a gas can be produced. This can be seen by bubbles forming in the reaction or by a coloured gas being produced (such as smoke).

 

 

Precipitate forming

Solid Formed  

 

If two solutions are mixed and a solid is formed, this is called a precipitate. A precipitate is an insoluble solid formed by reacting two liquids. For example, when milk goes very sour, a solid is formed.

 

 

Match burning

Temperature Change 

 

An energy change involves a change in temperature (either up or down). For example, the burning of a match involves a release of both heat and light.

 

 

The indicators of chemical reactions shown here all involve a chemical change - for example, the burning of a match. These are different to physical changes (such as boiling water) which always end up with the same substance, but sometimes in a different state (liquid to gas); it's easy to confuse those with a chemical reaction. which always ends up with new substances, so watch out!

 

Remember that, in a chemical reaction, one or more new substances are produced!

Using the table below, decide whether each of the following is a chemical change or a physical change.

 Chemical ChangePhysical Change
Wood burning
Water evaporating
Salt dissolving
Egg frying
Glue drying
Ice melting
Metal rusting

Which of the following is a chemical reaction?

gas burning on a hob

chocolate melting

clothes drying on a washing line

Which of the following is not a chemical reaction?

petrol burning in a car engine

fireworks exploding

paint drying

Read carefully the description of an experiment performed in a science lab:

 

Henrik added 5g of green copper carbonate to a colourless solution of hydrochloric acid. Immediately a lot of fizzing could be seen and the test tube became hot. After some time the reaction finished to leave a blue solution of copper chloride.

 

Use the table below to list the indicators of reaction that were observed by Henrik in this reaction.

 ObservedNot Observed
Colour change
Energy change
Gas produced
Solid formed

Read carefully the description of an experiment performed in a science lab:

 

Grace poured a small volume of the colourless solution potassium iodide into a clean test tube. To the same test tube she added an equal volume of the colourless solution lead chromate. Immediately a yellow solid was formed which, after some time, settled to the bottom of the test tube.

 

Use the table below to list the indicators of reaction that were observed by Grace in this reaction.

 ObservedNot Observed
Colour change
Energy change
Gas produced
Solid formed

Read carefully the description of an experiment performed in a science lab:

 

John was testing how well a new fuel burned. He placed a small quantity of the fuel in a spirit burner and placed it under a beaker of water. When he ignited the spirit burner he noted that the fuel burned with a smoky yellow flame and the temperature of the water increased slowly.

 

Use the table below to list the indicators of reaction that were observed by John in this reaction.

 ObservedNot Observed
Colour change
Energy change
Gas produced
Solid formed

Write this sentence again in the answer box below, filling in the gap with the correct word. Don't forget the full stop!

 

During a _________ reaction one or more new substances are formed.

Write this sentence again in the answer box below, filling in the gap with the correct word. Don't forget the full stop!

 

The melting of ice is an example of a _________ change.

Use the table below to decide whether each of the changes listed is a chemical change or a physical change.

 Chemical changePhysical change
Melting
Burning
Evaporating
Dissolving
Condensing

Sam was trying to get some sugar back from sugar solution.

 

She gently heated the sugar solution in an evaporating basin.

 

1. She observed steam evaporating from the basin and some white crystals forming.

 

2. As she continued to heat it she observed the crystals at first melt into a syrup.

 

3. With continued heating they formed a black sticky tar-like substance which smelled of burnt sugar.

 

Read this experiment through again and then match the different stages of the process (1,2,3) with whether Sam was observing a physical change at that point, or a chemical change.

 

 

 Chemical changePhysical change
1
2
3
  • Question 1

Using the table below, decide whether each of the following is a chemical change or a physical change.

CORRECT ANSWER
 Chemical ChangePhysical Change
Wood burning
Water evaporating
Salt dissolving
Egg frying
Glue drying
Ice melting
Metal rusting
EDDIE SAYS
A chemical reaction involves the production of one or more new products; for example, the rusting of iron is the process of iron slowly reacting with oxygen (corrosion) to form iron oxide. Ice melting on the other hand is a physical change; you start with water and end up with water but in a different state.
  • Question 2

Which of the following is a chemical reaction?

CORRECT ANSWER
gas burning on a hob
EDDIE SAYS
Chocolate melting and clothes drying are physical changes and not chemical changes. Gas burning, on the other hand, changes the gas into water and carbon dioxide (new substances!).
  • Question 3

Which of the following is not a chemical reaction?

CORRECT ANSWER
paint drying
EDDIE SAYS
Paint drying is simply a physical change, whereas petrol burning and fireworks exploding both involve chemical reactions, with new substances being formed in each case.
  • Question 4

Read carefully the description of an experiment performed in a science lab:

 

Henrik added 5g of green copper carbonate to a colourless solution of hydrochloric acid. Immediately a lot of fizzing could be seen and the test tube became hot. After some time the reaction finished to leave a blue solution of copper chloride.

 

Use the table below to list the indicators of reaction that were observed by Henrik in this reaction.

CORRECT ANSWER
 ObservedNot Observed
Colour change
Energy change
Gas produced
Solid formed
EDDIE SAYS
When producing reports on experiments scientists must list all their observations carefully as they all form part of the evidence of what took place during the experiment. Henrik would have mentioned the fizzing (gas produced), the tube getting hot (energy change) and the fact that the green solid ended up as a blue solution once the reaction was over (colour change).
  • Question 5

Read carefully the description of an experiment performed in a science lab:

 

Grace poured a small volume of the colourless solution potassium iodide into a clean test tube. To the same test tube she added an equal volume of the colourless solution lead chromate. Immediately a yellow solid was formed which, after some time, settled to the bottom of the test tube.

 

Use the table below to list the indicators of reaction that were observed by Grace in this reaction.

CORRECT ANSWER
 ObservedNot Observed
Colour change
Energy change
Gas produced
Solid formed
EDDIE SAYS
Here, Grace would have noted the colour change (colourless to yellow) as well as the fact that a solid (precipitate) was formed from the two solutions.
  • Question 6

Read carefully the description of an experiment performed in a science lab:

 

John was testing how well a new fuel burned. He placed a small quantity of the fuel in a spirit burner and placed it under a beaker of water. When he ignited the spirit burner he noted that the fuel burned with a smoky yellow flame and the temperature of the water increased slowly.

 

Use the table below to list the indicators of reaction that were observed by John in this reaction.

CORRECT ANSWER
 ObservedNot Observed
Colour change
Energy change
Gas produced
Solid formed
EDDIE SAYS
The yellow flame produced is not a sign of a chemical change but it is a sign of the energy released when the fuel burns.
  • Question 7

Write this sentence again in the answer box below, filling in the gap with the correct word. Don't forget the full stop!

 

During a _________ reaction one or more new substances are formed.

CORRECT ANSWER
During a chemical reaction one or more new substances are formed.
EDDIE SAYS
Got this now? A CHEMICAL CHANGE involves new substances being formed, like when flour, eggs, butter and sugar are baked to form a cake!
  • Question 8

Write this sentence again in the answer box below, filling in the gap with the correct word. Don't forget the full stop!

 

The melting of ice is an example of a _________ change.

CORRECT ANSWER
The melting of ice is an example of a physical change.
EDDIE SAYS
So what's ice? Solid water. What does it melt to form? Liquid water. Is it still water (H20)? Of course it is! So, physical change then. No new substances.
  • Question 9

Use the table below to decide whether each of the changes listed is a chemical change or a physical change.

CORRECT ANSWER
 Chemical changePhysical change
Melting
Burning
Evaporating
Dissolving
Condensing
EDDIE SAYS
Physical changes are more common in the home than chemical changes. Think about all different changes that happen in your home, especially when cooking dinner.
  • Question 10

Sam was trying to get some sugar back from sugar solution.

 

She gently heated the sugar solution in an evaporating basin.

 

1. She observed steam evaporating from the basin and some white crystals forming.

 

2. As she continued to heat it she observed the crystals at first melt into a syrup.

 

3. With continued heating they formed a black sticky tar-like substance which smelled of burnt sugar.

 

Read this experiment through again and then match the different stages of the process (1,2,3) with whether Sam was observing a physical change at that point, or a chemical change.

 

 

CORRECT ANSWER
 Chemical changePhysical change
1
2
3
EDDIE SAYS
To start off with, the water evaporated (liquid water to water vapour), so a physical change. Then, in 2, the sugar crystals at first melted (solid to liquid, but still sugar) until finally, in 3, the sugar burned: it reacted with oxygen in the air to form a mixture of gases (released) and black syrupy stuff, but no longer pure sugar.
---- OR ----

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