Detecting Chemical Reactions

• Topic is about detecting chemical reactions

Learning Objectives

• Learn different ways to tell if a chemical reaction occurred
• Identify test gases given off in reactions
• Be able to carry out practical tasks safely

Bell Work

• Consider differences between reactants and products, chemical and physical changes, acids and alkalis

What Happens in a Chemical Reaction

• In a chemical reaction, new products form from reactants

A Gas is Given Off

• One sign of a chemical reaction is when a gas is given off
• Reaction 1 details how magnesium reacts with acid producing hydrogen gas in Topic 8.1
• Hydrogen gas can be tested as detailed in Topic 8.1

Reaction 2

• Reaction of baking powder and vinegar produces carbon dioxide gas
• Carbon dioxide can be tested using limewater, turning limewater cloudy

Reaction 3

• Placing apple in hydrogen peroxide produces bubbling from oxygen gas
• Glowing splint inserted into the test tube mouth will relight if oxygen is present

Activity 8.4.1: Testing Gases

• Reactions are performed while testing the gases produced
• Apparatus shown in diagrams is needed
• Safety glasses must be worn

Testing for Carbon Dioxide

• Pour limewater into a test tube until half full
• Place calcium carbonate into another test tube
• Add hydrochloric acid to calcium carbonate
• Connect tubes using rubber bung and delivery tube, ensuring delivery tube is in the limewater

Testing for Oxygen

• Pour hydrogen peroxide into a test tube until half full
• Add a spatula of manganese dioxide
• Place a glowing splint into the neck of the test tube

Other Chemical Reaction Clues

• Reactants can disappear
• Magnesium ribbon reacting with acid produces hydrogen, as the magnesium combines with chlorine from hydrochloric acid to form magnesium chloride.

Color Change

• Gently heating black copper oxide with sulfuric acid forms a blue solution of copper sulfate
• The equation is copper oxide (black) + sulfuric acid -> copper sulfate + water
• If this is performed, avoid boiling the solution to prevent harmful fumes

Heat is Produced

• Potassium placed in water releases hydrogen gas and generates so much heat that hydrogen burns
• This reaction can only be performed by a teacher
• When zinc is added to hydrochloric acid, hydrogen gas is produced and the test tube feels hot

Change in pH

• Change in pH occurs when neutralizing an alkali, known as neutralization reaction
• The equation is sodium hydroxide + hydrochloric acid -> sodium chloride + water

A Precipitate is Formed

• Mixing silver nitrate and calcium chloride solutions results in a chemical reaction
• When the two liquids mix, a solid (precipitate) forms (silver chloride)
• The equation: silver nitrate + calcium chloride -> silver chloride + calcium nitrate
• Using limewater to test for carbon dioxide produced cloudiness
• Limewater is a solution of calcium hydroxide, creating a precipitate (calcium carbonate) when reacting with carbon dioxide
• The equation: calcium hydroxide + carbon dioxide -> calcium carbonate + water

Think Like a Scientist: Experiments

• Experiments are carried out in the laboratory as directed

Safety

• A risk assessment must occur before conducting reactions

Experiment A

• Needed: safety glasses, black copper oxide powder, dilute sulfuric acid, beaker, stirring rod
• Add black copper oxide powder to ~150 cm³ dilute sulfuric acid in a beaker, while stirring
• Observations and evidence of a chemical reaction will be noted

Experiment B

• Needed: safety glasses, test tube, test tube rack, small piece of zinc, dilute hydrochloric acid, beaker, stirring rod
• Half-fill a test tube with dilute hydrochloric acid, then add small zinc piece
• Observations and evidence of a chemical reaction will be noted

Experiment C

• Needed: safety glasses, limewater, test tube, test tube rack, straw
• Half-fill a test tube with limewater, use a straw and blow gently into it
• Observations and evidence of a chemical reaction will be noted

Experiment D

• Needed: safety glasses, chocolate piece, test tube, test tube rack, beaker, access to hot water
• Fill a beaker with hot water, place chocolate in a test tube, and stand it in the hot water beaker
• Observations and evidence of a chemical reaction will be noted

Experiment E

• Needed: safety glasses, copper sulfate solution, evaporating dish, pipe-clay triangle, tripod, Bunsen burner, tongs
• Place copper sulfate solution on evaporating dish, and heat gently until the solution starts to spit, turn off heat, and leave it to cool
• Observations and evidence of a chemical reaction will be noted

Experiment F

• Needed: safety glasses, sodium hydroxide, conical flask, measuring cylinder, universal indicator, hydrochloric acid, burette
• Place sodium hydroxide in a conical flask and add universal indicator drops
• Add acid slowly with a burette, until the indicator turns green
• Observations and evidence of a chemical reaction will be noted