C. Chemistry: Chemical Reactions · Chemical Reactions

Evidence of Chemical Change

C3.3 C2.5 C2.1 A1.1 A1.4 A1.5 A1.6About 75 minutes

Learning goals

I can identify reactants and products and explain that a reaction forms new substances.
I can describe five kinds of evidence of chemical change.
I can tell the difference between exothermic and endothermic reactions.
I can plan and carry out a safe inquiry to look for evidence of chemical change.

Words to know

chemical reaction
A change in which one or more new substances with new properties are formed.

reactant
A starting substance in a chemical reaction.

product
A new substance formed in a chemical reaction.

precipitate
A solid that forms when two solutions are mixed.

exothermic
Describes a reaction that releases energy (usually as heat) to its surroundings.

endothermic
Describes a reaction that absorbs energy (usually heat) from its surroundings.

control
A test in which the independent variable is not changed, used for comparison.

How can you tell?

On Canada Day, fireworks burst into colour over the water. At breakfast, pancake batter puffs up and turns golden brown. In a car, burning gasoline releases energy that moves you down the highway. Each of these is a chemical reaction: a change in which new substances with new properties are formed.

You cannot see atoms rearranging. So how do chemists know a reaction has happened? They look for evidence. In this lesson you will learn the main types of evidence for chemical change (expectation C3.3), and you will plan and carry out your own safe inquiry to look for them (C2.5).

Remember from Grade 9

In Grade 9 you learned that a physical change changes the form or state of a substance but not its identity (melting, dissolving, cutting), while a chemical change produces a new substance. You also used a precipitate and bubbles as clues. In Grade 10 you will name the parts of a reaction, write equations for it, and explain it with atoms.

Reactants and products

In a chemical reaction, the starting substances are the reactants and the new substances formed are the products. Atoms are not created or destroyed. They are rearranged: bonds between atoms break, and new bonds form.

For example, when hydrogen gas burns in oxygen, the reactants are hydrogen and oxygen, and the product is water. Water has completely different properties from either reactant. Hydrogen is a flammable gas, oxygen helps things burn, and water puts out fires.

In a physical change, the molecules stay the same; only their arrangement or spacing changes. In a chemical change, the atoms are rearranged into new molecules.

Five kinds of evidence

These observations are clues that a chemical reaction may have happened:

A change in colour. Example: a shiny copper roof slowly turns green; a sliced apple turns brown. (Mixing two coloured liquids does not count: that is just mixing.)
A gas is produced. Example: bubbles form when baking soda is added to vinegar. (Bubbles from boiling water do not count: that is water changing state.) A change in odour, such as the smell of rotten eggs from spoiled food, also means a new gas has formed.
A precipitate forms. A precipitate is a solid that forms when two solutions are mixed. Example: the cloudy solid that appears when hard water mixes with soap ("soap scum").
Heat is given off or taken in. A reaction that releases heat is exothermic (burning wood, a hand warmer). A reaction that absorbs heat, making its surroundings colder, is endothermic (baking soda reacting with vinegar).
Light is produced. Example: a burning candle, a glow stick, fireworks.

One clue is not proof. Several physical changes can look like chemical ones. Dissolving some substances in water makes it colder; boiling makes bubbles. Chemists become more confident when they see more than one kind of evidence, and when they test the products and find new properties.

Five common kinds of evidence of a chemical change. A change in odour is a sixth clue: it means a new gas has formed.

Exothermic reactions release energy as heat, so a thermometer in the surroundings rises. Endothermic reactions absorb heat, so it falls.

Planning an inquiry (your science skills)

Expectation C2.5 asks you to plan and conduct an inquiry. Here is the plan a scientist would write before starting:

Question: What do you want to find out? Make it testable.
Prediction or hypothesis: What do you think will happen, and why? ("If... then... because...")
Variables: the independent variable (what you change), the dependent variable (what you observe or measure) and the controlled variables (what you keep the same).
Materials and safety: list everything, including eye protection. In a school lab, check the WHMIS labels and the Safety Data Sheet (SDS) for every chemical you use.
Procedure: numbered steps that someone else could follow exactly.
Observations: recorded in a data table as you go, with qualitative (colour, bubbles) and quantitative (temperature, mass) results.
Analysis and conclusion: Did the evidence support your prediction? What were possible sources of error?

Lab safety for chemical reactions

Wear eye protection whenever you mix chemicals, even at home.
Never taste anything in a lab, and do not eat or drink where you are experimenting.
To smell a gas, waft it toward your nose with your hand; never sniff directly.
Mix only what the procedure says. Never mix cleaning products.
Dispose of chemicals the way your teacher or the SDS says. At home, rinse food-based mixtures down the sink with lots of water.

Worked example

Example 1: Chemical or physical?

For each observation, decide whether it is good evidence of a chemical change, and name the evidence. (a) Ice cream melts on a hot day. (b) A clear solution of silver nitrate is added to clear salt water, and a white solid appears. (c) A match bursts into flame. (d) Water boils in a kettle.

(a) The ice cream changes state but is still ice cream: physical. No new substance.
(b) Two solutions mixed and a solid formed: precipitate, so chemical.
(c) Light and heat are given off, and new gases and ash form: chemical (several kinds of evidence).
(d) Bubbles form, but they are water vapour, so water is only changing state: physical.

Chemical: (b) and (c). Physical: (a) and (d). The key question is always: was a new substance produced?

Worked example

Example 2: Plan an inquiry

Plan an inquiry to answer: "Which kinds of evidence of chemical change appear when baking soda reacts with vinegar?"

Hypothesis: If baking soda is added to vinegar, then bubbles of gas will form and the temperature will drop, because the reaction produces carbon dioxide and absorbs heat.
Variables: independent: whether baking soda is added (compare with a control of vinegar alone); dependent: observations of bubbles, colour and temperature; controlled: 50 mL of vinegar, 1 teaspoon of baking soda, same cup, same starting temperature.
Safety: eye protection; do the activity in a sink or on a tray; do not taste.
Procedure: 1. Pour 50 mL of vinegar into a cup and record its temperature. 2. Add 1 teaspoon of baking soda. 3. Stir gently and record observations every 30 s for 2 min, including temperature. 4. Repeat with a second cup of vinegar and no baking soda (the control).
Data table: columns for time (s), temperature (°C) and observations, with one table for each cup.

A complete plan has a testable question, a reasoned hypothesis, clear variables with a control, safety notes, numbered steps and a data table ready before you start. Expected result: lots of bubbles (carbon dioxide gas) and a temperature drop of a few degrees, while the control does not change.

Hands-on activity

Inquiry: hunting for evidence of chemical change

You need: a grown-up, eye protection (safety glasses or sunglasses), 4 clear cups or jars, a kitchen or aquarium thermometer if you have one, measuring spoons, white vinegar, baking soda, a packet of dry baker's yeast, 3% hydrogen peroxide from a pharmacy (the grown-up handles it), a glow stick, a pad of plain steel wool (without soap), dish gloves, a tray or the sink to work in.

Safety: wear eye protection for every step. 3% hydrogen peroxide can irritate skin and eyes: wear gloves and let the grown-up pour it. Wear gloves for the steel wool (splinters). Do not taste anything. Do not cut open the glow stick.

Plan first: copy this data table: Test, What I mixed, Temperature before, Temperature after, Observations, Evidence of chemical change? Write a prediction for each test.

Vinegar + baking soda: 50 mL vinegar, record the temperature, add 1 teaspoon of baking soda, record again.
Yeast + hydrogen peroxide: the grown-up pours 50 mL of 3% hydrogen peroxide into a cup. Record the temperature. Add 1/2 teaspoon of yeast and stir. Record what you see and the temperature after 1 minute.
Steel wool + vinegar: soak a small piece of steel wool in vinegar for 1 minute, squeeze it out, then wrap it around the thermometer bulb inside an empty jar and put a lid on loosely. Record the temperature every minute for 5 minutes.
Glow stick: in a dark room, bend the glow stick until it snaps inside, then shake it. Record what happens. Feel the outside: is it warm?
Control: stir 1 teaspoon of sugar into 50 mL of water. Record the temperature before and after.

What to look for: test 1 bubbles fiercely and the temperature drops (endothermic). Test 2 foams with oxygen gas and gets noticeably warmer (exothermic). Test 3 slowly gets warmer by a few degrees, and the steel wool may show brownish rust: the vinegar removes a protective coating so the iron can react with oxygen. Test 4 gives off light with very little heat. Test 5 shows the sugar disappearing but no bubbles, colour change or real temperature change: dissolving is a physical change. Clean-up: pour liquids down the sink with lots of water, put the steel wool in the garbage, and wash your hands.

Just for fun

A glow stick is a chemical reaction you can put in your pocket. If you put a glowing stick in the freezer, it dims but lasts longer, because cold slows the reaction down. Warm it up again later and it brightens. (Just do not leave it next to the ice cream.)

Your turn

What is the difference between a reactant and a product?

List five kinds of evidence of a chemical change.

What is a precipitate?

Is a burning candle exothermic or endothermic? How do you know?

Water is heated and bubbles form. Is this evidence of a chemical change? Explain.

Why do scientists include a control in an inquiry?

Name one safety rule for mixing chemicals and explain why it matters.

More practice: warm-up

What do we call a reaction that releases heat?

What do we call a reaction that absorbs heat?

In a chemical reaction, are atoms destroyed?

Give one example of a chemical change in a kitchen.

What does WHMIS stand for?

More practice: on your own

A clear solution and a blue solution are mixed, and the result is light blue. Is this evidence of a chemical reaction? Explain.

When two clear solutions are mixed, a yellow solid appears and settles. Name the evidence and explain what it suggests.

A student writes the hypothesis: "Baking soda and vinegar will react." Improve it.

Identify the independent, dependent and controlled variables: a student tests whether warmer hydrogen peroxide reacts faster with yeast by timing how long the foam takes to reach the top of a cup.

Rusting is slow and you rarely see bubbles or flames. What evidence tells you rusting is a chemical change?

Challenge

Dissolving a certain salt in water makes the water much colder, yet this is usually classified as a physical change. Explain why a temperature change is not enough to prove a chemical change.

Design a quick test to show that the gas from yeast + hydrogen peroxide is not just air. (Hint: think about what the gas does to a glowing splint.)

In the steel wool test, the temperature rose 4 °C in a closed jar but only 1 °C in an open dish. Suggest an explanation and a source of error.

Check your understanding

Quiz

Which is the best evidence that a chemical reaction has happened?

A reaction makes its container feel cold. The reaction is...

In a chemical reaction, the new substances formed are called...

Which is NOT evidence of a chemical change?

In an inquiry, what is the purpose of a control?

Why should you see more than one kind of evidence before concluding a reaction happened?

For parents and teachers

Big idea: C3.3 asks students to describe the evidence for chemical change (colour, gas, precipitate, heat, light), and C2.5 asks them to plan and conduct an inquiry to find it. Strand A skills are taught here: writing a hypothesis, identifying variables, using a control, recording data in a table and applying safe practice (A1.1, A1.4, A1.5, A1.6).

Common difficulties:

Treating every bubble or temperature change as a chemical change. Boiling and dissolving are physical.
Mixing up exothermic and endothermic. "Exo" = exit: heat leaves the reaction and the surroundings get warmer.
Writing vague hypotheses. Encourage "If... then... because..." with something measurable.

Safety at home: please supervise the inquiry. 3% hydrogen peroxide (sold in pharmacies) can irritate skin and eyes: you pour it, and everyone wears eye protection and gloves. Use only 3%, never stronger hair-salon peroxide. Steel wool can give splinters, so use gloves. Do not cut glow sticks open. Everything else is food-safe, but no tasting during experiments. Rinse liquids down the sink with plenty of water.