Exercises: Evidence of a chemical reaction

You mix a spoonful of baking soda with vinegar in a glass and, instantly, it fills with bubbles and intense fizzing. What evidence of a chemical reaction are you observing? What is being formed?

Pizza dough contains yeast and, when left to rest for a while, it doubles in size and becomes full of small holes. What evidence of a chemical change causes the dough to rise? What happens to the ingredients in the mixture?

You slice an avocado to make guacamole and, if it is not eaten promptly, its surface turns brown. What evidence of a chemical reaction is this? Do you think only the color has changed or have new substances also been formed?

You light the candle on a birthday cake: while it burns, it gives off light and heats the surrounding air. Identify the two pieces of evidence of a chemical reaction given off by the burning candle.

In the lab, you mix two clear, colorless liquids and suddenly a yellow solid appears and settles at the bottom of the test tube, as shown in the diagram.

What is this solid that forms called? Why does its appearance indicate that a chemical reaction has taken place?

A basketball player sprains an ankle and the physical therapist activates an instant cold pack by striking it: without a refrigerator or ice, the pack becomes freezing cold in seconds. What observable change shows that an endothermic process is taking place inside the pack?

Complete the sentence with the five main signs that reveal a chemical reaction: "Release of $\dots$, formation of a $\dots$ that settles at the bottom, change of $\dots$, variation in $\dots$ and emission of $\dots$."

The bathroom mirror steams up every time you take a hot shower. Is that evidence of a chemical reaction? Explain what is actually happening to the water vapor.

At a concert, you bend a glow stick bracelet, it makes a "click" sound and begins to glow with green light, without batteries or a plug.

1. a) What evidence of a chemical reaction does the glow stick bracelet provide?
2. b) A light bulb also gives off light and is not a chemical reaction: what is the difference between the two cases?

A granular drain cleaner, when poured down the sink with water, forms a lot of foam and, furthermore, the pipe becomes warm to the touch. Identify the two pieces of evidence of a chemical reaction shown by the product and explain what each one indicates.

A farmer analyzes irrigation water from his vegetable garden: he adds a few drops of a reagent to a sample and, if a white powder appears at the bottom of the tube, he knows that the water contains an excess of a certain substance.

1. a) What evidence of a chemical reaction does this test rely on?
2. b) Why does the appearance of that powder prove that a new substance has been formed?

For the ski trip, you take hand-warmer pouches that start heating up as soon as you open their plastic wrapper, without fire or batteries.

1. a) What evidence of a chemical reaction do they show?
2. b) Explain why they cannot simply be "storing heat" from the factory.

Red cabbage juice turns pink if you add a few drops of lemon juice and greenish if you add a soap solution, as shown in the diagram.

1. a) What chemical evidence does this kitchen trick rely on?
2. b) How can a juice that behaves like this be useful in the lab?

A tomato stain on Emily's white T-shirt disappears when bleach is applied, but the treated area never returns to its original state, no matter how much she washes it afterwards.

1. a) What evidence of a chemical reaction has occurred in the fabric?
2. b) Explain why removing the stain with bleach is not the same as rubbing it with water alone.

In the lab, you mix two liquids that were each at $20\ \celsius$, and the thermometer in the mixture rises to $35\ \celsius$ without placing any Bunsen burner or hot plate near it, as shown in the diagram.

1. a) What evidence of a chemical reaction is the thermometer detecting?
2. b) Where has that heat come from if no one heated the mixture?

When frying, the oil sizzles because water in the food evaporates suddenly; but if you leave it on the heat for too long, it starts to smoke, smell bad, and turn black.

1. a) Which sign of the process corresponds to a simple physical change?
2. b) Which signs reveal that the oil is undergoing a chemical reaction? Justify both answers.

Hydrogen peroxide from the first aid kit bubbles intensely when applied to a wound, but remains stable inside the closed bottle.

1. a) What evidence of a chemical reaction does it show when coming into contact with the skin?
2. b) What do you think the wound provides to trigger the reaction at that moment?
3. c) If the bottle were left open for years, it would eventually turn into ordinary water: explain what has happened to the original substance and why this confirms that it was a chemical reaction.

No single piece of evidence guarantees that a chemical reaction has taken place. Give a physical counterexample (an everyday phenomenon that shows the same sign without being chemical) for each of these four signs:

1. a) A gas is given off.
2. b) A solid appears where there was none before.
3. c) The color changes.
4. d) The temperature changes.

Finish by explaining what you should always check, besides the sign, to confirm that a chemical reaction has occurred.

A freshly washed and thoroughly dried steel pot is placed over the blue flame of a gas stove: tiny droplets of water immediately appear on its bottom, which disappear again as the pot heats up. The setup is shown in the figure:

1. a) Where did that water come from if the pot was completely dry?
2. b) Why are those droplets a clue that the combustion of the gas produces new substances?
3. c) Why do they disappear afterwards? Classify this last process as physical or chemical.

In the garden, the compost pile is warm inside even on a cold winter day, and in the morning, it appears to be "smoking".

1. a) What chemical evidence explains this heat if nobody is heating the pile?
2. b) The "smoke" is actually water vapor condensing on contact with cold air: is this evidence of a chemical reaction? Justify the difference between the two signs given by the compost.

High-mountain emergency rations heat themselves when water is poured into a double bag: the food goes from about $15\ \celsius$ to about $60\ \celsius$ in ten minutes, without fire or electricity.

1. a) What evidence of a chemical reaction is taking place inside the bag?
2. b) Why is it impossible for it to be hot water "hidden" in the packaging?
3. c) After a while, the bag stops producing heat even though there is still water inside: explain what has been used up.

Storm matches ignite even in strong winds. Striking one reveals several signs: a bright flame, heat, a strong smell, grayish smoke and a crackling sound.

1. a) Make a list of the signs that are indeed evidence of a chemical reaction and justify each choice.
2. b) Which ones accompany the phenomenon but, on their own, do not prove that new substances have been formed? Explain your reasoning.

Over time, a hard white crust appears at the bottom of the electric kettle. Your brother claims: "A solid has formed where there wasn't one before, so it's a precipitate from a chemical reaction."

1. a) Explain to him why his reasoning is not valid: the appearance of a solid does not by itself prove a reaction (a solid also appears when salt crystallises as seawater evaporates). Then state what actually happens: tap water contains dissolved calcium hydrogencarbonate, $\mathrm{Ca(HCO_3)_2}$, which decomposes on heating into calcium carbonate (the crust), water and carbon dioxide. Is it, then, a physical or a chemical change?
2. b) What fundamental difference is there between that crust and the precipitate formed when mixing two liquids in the lab?

On a barbecue, charcoal burns leaving embers that glow and give off heat, smoke with a characteristic smell is released, and in the end, only very light gray ash remains; meanwhile, the water in a glass forgotten near the grill evaporates due to the heat.

1. a) Sort all the signs of the process into two lists: physical changes and evidence of a chemical reaction.
2. b) Justify why the ash is evidence that new substances were formed from the charcoal.

Unit challenge. Become a scientific researcher in your kitchen and analyze the boundary between physical and chemical changes using two everyday transformations: dissolving table salt in water and the reaction between vinegar and baking soda.

1. Experimentation: In four glasses containing $50\text{ mL}$ of vinegar each, add respectively $1\text{ g}$, $2\text{ g}$, $3\text{ g}$, and $4\text{ g}$ of baking soda (measured with a kitchen scale, with an uncertainty of $\pm 0.1\text{ g}$). Repeat a trial by dissolving $2\text{ g}$ of salt in $50\text{ mL}$ of water.

2. Taking measurements: Time the duration of the effervescence in each case using your cell phone ($t \pm 0.5\text{ s}$) and measure the initial and final temperature of the liquid with a kitchen thermometer ($T \pm 0.5\text{ }^\circ\text{C}$).

3. Data processing: Create a data table with your values and uncertainties, and plot a graph of effervescence time versus mass of baking soda.

4. Final report: Write a short report where you classify both processes with proper justification, identifying the experimental evidence observed (temperature change, gas evolution).

The following will be assessed:

• Correct identification and reasoning regarding the evidence of chemical change versus physical change.

• Thoroughness in data recording, handling of uncertainties, and preparation of the data table and graph.

• Quality of the analysis, clarity of presentation, and conclusions of the scientific report.