Changes Around Us: Physical and Chemical · Lesson 2 of 6
How Chemical Changes Form New Substances
“Use simple investigations to recognise new substances and identify carbon dioxide with lime water.”
• Explain what a chemical change is and how it differs from a physical change. • Describe why carbon dioxide turns lime water milky. • Explain the vinegar and baking soda reaction using observations and a word equation. • Interpret gas-transfer diagrams and distinguish a produced gas from air already present. • Connect the yeast balloon investigation with physical and chemical changes.
Look for a change in the substance itself
In a physical change, the starting material remains the same substance. Some changes go further: the starting substances react and produce something new with different properties. To recognise such a change, we must connect an observation with evidence for a new substance. A different shape or a few bubbles are not enough on their own.
A change in which one or more new substances are formed.
The process through which substances change into new substances.
The starting substances take part in the reaction, and the substances formed are its products. You do not need to memorise complicated chemical symbols here. A word equation can name what takes part and what is formed. We will build an equation only after understanding the investigation it represents.
Compare breath in water and in lime water
Imagine two transparent glasses. One contains ordinary water and the other contains lime water, a clear solution containing calcium hydroxide. A teacher passes exhaled air into each using separate straws. The comparison is useful because the same kind of air enters two different liquids.
In ordinary water, bubbles rise because air is passing through the liquid. The water does not develop a new milky appearance. In lime water, bubbles are also seen, but the liquid becomes cloudy or milky. On standing, a white solid can settle. That solid is evidence of a substance that was not present in the clear solution before.
Exhaled air contains carbon dioxide. It reacts with the calcium hydroxide in lime water to form calcium carbonate and water. Calcium carbonate is a white substance that does not dissolve in this water. Tiny particles scatter light, making the liquid look milky; some later settle at the bottom.
A substance that does not dissolve in the liquid being considered.
Because this reaction produces a visible result, freshly prepared lime water can be used to test for carbon dioxide in these investigations. We do not say that the gas is carbon dioxide merely because it makes bubbles. We identify it by the change it causes in lime water.
Use teacher supervision for the lime-water investigation. Blow only as directed; never suck liquid through a straw, taste lime water, or share straws.
Problem
A learner sees bubbles in both water and lime water. Only the lime water becomes milky. Which observation shows a chemical change?
- 1.Bubbles in ordinary water show that gas is moving through it; they need not be newly made gas.
- 2.The milky appearance and settling white solid in lime water show calcium carbonate forming.
- 3.The formation of calcium carbonate is a chemical change. The comparison helps separate gas movement from a reaction.
Vinegar and baking soda produce a gas
Vinegar and lemon juice contain acids. Baking soda, also called sodium hydrogen carbonate, reacts with either of these liquids. When they are mixed, fizzing and bubbling are observed. The next question is not just “is there gas?” but “which gas is it, and was it newly formed?”
In the teacher-led investigation, a small amount of vinegar or lemon juice is mixed with baking soda. A delivery tube carries the gas into a second container of fresh lime water. The lime water turns milky. We can therefore connect two observations: the first mixture makes a gas, and that gas gives the carbon-dioxide test. The gas-producing container is fitted so that gas above the mixture enters the delivery tube; the other end of the tube is below the lime-water surface.
The apparatus can use test tubes or two small bottles with a suitable connecting tube. Its purpose is the same in both versions: keep the reacting mixture separate from the testing liquid while transferring the gas. This prevents us from confusing the appearance of the original mixture with the result of the gas test.
Now compare baking soda mixed with water alone. It can dissolve without the vigorous reaction seen with vinegar. Under the ordinary conditions of this comparison, dissolving is treated as a physical change: the baking soda spreads through the water, and the carbon-dioxide-producing acid reaction is absent. Comparing mixtures is more informative than describing just one bubbling liquid.
| Starting mixture | Expected result in the lime-water test | Reason |
|---|---|---|
| Vinegar + baking soda | Lime water turns milky. | The reaction produces carbon dioxide. |
| Lemon juice + vinegar | No carbon-dioxide milkiness from this mixture. | Mixing the two acidic liquids does not provide the baking-soda reaction. |
| Vinegar + common salt | No carbon-dioxide milkiness from this mixture. | Common salt does not replace baking soda in this reaction. |
| Lemon juice + baking soda | Lime water turns milky. | This acid and baking soda also produce carbon dioxide. |
Problem
Four setups use the mixtures in the table above. Which two are expected to send carbon dioxide into lime water?
- 1.Find the mixtures containing baking soda together with vinegar or lemon juice.
- 2.These are vinegar with baking soda and lemon juice with baking soda. Both can produce the new gas.
- 3.The gas makes the receiving lime water milky. The other two mixtures are not expected to give this result under the investigated conditions.
Boiling water, blowing through a straw, and an acid reacting with baking soda can all show bubbles. These bubbles have different explanations. A gas test and the starting substances provide evidence; bubbles alone do not identify carbon dioxide or prove a chemical change.
Explore how yeast inflates a balloon
The gas-test idea also helps explain bread making. Yeast is a living organism used to help dough rise. In a suitable sugar-and-water mixture, yeast produces carbon dioxide through a chemical process. Gas collecting in a balloon makes it expand, so we can observe a physical effect of a chemical change.
In this supervised project, sugar is first dissolved in water in a small bottle. Yeast is added, and a balloon is fitted over the mouth to collect gas. After about an hour under suitable conditions, the balloon may inflate. A teacher can transfer the collected gas into another container of fresh lime water and gently mix it with the liquid. If carbon dioxide is present, the liquid becomes milky, supporting the conclusion that carbon dioxide was formed. The amount of inflation and time needed can vary with conditions.
Separate the stages carefully. Dissolving the sugar and stretching the balloon are physical changes. Yeast producing carbon dioxide from the sugar mixture involves chemical change. In bread dough, the gas forms pockets that help make the dough rise and contribute to a soft, airy texture. We use this project to connect a chemical reaction with an everyday application, without needing the detailed chemistry of yeast.
Problem
A sugar solution looks clear at first; later, after yeast is added, a balloon expands. Does balloon expansion alone prove that new gas was formed?
- 1.Stretching the balloon changes its shape, so that part is physical.
- 2.To explain the gas, consider the yeast acting on the sugar mixture and test the collected gas with lime water.
- 3.Milkiness in the gas test supports carbon dioxide formation. The whole investigation combines a chemical process with a physical change in the balloon.
Quiz
What is the defining feature of a chemical change?
Which substance makes lime water milky in the breath investigation?
What does the arrow in a word equation mean?
Which mixture is expected to produce carbon dioxide in this investigation?
Why is a second bottle of lime water useful?
Which stage of the yeast project is a physical change?
Practice Problems
- Define chemical change and give a reasoned comparison with physical change.
- Explain why bubbles in ordinary water and milkiness in lime water are different kinds of evidence.
- Write the lime-water word equation and name the insoluble substance.
- Describe the investigation using vinegar and baking soda as three parts: question, observation, and conclusion.
- A gas from a mixture turns lime water milky. State what you infer and which observation supports it.
- Compare all four starting mixtures in the table and explain why common salt cannot replace baking soda in this investigation.
- Identify physical and chemical stages in the yeast balloon project. Explain why the lime-water test improves the evidence.
- Why must the gas-delivery tube enter the lime water rather than end above its surface?
Key Takeaways
• A chemical change forms new substances through a chemical reaction. • Carbon dioxide reacts with calcium hydroxide in lime water to form insoluble calcium carbonate and water. • Vinegar or lemon juice reacts with baking soda to produce carbon dioxide and other substances. • Gas movement and gas formation are different; use the complete investigation to interpret bubbles. • The yeast project links a chemical process to physical balloon expansion and bread making.