NCERT Solutions for Class 7th Science Chapter 5 .2 A Substance May Change in Appearance and Not Remain the Same! — Activity 5.4: Let us experiment

Book page 615 Updated on2026-09-19

Q1.

Take a teaspoonful of vinegar or lemon juice in a test tube. Add a pinch of baking soda (sodium hydrogen carbonate) to it. What do you observe?

Answer

The mixture fizzes at once. We hear a fizzing, bubbling sound and see a froth of gas bubbles rising and filling the upper part of the test tube. The test tube also feels slightly cool.

Why it happens: Vinegar (and lemon juice) is acidic, baking soda is a hydrogen carbonate. The moment they meet they react and a gas is set free. A gas coming out of a mixture of two substances that had no gas in them is one of the clearest signs that a new substance has been made — that is, of a chemical change.

Tip: Add the baking soda in a small pinch. If you add a spoonful, the froth climbs out of the test tube and the gas is lost before you can test it.

Q2.

Pass this gas through freshly prepared lime water kept in another test tube, as shown in Fig. 5.3a. What do you observe?

Fig. 5.3(a), page 61 — the gas from vinegar + baking soda is passed through lime water in the second test tube.

Answer

The clear, colourless lime water turns milky, exactly as it did in Activity 5.3 when we blew air into it.

Gas from vinegar + baking soda → passed into lime water
Clear lime water → milky white
On standing, a white insoluble solid settles at the bottom

Why it happens: The gas reacts with the calcium hydroxide of the lime water and forms tiny particles of calcium carbonate, which do not dissolve. Suspended in the liquid, they scatter light and the lime water looks milky.

Check it yourself: The lime water must be freshly prepared and clear. Old lime water has already reacted with the carbon dioxide of the air and will not give a sharp result.

Q3.

The lime water turns milky. What do you infer about the gas formed by mixing vinegar and baking soda?

Answer

We infer that the gas is carbon dioxide.

Why it happens: Turning lime water milky is the accepted test for carbon dioxide. In Activity 5.3, the gas we breathed out was known to contain carbon dioxide and it turned lime water milky. Here an unknown gas does exactly the same thing, so it must be the same gas. Since carbon dioxide was not present in either the vinegar or the baking soda to begin with, it is a new substance — and the reaction is a chemical change:

Vinegar + Baking soda → Carbon dioxide + Other substances

You can perform the same activity with two small used bottles and a flexible straw instead of test tubes, as shown in Fig. 5.3b.

Q4.

Repeat the above activity using baking soda and water. Do you observe any bubble formation? Is this a physical or a chemical change?

Answer

No fizzing and no stream of gas bubbles are seen. The baking soda simply dissolves in the water and the liquid stays clear. (A few small air bubbles that were clinging to the powder may float up, but the fizzing sound is absent.) This is a physical change.

	Baking soda + vinegar	Baking soda + water
What is seen	Loud fizzing, plenty of gas bubbles	Powder dissolves quietly
Lime water test	Lime water turns milky	Lime water stays clear
New substance formed?	Yes — carbon dioxide	No
Type of change	Chemical change	Physical change

Why it happens: Baking soda needs an acid to release carbon dioxide. Vinegar and lemon juice are acids, plain water is not. In water the particles of baking soda only spread out among the water particles — that is dissolving, and the baking soda can be recovered by evaporating the water. Since nothing new is formed, the change is physical.

Did you know? This is why a recipe for dhokla or cake asks you to add curd, lemon juice or eno along with the soda. Without the acid there is no gas, and the batter will not rise.

Was this helpful?