Class 7 Science Curiosity Chapter 2 MCQ – Exploring Substances: Acidic, Basic and Neutral

A solution that turns red litmus blue is basic. Vinegar is acidic, so adding enough of it can neutralise the base and make the mixture acidic. In an acidic mixture, the blue litmus changes back to red.

Each observation identifies a basic solution. Bases turn red litmus blue, change yellow turmeric to red, and make red rose extract green. Although three different indicators are used, all three tests point to the same nature.

Turning blue litmus red is the clear sign of an acidic liquid. An acid does not change red litmus or yellow turmeric. The three observations therefore agree with one another and identify the sample as acidic.

An olfactory indicator is identified through changes in smell rather than colour. This makes it suitable for an activity involving a blindfolded student. The chapter describes onion strips as an example for exploring such changes in acidic and basic substances.

The green colour with red rose extract shows that liquid X is initially basic. Amla juice is acidic. Adding enough of it first counters the base and can then make the mixture acidic, as stated in the question.

Litmus is a natural substance obtained from lichens. It is used as a solution or on paper strips to test substances. Its colour changes help us identify whether a substance is acidic or basic in nature.

Neutral substances are neither acidic nor basic. In the chapter’s tests, examples such as sugar solution and common salt solution do not change either red or blue litmus, helping distinguish them from the acidic and basic groups.

The edible acidic samples discussed in the chapter, such as lemon juice and tamarind water, taste sour. However, tasting is not a safe way to identify an unknown substance; the chapter instead uses indicators to test its nature.

Baking soda solution feels soapy or slippery in the activity. The chapter explains that basic substances generally have this property. This observation supports the other evidence, including the change of red litmus to blue, that baking soda solution is basic.

The chapter gives bitter gourd as an example of something that tastes bitter but is not basic. Therefore, taste alone cannot reliably identify a base. Appropriate indicators provide a better way of checking the nature of a substance.

Lime water is a solution of calcium hydroxide in water. The chapter specifically warns readers not to confuse it with the citrus fruit called lime. Unlike sour fruit juice, lime water is basic in the indicator tests.

These different responses allow the flower extract to work as a natural indicator for the substances being tested.

Yellow turmeric changes to red in a basic solution, but it stays yellow in acidic and neutral solutions. Therefore, an unchanged strip does not tell us which of these two possibilities is correct without another useful test.

The hot water takes up colour from the crushed rose petals. After filtration, the collected coloured liquid is called the filtrate. This flower extract is used as an indicator because it responds differently to acidic and basic solutions.

Yellow turmeric turns red when it comes into contact with a basic solution. This colour change explains why a turmeric-containing curry stain changes colour when soap is applied, since soap solution is one of the basic samples discussed.

The chapter represents neutralisation as acid plus base producing salt and water, along with heat. Suitable amounts are needed for the mixture to become neutral. Simply mixing any quantities does not guarantee that no acid or base remains.

The chapter explains that a red ant injects an acidic liquid containing formic acid. Baking soda is basic, so the example uses moist baking soda to counter the acid. This illustrates neutralisation in an everyday situation described in the lesson.

Lime is basic and can counter the acidic nature of soil. The chapter describes this treatment when excessive chemical fertiliser use makes soil acidic. The purpose is to correct the soil’s nature so that plants can grow better.

Organic matter, including manure and composted leaves, releases acids as it breaks down. These acids can neutralise the basic nature of soil. The chapter therefore presents organic matter as a suitable treatment for soil that is too basic.

Acidic factory waste can harm organisms living in a lake. The chapter explains that adding suitable basic substances can neutralise the waste before release. This application uses the ability of acids and bases to counter each other.

The chapter states that hydrangeas produce blue flowers in acidic soil and pink or red flowers in basic soil. Changing the soil towards basic conditions therefore matches the desired colour change in this particular example of plant growth.

Both acidic and neutral samples can leave red litmus unchanged. Therefore, this single result cannot distinguish between them. Testing another piece of blue litmus would help, because an acid turns it red while a neutral sample leaves it blue.

Vinegar is acidic and sugar solution is neutral in the chapter’s examples. Both leave yellow turmeric unchanged. The same result for both bottles shows that this particular test alone cannot tell which of the two liquids is present.

Turmeric is red in a basic solution but yellow in an acidic one. Soap first makes the strip red. When enough lemon juice changes the liquid to acidic, the strip returns to the yellow colour associated with acidic conditions.

A green rose indicator identifies basic conditions, whereas red identifies acidic conditions. Vinegar supplies acid. The final red colour shows that enough vinegar has been added to overcome the original basic nature and leave an acidic mixture.

Lemon juice is acidic, while fresh yellow turmeric turns red in basic conditions. Turmeric remains yellow in an acidic liquid. The chosen combination therefore does not produce red writing, showing why a suitable indicator must be matched to the writing substance.

Lime water is basic, while lime fruit juice is acidic. Litmus tests reveal this difference: a base turns red litmus blue, while an acid turns blue litmus red. Their liquid appearance or container shape cannot establish their nature.

The change of blue litmus to red identifies the liquid as acidic. Red rose extract is another indicator and also gives a red colour in acidic conditions. The two tests therefore provide matching evidence about the same liquid.

The chapter explains that plants may grow poorly even when the soil is neutral because some nutrients are missing. Poor growth alone does not prove acidity or basicity, so the cause should be considered before selecting a treatment.

The chapter warns against tasting unknown substances. An appropriate indicator allows the liquid’s acidic or basic nature to be investigated through an observable response, such as a colour change, without using taste as the method of identification.