In-text Questions — Hard Water and Detergents — Carbon and its Compounds

1. 12 marksNCERT Cl-10 Science, In-text Qs after §4.5, Q1

   Would you be able to check if water is hard by using a detergent?

   Hint. Recall exactly why detergents were invented in the first place — what problem with hard water do they specifically avoid?

   Step 1 — Recall what makes detergents different from soap. Detergents don't form insoluble precipitates with the calcium and magnesium ions found in hard water, unlike soap.

   Step 2 — Apply this to the hardness test. Since a detergent foams equally well in both hard and soft water, there is no visible difference (no curdy scum, no reduced foam) to tell the two apart.

   ✦ Answer: No — a detergent lathers the same way in both hard and soft water, since it doesn't react with calcium/magnesium ions the way soap does, so it can't be used to detect hardness. Soap, which does show reduced foam and scum in hard water, is the tool for that test.

   Where students slip. Assuming any cleaning agent can reveal hardness — the whole point of a detergent is that it behaves identically in hard and soft water, which is exactly why it can't be used to distinguish them.

2. 22 marksNCERT Cl-10 Science, In-text Qs after §4.5, Q2

   People use a variety of methods to wash clothes. Usually after adding the soap, they 'beat' the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?

   Hint. Soap alone can surround dirt in micelles — but something still has to physically pull that dirt out of the fabric fibres.

   Step 1 — Recall what soap alone does. Soap forms micelles around oily dirt, but the dirt still has to be physically separated from the cloth fibres it's clinging to.

   Step 2 — Explain what agitation adds. Beating, scrubbing or machine tumbling provides the mechanical action needed to dislodge dirt from the fabric and disperse it into the surrounding water as micelles.

   Step 3 — Connect this to the final result. Without this physical action, much of the loosened dirt would remain trapped in the fibres even with soap present, so agitation is what actually completes the cleaning.

   ✦ Answer: Soap can only surround dirt with micelles once it's free in the water — agitation supplies the mechanical force needed to actually dislodge the dirt from the cloth fibres so it can be carried away in those micelles.

   Where students slip. Treating soap as sufficient on its own — soap changes the dirt's chemistry (making it water-soluble via micelles), but the physical work of pulling dirt out of fabric still needs mechanical agitation.

Solutions written by the tuition.in editorial team and checked against the NCERT Class 10 Science textbook, Reprint 2026-27 (jesc104.pdf) — five in-text question sets (13 questions total, not 18 as some older manifests claim) plus one end-of-chapter Exercise (15 questions, not 20). Unchanged by rationalisation.. Questions are referenced from the NCERT textbook for identification.