Chemistry: Matter, Acids-Bases and Metals
Chemistry: Matter, Acids-Bases and Metals
Chemistry supplies roughly 2 to 3 of the science questions in Part-II. The favourite areas are fixed: physical versus chemical change, the pH scale, everyday acids, and the properties of metals. Every one of these can be locked down with a short table, which makes this a high-return topic.
Matter and Its Classification
Matter exists in three common states, and the arrangement of particles explains their behaviour.
| State | Particle arrangement | Shape and volume |
|---|---|---|
| Solid | Closely packed, fixed positions | Fixed shape, fixed volume |
| Liquid | Close but able to slide | No fixed shape, fixed volume |
| Gas | Far apart, free moving | No fixed shape or volume |
An element has only one kind of atom, a compound has two or more elements chemically combined in a fixed ratio, and a mixture has substances merely mixed and separable by physical means. Air is a mixture; water is a compound.
Physical and Chemical Change
A physical change alters only form or state and is usually reversible. A chemical change produces a new substance and is usually irreversible.
- Melting of ice, dissolving sugar, tearing paper — physical.
- Burning of paper, rusting of iron, curdling of milk, cooking food — chemical.
✗ Rusting of iron is a physical change | ✓ Rusting produces a new substance, so it is a chemical change
Acids, Bases and Salts
Acids taste sour and turn blue litmus red. Bases taste bitter, feel soapy and turn red litmus blue. When they react, they neutralise each other to form salt and water.
Acid + Base → Salt + Water
The pH scale runs from 0 to 14. Below 7 is acidic, exactly 7 is neutral, above 7 is basic.
| Acid | Found in |
|---|---|
| Citric acid | Lemon, orange |
| Acetic acid | Vinegar |
| Lactic acid | Curd |
| Oxalic acid | Tomato |
| Formic acid | Ant sting, nettle |
| Tartaric acid | Tamarind |
✗ pH 7 means the solution is acidic | ✓ pH 7 is neutral; below 7 is acidic
Metals and Non-Metals
| Property | Metals | Non-metals |
|---|---|---|
| Lustre | Shiny | Dull |
| Malleable and ductile | Yes | No, brittle |
| Conductivity | Good | Poor (graphite is the exception) |
| Nature of oxides | Basic | Acidic |
Two exceptions are asked repeatedly: mercury is the only metal liquid at room temperature, and bromine is the only non-metal that is liquid. Rusting needs both oxygen and moisture, which is why coating iron with zinc, called galvanisation, prevents it.
pH 7 उदासीन है — 7 से कम अम्लीय, 7 से ज्यादा क्षारीय।
TRE pointer: Everyday-acid questions are almost guaranteed in a teaching paper because they sit in the class 7 textbook and connect to daily life. The favourite trap is a statement-based question mixing one correct acid-source pair with one wrong one. Keep mercury and bromine ready as the standard exception answers, and remember that with five options and −1/3 negative marking, a half-remembered acid pair is better left to option E.
60-Second Recap
- Element, compound and mixture: air is a mixture, water is a compound.
- Physical change is reversible; chemical change makes a new substance.
- Rusting, burning, cooking and curdling are chemical changes.
- pH: below 7 acidic, exactly 7 neutral, above 7 basic. Acid + base gives salt and water.
- Citric in lemon, acetic in vinegar, lactic in curd, oxalic in tomato, formic in ant sting.
- Mercury is the only liquid metal; bromine the only liquid non-metal; graphite conducts.
Frequently Asked Questions
Which acid is present in curd and which in lemon?
Curd contains lactic acid and lemon contains citric acid. The other pairs commonly asked are acetic acid in vinegar, oxalic acid in tomato, formic acid in an ant sting, and tartaric acid in tamarind. These six pairs cover almost every question set on everyday acids.
Is rusting a physical or chemical change?
Chemical. Rusting converts iron into hydrated iron oxide, a completely new substance, and the change cannot be reversed by simple physical means. It requires both oxygen and moisture, which is why galvanising iron with a zinc coating stops the process.
Which is the only metal that is liquid at room temperature?
Mercury. Its counterpart among non-metals is bromine, the only non-metal that is liquid at room temperature. Examiners often place these two in the same question, so learning them as a pair is safer than learning either one alone.
What does a pH value of 7 mean?
It means the solution is neutral, like pure water. Values below 7 indicate an acidic solution and values above 7 indicate a basic or alkaline one, with the scale running from 0 to 14. Treating 7 as acidic is the single most common error in this topic.