pH Scale and Important Salts
NCERT Class 10 • Science • Chapter "Acids Bases and Salts"
pH Scale and Important Salts
The strength of an acid or base is measured on the pH scale, which runs from 0 to 14. It tells us how many hydrogen ions a solution contains, and it matters far beyond the laboratory, since soil, blood, rain and the mouth all have a pH that must stay within a range.
The pH Scale
- pH 7 is neutral, below 7 is acidic and above 7 is basic.
- The lower the pH, the greater the concentration of hydrogen ions and the stronger the acid.
- A universal indicator shows different colours across the whole range, so the approximate pH can be read directly.
- Living things survive only in a narrow pH range; human blood stays close to neutral.
- Acid rain has a pH below about 5.6 and damages crops, buildings and aquatic life.
- Soil pH decides which crops can grow, and it is corrected by adding lime for acidic soil or gypsum for alkaline soil.
- Tooth decay begins when the pH in the mouth falls below about 5.5, which is why toothpaste is mildly basic.
Important Salts
- Common salt, sodium chloride, is the raw material for many chemicals and is obtained from sea water and rock salt.
- Electrolysis of brine, called the chlor alkali process, gives sodium hydroxide, chlorine and hydrogen.
- Baking soda, sodium hydrogen carbonate, is mildly basic; it is used in cooking, in antacids and in fire extinguishers.
- Baking powder is baking soda with a mild edible acid, and it releases carbon dioxide that makes cakes rise.
- Washing soda, sodium carbonate with ten molecules of water, is used for cleaning and for removing the hardness of water.
- Bleaching powder is made by the action of chlorine on dry slaked lime and is used for bleaching and for disinfecting water.
- Plaster of Paris is obtained by heating gypsum, and it sets into a hard solid on adding water, which is why it is used for plaster casts.
| Salt | Common Name | Main Use |
|---|---|---|
| Sodium chloride | Common salt | Food and raw material |
| Sodium hydrogen carbonate | Baking soda | Cooking, antacid |
| Sodium carbonate | Washing soda | Cleaning, softening water |
| Calcium sulphate hemihydrate | Plaster of Paris | Casts and moulds |
pH scale — a scale from 0 to 14 that measures the concentration of hydrogen ions in a solution.
Exam me kaise aata hai
- The pH of a neutral solution is — 7
- Tooth decay begins below which pH — 5.5
- Plaster of Paris is obtained by heating — gypsum
- Which gas makes a cake rise — carbon dioxide
UPSC/State PSC ke liye
- Acid rain harms aquatic life because a small fall in pH is a large rise in hydrogen ion concentration, since the scale is logarithmic.
- Plaster of Paris must be stored in a moisture proof container, because even damp air will set it before use.
Yahan confuse hote hain
✗ A higher pH means a stronger acid | A lower pH means a stronger acid | ✓
✗ Baking soda and baking powder are the same | Baking powder is baking soda plus a mild edible acid | ✓
✗ Washing soda and baking soda are the same salt | They are different salts with different uses | ✓
Ek nazar me
- pH runs from 0 to 14, with 7 neutral, below acidic and above basic.
- Soil, blood, rain and the mouth all depend on a narrow pH range.
- Baking soda for cooking and antacid; washing soda for cleaning.
- Bleaching powder disinfects water; plaster of Paris comes from gypsum.