Oracle IAS, the best coaching institute for UPSC/IAS/PCS preparation in Dehradun brings to you UKPCS Science Chemistry (paper 6).
- pH (potential of hydrogen) is a scale of acidity from 0 to 14. It tells how acidic or alkaline a substance is.
- More acidic solutions, have lower pH. More alkaline solutions, have higher pH. Substances that aren’t acidic or alkaline (that is, neutral solutions) usually have a pH of 7.
- Acids have a pH that is less than 7. Alkalis have a pH that is greater than 7.pH is a measure of the concentration of protons (H+) in a solution.
- Typical indicators are phenolphthalein, methyl orange, methyl red, bromothymol blue, and thymol blue. They each change colour at different points on the pH scale, and can be used together as a universal indicator.
- Another way is to use litmus paper, which is based on a natural pH indicators.
- Neutral solutions
Neutral solutions have a pH of 7. They do not change the colour of litmus paper, but they turn universal indicator green. Water is neutral.
When acids react with carbonates, such as calcium carbonate (found in chalk, limestone and marble), a salt, water and carbon dioxide are made. In general:
acid + metal carbonate → salt + water + carbon dioxide
Notice that an extra product – carbon dioxide – is made. It causes bubbling during the reaction, and can be detected using limewater. You usually see this reaction if you study the effects of acid rain on rocks and building materials.
- Reactive metals
Acids will react with reactive metals, such as magnesium and zinc, to make a saltand hydrogen. In general:
acid + metal → salt + hydrogen
The hydrogen causes bubbling during the reaction, and can be detected using a lighted splint. You usually see this reaction if you study the reactivity series of metals.
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