Introduction:
Curd and sour substances, such as lemon juice or vinegar, are mild acids that contain citric acid or acetic acid. These acids can react with brass and copper, resulting in the production of toxic substances. In this response, we will discuss the reasons for this reaction and the toxic substances that are formed.

Reaction between curd/sour substances and brass/copper:
When curd or sour substances come into contact with brass or copper, a chemical reaction takes place. This reaction occurs because brass and copper are metals that are prone to oxidation. The mild acids in curd and sour substances accelerate the oxidation process by providing hydrogen ions (H+) that can react with the metal surface.

Formation of toxic substances:
The reaction between curd/sour substances and brass/copper results in the formation of toxic substances such as copper acetate or copper citrate. These substances are toxic because they can have harmful effects on human health if ingested or inhaled.

Copper acetate:
Copper acetate is a compound formed when copper reacts with acetic acid, which is present in vinegar. It is a greenish-blue crystalline solid that is highly toxic. Ingesting or inhaling copper acetate can cause symptoms such as nausea, vomiting, abdominal pain, and liver damage. Prolonged exposure to copper acetate can have serious health consequences.

Copper citrate:
Copper citrate is formed when copper reacts with citric acid, which is present in lemon juice. It is a compound that is also toxic and can cause health issues when ingested or inhaled. Symptoms of copper citrate poisoning include gastrointestinal disturbances, kidney damage, and liver dysfunction.

Precautions:
To avoid the formation of toxic substances and ensure safety, it is important to take certain precautions when handling curd or sour substances with brass or copper. Some precautions include:

1. Avoid storing curd or sour substances in brass or copper containers, as this can lead to prolonged exposure and the formation of toxic substances.
2. Use glass or stainless steel containers for storing curd or sour substances.
3. Avoid using brass or copper utensils for cooking or serving acidic foods.
4. Clean brass or copper utensils thoroughly after use with curd or sour substances to remove any residue that may lead to the formation of toxic substances.

Conclusion:
In conclusion, curd and sour substances contain mild acids that can react with brass and copper, resulting in the formation of toxic substances such as copper acetate or copper citrate. These substances can have harmful effects on human health if ingested or inhaled. It is important to take precautions and avoid storing or using curd or sour substances with brass or copper containers or utensils to ensure safety.