Soap in hard water forms scum

The idea

Soap reacts with the calcium and magnesium salts that cause hardness of water, forming an insoluble scum, so foam forms with difficulty and a larger amount of soap is needed.

Foam that will not come

While bathing, you may see that foam forms with difficulty and an insoluble substance, scum, remains after washing.

What do you think is the cause?

• Salts of calcium and magnesium in the water
• Too little water in the bucket
• The soap is too old

Soap meets hard water

Press Next step.

Scum uses up the soap

Part of the soap is used up making the scum, so less is left to clean and foam. That is why more soap is needed.

Soap reacts with the calcium and magnesium salts that cause hardness of water, forming an insoluble scum, so foam forms with difficulty and a larger amount of soap is needed.

Notes

Soap reacts with the calcium and magnesium salts that cause hardness of water, forming an insoluble scum, so foam forms with difficulty and a larger amount of soap is needed.

Check yourself

Which salts cause the hardness of water?

What does soap form when it reacts with the salts in hard water?

What happens to the foam when soap is used in hard water?

What is needed when soap is used in hard water?

• Sodium and potassium salts. Sodium or potassium salts of long-chain acids are soaps, not the cause of hardness.
• Calcium and magnesium salts — correct. Yes!
• Ammonium salts. Calcium and magnesium salts cause hardness of water.
• A clear solution. The book says an insoluble substance, scum, remains.
• A micelle that cleans better. The reaction forms an insoluble scum, so foam forms with difficulty.
• An insoluble scum — correct. Yes!
• It forms with difficulty — correct. Yes!
• It forms very easily. Foam forms with difficulty in hard water.
• It forms but turns into oil. The scum is an insoluble substance. The foam forms with difficulty.
• A smaller amount of soap. Some soap is used up in forming scum, so a larger amount is needed.
• A larger amount of soap — correct. Yes!
• No soap at all. Soap is still used, but a larger amount is needed.