You fill a glass from the kitchen tap in St. Louis, and it looks clear. Later, the sides or bottom have a faint orange tint, or the water looks cloudy with tiny flecks. Many homeowners assume the pipes are rusting or that something is wrong with the water heater. Often the cause is simpler: the water carried dissolved iron that stayed invisible until it met air.

Why the water looks clear at first

Iron in groundwater is often dissolved, in a form called ferrous iron. Dissolved iron does not show color while the water stays sealed in pipes, away from oxygen. Once water is poured into a glass, it picks up oxygen from the air. The iron reacts and changes into ferric iron, which does not stay dissolved. It forms solid particles that look orange, yellow, or brown. This change takes time, so the glass may look fine for a few minutes and then slowly stain.

The same water can look clear at the tap, cloudy in a pitcher on the counter, and stained after a day in the sink. Each stage gives you a clue.

What the change suggests

  • Clear at the tap, then orange after sitting: dissolved iron is a likely cause.
  • Orange from the start: particles are already in the water, so look at the pipes, water heater, or any filter upstream.
  • Gritty or sandy material that settles at the bottom: this points more toward sediment than iron.
  • Only hot water changes color: the water heater or its tank becomes the main suspect.

A simple glass test

  1. Run cold water at the kitchen tap for several minutes so the line clears.
  2. Fill a clear glass and look at it in daylight against a white background.
  3. Set it on the counter and check again after an hour, then the next morning.
  4. Repeat with hot water, and with cold water from a bathroom tap at the far end of the house.

If cold water stays clear at first and then stains, dissolved iron is a strong possibility. If the cold water is orange from the start, the particles are already present. If only the hot tap stains, focus on the water heater.

Check the aerator before you blame the supply

Faucet aerators and screens can trap small particles and debris. A cloudy spray that clears in the glass is often trapped air, which is harmless. Unscrew the aerator, rinse it under clean water, and look at the screen. If the screen is coated or has debris, clean it and repeat the glass test. If the water still changes after the aerator is clean, the source is likely upstream.

Testing at home or in a lab

Home test strips and drop kits can give a rough reading for iron, but they are a starting point, not a final answer. For a more reliable result, send a sample to a certified water testing lab. Ask the lab to test for total iron, and ask whether it can also measure dissolved iron, since that is the form most likely to stain after sitting. Follow the lab's sample instructions closely, and note whether the sample came from cold or hot water.

What to do next

  1. Repeat the glass test on cold and hot water to confirm the pattern.
  2. Clean the aerators and screens at the faucets you use most.
  3. Send a sample to a certified lab if the pattern points to iron.
  4. If iron is confirmed, talk with a qualified water treatment professional about options, and ask how they reached their recommendation.

Iron is a common concern for homes across the St. Louis area, and a clear glass that turns orange is one of the most useful clues you can observe yourself. Careful observation, a few checks at the faucets, and a lab sample will usually show you whether iron is the cause before you decide on any changes.