A dishwasher does not scrub, it dissolves

The points below about dishwashers are ordered by how much difference they make, not by how often they get repeated.

Chemistry does the work, water does the delivery

Dishwasher detergent is strongly alkaline, which breaks down fats and proteins in a way that mechanical scrubbing does not. It is deliberately too aggressive for skin contact, which is part of why it cleans without hands or a brush. Enzymes in the detergent attack specific soils, cutting starches and proteins into fragments small enough to wash away.

The spray arms exist to distribute this chemistry over every surface and to carry the loosened material to the drain. This is why water pressure matters far less than coverage, and why a blocked arm ruins a cycle completely.

The arms only reach what they can see

Spray arms rotate because the jets are angled, and the reaction from the water leaving them drives the rotation. Anything the jets cannot reach directly is washed only by splashing, which is far less effective at removing residue. A large flat item leaning across the lower basket shadows everything behind it for the entire cycle.

Bowls and cups facing upward collect water and dirty residue rather than draining it, which is why they are loaded inverted. Checking that both arms spin freely with the machine loaded catches the most common cause of a poor result.

Temperature staging and the pre-rinse mistake

A cycle usually begins with a cooler stage where enzymes are active, then heats for the main wash where alkalinity dominates. Enzymes denature above a certain temperature, so starting hot destroys them before they have done anything useful.

This is why a longer cycle at a moderate temperature often cleans better than a short hot one. Enzymes also need something to act on, and rinsing plates thoroughly beforehand leaves them with almost no work. Scraping solids into the bin while leaving the residue in place is the behaviour the chemistry was designed around.

Hard water, salt and the resin

Dissolved calcium and magnesium react with detergent and leave deposits on glass and on the machine's own components. Many machines contain a chamber of ion exchange resin that swaps those minerals for sodium as water passes through.

In the datasheet, the resin becomes saturated and is regenerated using a strong brine, which is what the coarse salt reservoir supplies. That salt is not table salt and is not a cleaning agent; it exists only to recharge the resin between cycles. Machines in soft water areas need little or none, which is why the setting is adjustable rather than fixed.

Rinse aid changes the shape of a droplet

Water on a smooth surface beads up because surface tension pulls it into droplets that dry in place. Anything dissolved in those droplets is left behind as a spot when the water evaporates, which is the classic glass marking. Rinse aid is a surfactant that lowers surface tension so water forms a thin sheet and runs off instead.

Less water remaining means less residue and faster drying, without any additional heat being applied at all. Too much produces a film of its own, which is why the dispenser has an adjustable dose rather than one setting.

Implementations differ, and vendors are not obliged to document the differences.

Drying without a heating element

Heated drying uses an element or a fan to evaporate remaining water, which is effective and consumes noticeable energy. Condensation drying instead relies on the load being hot and the stainless interior being cooler, so moisture condenses on the walls.

Mechanically, it is quieter and cheaper but slower, and plastic items dry poorly because they hold little heat to drive evaporation. Some machines pop the door open at the end, letting humid air escape and cool air replace it. Others pass air over a mineral that adsorbs moisture and releases it again during the next wash when heated.

Everything above, in order of what to do first

1. Chemistry does the work, water does the delivery. Dishwasher detergent is strongly alkaline, which breaks down fats and proteins in a way that mechanical scrubbing does not.
2. The arms only reach what they can see. Spray arms rotate because the jets are angled, and the reaction from the water leaving them drives the rotation.
3. Temperature staging and the pre-rinse mistake. A cycle usually begins with a cooler stage where enzymes are active, then heats for the main wash where alkalinity dominates.
4. Hard water, salt and the resin. Dissolved calcium and magnesium react with detergent and leave deposits on glass and on the machine's own components.
5. Rinse aid changes the shape of a droplet. Water on a smooth surface beads up because surface tension pulls it into droplets that dry in place.
6. Drying without a heating element. Heated drying uses an element or a fan to evaporate remaining water, which is effective and consumes noticeable energy.

The takeaway

Scrape, do not rinse, and make sure the arms can turn.

The constraint is almost always physical, and marketing rarely mentions which one.